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| 19-1481; Rev 0; 4/99 KIT ATION EVALU ABLE AVAIL +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC General Description Features o 18-Bit Resolution, Sigma-Delta ADC o 16-Bit Accuracy with No Missing Codes to 480sps o Matched On-Board Current Sources (200A) for Sensor Excitation o Low Quiescent Current 250A (operating mode) 2A (power-down mode) o 3 Fully Differential or 5 Pseudo-Differential Signal Input Channels o 2 Additional, Fully Differential Calibration Channels/Auxiliary Input Channels o Programmable Gain and Offset o Fully Differential Reference Inputs o Converts Continuously or On Command o Automatic Channel Scanning and Continuous Data Output Mode o Operates with Analog and Digital Supplies from +2.7V to +3.6V o SPITM/QSPITM-Compatible 3-Wire Serial Interface o 28-Pin SSOP Package MAX1403 The MAX1403 18-bit, low-power, multichannel, serialoutput analog-to-digital converter (ADC) features matched 200A current sources for sensor excitation. This ADC uses a sigma-delta modulator with a digital decimation filter to achieve 16-bit accuracy. The digital filter's user-selectable decimation factor allows the conversion resolution to be reduced in exchange for a higher output data rate. True 16-bit performance is achieved at an output data rate of up to 480sps. In addition, the modulator sampling frequency may be optimized for either lowest power dissipation or highest throughput rate. The MAX1403 operates from +3V. This device offers three fully differential input channels that may be independently programmed with a gain between +1V/V and +128V/V. Furthermore, it can compensate an input-referred DC offset up to 117% of the selected full-scale range. These three differential channels may also be configured to operate as five pseudodifferential input channels. Two additional, fully differential system-calibration channels are provided for gain and offset error correction. The MAX1403 can be configured to sequentially scan all signal inputs and provide the results via the serial interface with minimum communications overhead. When used with a 2.4576MHz or 1.024MHz master clock, the digital decimation filter can be programmed to produce zeros in its frequency response at the line frequency and associated harmonics, ensuring excellent line rejection without the need for further postfiltering. The MAX1403 is available in a 28-pin SSOP package. Pin Configuration TOP VIEW CLKIN 1 28 SCLK 27 DIN 26 DOUT 25 INT 24 VDD Applications Portable Industrial Instruments Portable Weigh Scales Loop-Powered Systems Pressure Transducers CLKOUT 2 CS 3 RESET 4 DS1 5 DS0 6 OUT2 7 OUT1 8 AGND 9 MAX1403 23 DGND 22 CALOFF+ 21 CALOFF20 REFIN+ 19 REFIN18 CALGAIN+ 17 CALGAIN16 AIN6 15 AIN5 Ordering Information PART MAX1403CAI MAX1403EAI TEMP. RANGE 0C to +70C -40C to +85C PIN-PACKAGE 28 SSOP 28 SSOP V+ 10 AIN1 11 AIN2 12 AIN3 13 AIN4 14 SPI and QSPI are trademarks of Motorola, Inc. SSOP ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 ABSOLUTE MAXIMUM RATINGS V+ to AGND, DGND .................................................-0.3V to +6V VDD to AGND, DGND ...............................................-0.3V to +6V AGND to DGND.....................................................-0.3V to +0.3V Analog Inputs to AGND................................-0.3V to (V+ + 0.3V) Analog Outputs to AGND .............................-0.3V to (V+ + 0.3V) Reference Inputs to AGND...........................-0.3V to (V+ + 0.3V) CLKIN and CLKOUT to DGND...................-0.3V to (VDD + 0.3V) All Other Digital Inputs to DGND..............................-0.3V to +6V All Digital Outputs to DGND .......................-0.3V to (VDD + 0.3V) Maximum Current Input into Any Pin ..................................50mA Continuous Power Dissipation (TA = +70C) 28-Pin SSOP (derate 9.52mW/C above +70C) ........524mW Operating Temperature Ranges MAX1403CAI .....................................................0C to +70C MAX1403EAI...................................................-40C to +85C Storage Temperature Range .............................-60C to +150C Lead Temperature (soldering, 10sec) .............................+300C Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (V+ = +2.7V to +3.6V, VDD = +2.7V to +3.6V, VREFIN+ = +1.25V, REFIN- = AGND, fCLKIN = 2.4576MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER STATIC PERFORMANCE Noise-Free Resolution Output Noise Integral Nonlinearity (Note 1) Nominal Gain (Note 2) Unipolar Offset Error Unipolar Offset Drift Bipolar Zero Error Bipolar Zero Drift Positive Full-Scale Error (Note 3) Full-Scale Drift (Note 4) Gain Error (Note 5) Gain-Error Drift (Note 6) Bipolar Negative Full-Scale Error Bipolar Negative Full-Scale Drift For gains of 1, 2, 4 For gains of 8, 16, 32, 64, 128 For gains of 1, 2, 4, 8, 16, 32, 64 For gain of 128 For gains of 1, 2, 4 For gains of 8, 16, 32, 64, 128 For gains of 1, 2, 4, 8, 16, 32, 64 For gain of 128 For gains of 1, 2, 4, 8, 16, 32, 64 For gain of 128 For gains of 1, 2, 4, 8, 16, 32, 64 For gain of 128 For gains of 1, 2, 4 For gains of 8, 16, 32, 64, 128 -2.5 -3.5 0.8 0.3 -2 -3 1 5 2.5 3.5 -2.5 -3.5 0.8 0.3 2 3 Relative to nominal of 1% FSR For gains of 1, 2, 4 For gains of 8, 16, 32, 64, 128 -2.0 0.8 0.3 2.5 3.5 -1 0.5 0.3 2.0 INL No missing codes guaranteed by design; for filter settings with FS1 = 0 Depends on filter setting and selected gain Bipolar mode; FS1 = 0; MF1, MF0 = 0 FS1 = 0; MF1, MF0 = 1, 2, 3 16 (Tables 16a, 16b) -0.0015 0.001 0.98 2 %FSR V/C %FSR V/C %FSR V/C %FSR ppm/C %FSR V/C 0.0015 %FSR Bits SYMBOL CONDITIONS MIN TYP MAX UNITS 2 _______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 ELECTRICAL CHARACTERISTICS (continued) (V+ = +2.7V to +3.6V, VDD = +2.7V to +3.6V, VREFIN+ = +1.25V, REFIN- = AGND, fCLKIN = 2.4576MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER OFFSET DAC Offset DAC Range (Note 7) Offset DAC Resolution Offset DAC Full-Scale Error Offset DAC Zero-Scale Error Additional Noise from Offset DAC (Note 8) DAC code = 0000 Unipolar mode Bipolar mode Unipolar mode Bipolar mode Input referred -2.5 0 0 -116.7 -58.35 16.7 8.35 2.5 116.7 58.35 %FSR %FSR %FSR %FSR VRMS SYMBOL CONDITIONS MIN TYP MAX UNITS ANALOG INPUTS/REFERENCE INPUTS (Specifications for AIN and REFIN, unless otherwise noted.) At DC Common-Mode Rejection CMR For filter notch 50Hz, 0.02 * fNOTCH, MF1 = 0, MF0 = 0, fCLKIN = 2.4576MHz (Note 9) For filter notch 60Hz, 0.02 * fNOTCH, MF1 = 0, MF0 = 0, fCLKIN = 2.4576MHz (Note 9) Normal Mode 50Hz Rejection (Note 9) Normal Mode 60Hz Rejection (Note 9) Common-Mode Voltage Range (Note 10) Absolute Input Voltage Range Absolute and Common-Mode AIN Voltage Range DC Input Leakage Current (Note 11) AIN Input Current (Note 11) NMR NMR For filter notch 50Hz, 0.02 * fNOTCH, MF1 = 0, MF0 = 0, fCLKIN = 2.4576MHz For filter notch 60Hz, 0.02 * fNOTCH, MF1 = 0, MF0 = 0, fCLKIN = 2.4576MHz REFIN and AIN for BUFF = 0 REFIN and AIN for BUFF = 0 BUFF = 1 REFIN and AIN for BUFF = 0 BUFF = 1 Gain = 1 AIN Input Capacitance (Notes 12) BUFF = 0 Gain = 2 Gain = 4 Gain = 8, 16, 32, 64, 128 BUFF = 1, all gains AIN Differential Voltage Range (Note 13) Unipolar input range (U/B bit = 1) Bipolar input range (U/B bit = 0) 34 38 45 60 30 0 to VREF / gain VREF / gain V pF TA = +25C TA = TMIN to TMAX 90 150 150 100 100 VAGND VAGND - 30mV VAGND + 200mV 40 10 10 V+ V+ + 30mV V+ - 1.5 dB dB V V V pA nA nA dB _______________________________________________________________________________________ 3 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 ELECTRICAL CHARACTERISTICS (continued) (V+ = +2.7V to +3.6V, VDD = +2.7V to +3.6V, VREFIN+ = +1.25V, REFIN- = AGND, fCLKIN = 2.4576MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER AIN and REFIN Input Sampling Frequency REFIN+ - REFIN- Voltage (Note 14) LOGIC INPUTS Input Current Input Low Voltage Input High Voltage Input Hysteresis LOGIC OUTPUTS Output Low Voltage (Note 15) Output High Voltage (Note 15) Floating-State Leakage Current Floating-State Output Capacitance Current Initial Tolerance Drift TRANSDUCER EXCITATION CURRENTS Current Initial Tolerance Drift Match Drift Match Compliance Voltage Range POWER REQUIREMENTS V+ Voltage VDD Voltage Power-Supply Rejection V+ (Note 17) V+ VDD PSR For specified performance 2.7 2.7 (Note 18) 3.6 3.6 V V dB VAGND OUT1 to OUT2 5 V+ - 1.0 100 1 IEXC 200 15 uA % ppm/C % ppm/C V VOL VOH IL CO DOUT and INT, ISINK = 100A CLKOUT, ISINK = 10A DOUT and INT, ISOURCE = 100A CLKOUT, ISOURCE = 10A VDD - 0.3 VDD - 0.3 -10 9 10 0.4 0.4 V V A pF IIN VIL VIH VHYS All inputs except CLKIN CLKIN only All inputs except CLKIN CLKIN only All inputs except CLKIN 2 2.4 200 -10 +10 0.4 0.4 A V V mV SYMBOL fS 5% for specified performance; functional with lower VREF CONDITIONS MIN TYP (Table 15) 1.25 MAX UNITS Hz V TRANSDUCER BURN-OUT (Note 16) IBO 0.1 10 0.05 A % %/C 4 _______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC ELECTRICAL CHARACTERISTICS (continued) (V+ = +2.7V to +3.6V, VDD = +2.7V to +3.6V, VREFIN+ = +1.25V, REFIN- = AGND, fCLKIN = 2.4576MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS MAX1403 ANALOG POWER-SUPPLY CURRENT (Measured with digital inputs at either DGND or VDD, external CLKIN, burn-out and transducer excitation currents disabled, X2CLK = 0, CLK = 0 for 1.024MHz, CLK = 1 for 2.4576MHz.) V+ Standby Current (Note 19) PD bit = 1, external clock stopped Normal mode, MF1 = 0, MF0 = 0 1.024MHz 2.4576MHz 1.024MHz 2.4576MHz 1.024MHz 2.4576MHz 1.024MHz 2.4576MHz Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on 1 175 370 250 610 245 610 0.42 1.2 0.42 1.2 1.8 4.8 1.8 4.8 1.8 4.8 2.2 6 2.2 6 mA 0.55 1.5 10 210 420 300 700 A A 2X mode, MF1 = 0, MF0 = 1 V+ Current IV+ 4X mode, MF1 = 1, MF0 = 0 8X mode, MF1 = 1, MF0 = 1 DIGITAL POWER-SUPPLY CURRENT (Measured with digital inputs at either DGND or VDD, external CLKIN, burn-out and transducer excitation currents disabled, X2CLK = 0, CLK = 0 for 1.024MHz, CLK = 1 for 2.4576MHz.) VDD Standby Current (Note 19) PD bit = 1, external clock stopped Normal mode, MF1 = 0, MF0 = 0 2X mode, MF1 = 0, MF0 = 1 Digital Supply Current IDD 4X mode, MF1 = 1, MF0 = 0 8X mode, MF1 = 1, MF0 = 1 1.024MHz 2.4576MHz 1.024MHz 2.4576MHz 1.024MHz 2.4576MHz 1.024MHz 2.4576MHz 1 70 150 0.08 0.17 0.11 0.22 0.15 0.32 0.50 0.40 0.35 mA 10 200 300 A A _______________________________________________________________________________________ 5 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 ELECTRICAL CHARACTERISTICS (continued) (V+ = +2.7V to +3.6V, VDD = +2.7V to +3.6V, VREFIN+ = +1.25V, REFIN- = AGND, fCLKIN = 2.4576MHz, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS POWER DISSIPATION (V+ = VDD = +3.3V, digital inputs = 0 or VDD, external CLKIN, burn-out and transducer excitation currents disabled, X2CLK = 0, CLK = 0 for 1.024MHz, CLK = 1 for 2.4576MHz.) Normal mode, MF1 = 0, MF0 = 0 1.024MHz 2.4576MHz 1.024MHz 2.4576MHz 1.024MHz 2.4576MHz 1.024MHz 2.4576MHz Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on Buffers off Buffers on 0.81 1.45 1.32 2.51 1.08 2.28 1.95 4.53 1.75 4.32 6.67 16.6 6.44 16.4 7.0 16.9 7 8.91 21.45 70 W 8.58 21.2 2.97 6.11 mW 1.36 2.05 1.98 3.30 2X mode, MF1 = 0, MF0 = 1 Power Dissipation PD 4X mode, MF1 = 1, MF0 = 0 8X mode, MF1 = 1, MF0 = 1 Standby Power Dissipation (Note 19) Note 1: Contact factory for INL limits applicable with FS1 = 0 and MF1, MF0 = 1, 2, or 3. Note 2: Nominal gain is 0.98. This ensures a full-scale input voltage may be applied to the part under all conditions without causing saturation of the digital output data. Note 3: Positive Full-Scale Error includes zero-scale errors (unipolar offset error or bipolar zero error) and applies to both unipolar and bipolar input ranges. This error does not include the nominal gain of 0.98. Note 4: Full-Scale Drift includes zero-scale drift (unipolar offset drift or bipolar zero drift) and applies to both unipolar and bipolar input ranges. Note 5: Gain Error does not include zero-scale errors. It is calculated as (full-scale error - unipolar offset error) for unipolar ranges and as (full-scale error - bipolar zero error) for bipolar ranges. This error does not include the nominal gain of 0.98. Note 6: Gain-Error Drift does not include unipolar offset drift or bipolar zero drift. It is effectively the drift of the part if zero-scale error is removed. Note 7: Use of the offset DAC does not imply that any input may be taken below AGND. Note 8: Additional noise added by the offset DAC is dependent on the filter cutoff, gain, and DAC setting. No noise is added for a DAC code of 0000. Note 9: Guaranteed by design or characterization; not production tested. Note 10: The absolute input voltage must be within the input voltage range specification. Note 11: All AIN and REFIN pins have identical input structures. Leakage is production tested only for the AIN3, AIN4, AIN5, CALGAIN, and CALOFF inputs. Note 12: The dynamic load presented by the MAX1403 analog inputs for each gain setting is discussed in detail in the Switching Network section. Values are provided for the maximum allowable external series resistance. Note 13: The input voltage range for the analog inputs is with respect to the voltage on the negative input of its respective differential or pseudo-differential pair. Table 5 shows which inputs form differential pairs. Note 14: VREF = VREFIN+ - VREFIN-. Note 15: These specifications apply to CLKOUT only when driving a single CMOS load. 6 _______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Note 16: The burn-out currents require a 500mV overhead between the analog input voltage and both V+ and AGND to operate correctly. Note 17: Measured at DC in the selected passband. PSR at 50Hz will exceed 120dB with filter notches of 25Hz or 50Hz and FAST bit = 0. PSR at 60Hz will exceed 120dB with filter notches of 20Hz or 60Hz and FAST bit = 0. Note 18: PSR depends on gain. For a gain of +1V/V, PSR is 70dB typical. For a gain of +2V/V, PSR is 75dB typical. For a gain of +4V/V, PSR is 80dB typical. For gains of +8V/V to +128V/V, PSR is 85dB typical. Note 19: Standby power-dissipation and current specifications are valid only with CLKIN driven by an external clock and with the external clock stopped. If the clock continues to run in standby mode, the power dissipation will be considerably higher. When used with a resonator or crystal between CLKIN and CLKOUT, the actual power dissipation and IDD in standby mode will depend on the resonator or crystal type. TIMING CHARACTERISTICS (V+ = +2.7V to +3.6V, VDD = +2.7V to +3.6V, AGND = DGND, fCLKIN = 2.4576MHz, input logic 0 = 0V, logic 1 = VDD, TA = TMIN to TMAX, unless otherwise noted.) (Notes 20, 21, 22) PARAMETER Master Clock Frequency SYMBOL fCLKIN CONDITIONS Crystal oscillator or clock exterX2CLK = 0 nally supplied for specified perforX2CLK = 1 mance (Notes 23, 24) tCLKIN = 1 / fCLKIN, X2CLK = 0 tCLKIN = 1 / fCLKIN, X2CLK = 0 X2CLK = 0, N = 2(2 * MF1 + MF0) INT High Time tINT X2CLK = 1, N = 2(2 * MF1 + MF0) RESET Pulse Width Low INT to CS Setup Time (Note 9) SCLK Setup to Falling Edge CS CS Falling Edge to SCLK Falling Edge Setup Time SCLK Falling Edge to Data Valid Delay (Notes 25, 26) SCLK High Pulse Width SCLK Low Pulse Width CS Rising Edge to SCLK Rising Edge Hold Time (Note 22) Bus Relinquish Time After SCLK Rising Edge (Note 27) SCLK Rising Edge to INT High (Note 28) t2 t3 t4 t5 t6 t7 t8 t9 t10 t11 MIN 0.4 0.8 0.4 * tCLKIN 0.4 * tCLKIN 280 / N TYP MAX 2.5 MHz 5.0 ns ns UNITS Master Clock Input Low Time Master Clock Input High Time fCLKIN LO fCLKIN HI * tCLKIN 560 / N * tCLKIN 100 0 30 30 0 0 100 100 0 10 100 200 100 100 ns ns ns ns ns ns ns ns ns ns ns SERIAL-INTERFACE READ OPERATION SERIAL-INTERFACE WRITE OPERATION SCLK Setup to Falling Edge CS t12 30 ns _______________________________________________________________________________________ 7 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 TIMING CHARACTERISTICS (continued) (V+ = +2.7V to +3.6V, VDD = +2.7V to +3.6V, AGND = DGND, fCLKIN = 2.4576MHz, input logic 0 = 0V, logic 1 = VDD, TA = TMIN to TMAX, unless otherwise noted.) (Notes 20, 21, 22) PARAMETER CS Falling Edge to SCLK Falling Edge Setup Time Data Valid to SCLK Rising Edge Setup Time Data Valid to SCLK Rising Edge Hold Time SCLK High Pulse Width SCLK Low Pulse Width CS Rising Edge to SCLK Rising Edge Hold Time SYMBOL t13 t14 t15 t16 t17 t18 CONDITIONS MIN 30 30 0 100 100 0 TYP MAX UNITS ns ns ns ns ns ns AUXILIARY DIGITAL INPUTS (DS0 and DS1) DS0/DS1 to SCLK Falling Edge Setup Time (Notes 21, 29) DS0/DS1 to SCLK Falling Edge Hold Time (Notes 21, 29) t19 t20 40 0 ns ns Note 20: All input signals are specified with tr = tf = 5ns (10% to 90% of VDD) and timed from a voltage level of 1.6V. Note 21: See Figure 4. Note 22: Timings shown in tables are for the case where SCLK idles high between accesses. The part may also be used with SCLK idling low between accesses, provided CS is toggled. In this case SCLK in the timing diagrams should be inverted and the terms "SCLK Falling Edge" and "SCLK Rising Edge" exchanged in the specification tables. If CS is permanently tied low, the part should only be operated with SCLK idling high between accesses. Note 23: CLKIN duty cycle range is 45% to 55%. CLKIN must be supplied whenever the MAX1403 is not in standby mode. If no clock is present, the device can draw higher current than specified. Note 24: The MAX1403 is production tested with fCLKIN at 2.5MHz (1MHz for some IDD tests). Note 25: Measured with the load circuit of Figure 1 and defined as the time required for the output to cross the VOL or VOH limits. Note 26: For read operations, SCLK active edge is falling edge of SCLK. Note 27: Derived from the time taken by the data output to change 0.5V when loaded with the circuit of Figure 1. The number is then extrapolated back to remove effects of charging or discharging the 50pF capacitor. This ensures that the times quoted in the timing characteristics are true bus-relinquish times and are independent of external bus loading capacitances. Note 28: INT returns high after the first read after an output update. The same data can be read again while INT is high, but be careful not to allow subsequent reads to occur close to the next output update. Note 29: Auxiliary inputs DS0 and DS1 are latched on the first falling edge of SCLK during a data-read cycle. 100A at VDD = +3.3V TO OUTPUT PIN 50pF 100A at VDD = +3.3V Figure 1. Load Circuit for Bus-Relinquish Time and VOL and VOH Levels 8 _______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC Typical Operating Characteristics (V+ = +3V, VDD = +3V, VREFIN+ = +1.25V, REFIN- = AGND, fCLKIN = 2.4576MHz, transducer excitation currents disabled, TA = +25C, unless otherwise noted.) OUT1 AND OUT2 COMPLIANCE MAX1402 toc01 MAX1403 DIFFERENTIAL NONLINEARITY MAX1403-02 INTEGRAL NONLINEARITY 480sps GAIN = +1V/V 262, 144 pts MAX1403-03 250 15 10 5 480sps GAIN = +1V/V 262, 144 pts 15 10 5 INL (ppm) 0 -5 -10 -15 200 OUTPUT CURRENT (A) 150 DNL (ppm) V+ = +3.3V 0 -5 100 50 -10 -15 1.5 2.0 2.5 3.0 3.5 -1.0 -0.5 0 0.5 1.0 DIFFERENTIAL INPUT VOLTAGE (V) 0 0 0.5 1.0 COMPLIANCE VOLTAGE (V) -1.0 -0.5 0 0.5 1.0 DIFFERENTIAL INPUT VOLTAGE (V) VDD SUPPLY CURRENT vs. TEMPERATURE (20sps OUTPUT DATA RATE UNBUFFERED) MAX1403 toc04 VDD SUPPLY CURRENT vs. TEMPERATURE (60sps OUTPUT DATA RATE UNBUFFERED) MAX1403 toc05 VDD SUPPLY CURRENT vs. TEMPERATURE (120sps OUTPUT DATA RATE UNBUFFERED) MAX1403 toc06 350 300 VDD SUPPLY CURRENT (A) 250 200 150 100 50 0 -50 -25 0 25 50 75 VDD = +3.6V (NOTE 30) 350 300 VDD SUPPLY CURRENT (A) 250 200 150 100 50 0 VDD = +3.6V (NOTE 30) -50 -25 0 25 50 75 350 300 VDD SUPPLY CURRENT (A) 250 200 150 100 50 0 VDD = +3.6V (NOTE 30) -50 -25 0 25 50 75 100 100 100 TEMPERATURE (C) TEMPERATURE (C) TEMPERATURE (C) V+ SUPPLY CURRENT vs. TEMPERATURE (20sps OUTPUT DATA RATE) MAX1403 toc07 V+ SUPPLY CURRENT vs. TEMPERATURE (60sps OUTPUT DATA RATE) 600 V+ SUPPLY CURRENT (A) 500 400 300 200 UNBUFFERED 100 0 BUFFERED MAX1403 toc08 V+ SUPPLY CURRENT vs. TEMPERATURE (120sps OUTPUT DATA RATE) MAX1403 toc09 400 350 V+ SUPPLY CURRENT (A) 300 250 200 150 100 50 0 -50 -25 0 25 50 75 UNBUFFERED BUFFERED 1200 1000 V+ SUPPLY CURRENT (A) BUFFERED 800 600 400 200 0 UNBUFFERED 100 -50 -25 0 25 50 75 100 -50 -25 0 25 50 75 100 TEMPERATURE (C) TEMPERATURE (C) TEMPERATURE (C) _______________________________________________________________________________________ 9 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Typical Operating Characteristics (continued) (V+ = +3V, VDD = +3V, VREFIN+ = +1.25V, REFIN- = AGND, fCLKIN = 2.4576MHz, transducer excitation currents disabled, TA = +25C, unless otherwise noted.) VDD SUPPLY CURRENT vs. TEMPERATURE (240sps OUTPUT DATA RATE UNBUFFERED) MAX1403 toc10 VDD SUPPLY CURRENT vs. TEMPERATURE (480sps OUTPUT DATA RATE UNBUFFERED) 600 VDD SUPPLY CURRENT (A) 500 400 300 200 100 0 VDD = +3.6V (NOTE 30) -50 -25 0 25 50 75 100 MAX1403 toc11 450 400 VDD SUPPLY CURRENT (A) 350 300 250 200 150 100 50 0 -50 -25 0 25 50 75 VDD = +3.6V (NOTE 30) 100 TEMPERATURE (C) TEMPERATURE (C) V+ SUPPLY CURRENT vs. TEMPERATURE (240sps OUTPUT DATA RATE) MAX1403 toc12 V+ SUPPLY CURRENT vs. TEMPERATURE (480sps OUTPUT DATA RATE) 5000 BUFFERED MAX1403 toc13 5000 V+ SUPPLY CURRENT (A) BUFFERED 3000 2000 UNBUFFERED V+ SUPPLY CURRENT (A) 4000 4000 3000 2000 1000 1000 UNBUFFERED 0 -50 -25 0 25 50 75 100 TEMPERATURE (C) 0 -50 -25 0 25 50 75 100 TEMPERATURE (C) Note 30: Minimize capacitive loading at CLKOUT for lowest VDD supply current. Typical Operating Characteristics show VDD supply current with CLKOUT loaded by 120pF. 10 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC Pin Description PIN 1 NAME CLKIN FUNCTION Clock Input. A crystal can be connected across CLKIN and CLKOUT. Alternatively, drive CLKIN with a CMOS-compatible clock at a nominal frequency of 2.4576MHz or 1.024MHz, and leave CLKOUT unconnected. Frequencies of 4.9152MHz and 2.048MHz may be used if the X2CLK control bit is set to 1. Clock Output. When deriving the master clock from a crystal, connect the crystal between CLKIN and CLKOUT. In this mode, the on-chip clock signal is not available at CLKOUT. Leave CLKOUT unconnected when CLKIN is driven with an external clock. Chip-Select Input. This active-low logic input is used to enable the digital interface. With CS hard-wired low, the MAX1403 operates in its 3-wire interface mode with SCLK, DIN, and DOUT used to interface to the device. CS is used either to select the device in systems with more than one device on the serial bus, or as a frame-synchronization signal for the MAX1403, when a continuous SCLK is used. Active-Low Reset Input. Drive low to reset the control logic, interface logic, digital filter, and analog modulator to power-on status. RESET must be high and CLKIN must be toggling in order to exit reset. Digital Input for Auxiliary Data Input Bit 1. The status of this bit is reflected in the output data by bit D4. Used to communicate the status of DS1 via the serial interface. Digital Input for Auxiliary Data Input Bit 0. The status of this bit is reflected in the output data by bit D3. Used to communicate the status of DS0 via the serial interface. Transducer Excitation Current Source 2 Transducer Excitation Current Source 1 Analog Ground. Reference point for the analog circuitry. AGND connects to the IC substrate. Analog Positive Supply Voltage (+2.7V to +3.6V). Analog Input Channel 1. May be used as a pseudo-differential input with AIN6 as common, or as the positive input of the AIN1/AIN2 differential analog input pair (see On-Chip Registers section). Analog Input Channel 2. May be used as a pseudo-differential input with AIN6 as common, or as the negative input of the AIN1/AIN2 differential analog input pair (see On-Chip Registers section). Analog Input Channel 3. May be used as a pseudo-differential input with AIN6 as common, or as the positive input of the AIN3/AIN4 differential analog input pair (see On-Chip Registers section). Analog Input Channel 4. May be used as a pseudo-differential input with AIN6 as common, or as the negative input of the AIN3/AIN4 differential analog input pair (see On-Chip Registers section). Analog Input Channel 5. Used as a differential or pseudo-differential input with AIN6 (see On-Chip Registers section). Analog Input 6. May be used as a common point for AIN1 through AIN5 in pseudo-differential mode, or as the negative input of the AIN5/AIN6 differential analog input pair (see On-Chip Registers section). Negative Gain Calibration Input. Used for system gain calibration. It forms the negative input of a fully differential input pair with CALGAIN+. Normally these inputs are connected to reference voltages in the system. When system gain calibration is not required and the auto-sequence mode is used, the CALGAIN+/CALGAIN- input pair provides an additional fully differential input channel. Positive Gain Calibration Input. Used for system gain calibration. It forms the positive input of a fully differential input pair with CALGAIN-. Normally these inputs are connected to reference voltages in the system. When system gain calibration is not required and the auto-sequence mode is used, the CALGAIN+/CALGAIN- input pair provides an additional fully differential input channel. MAX1403 2 CLKOUT 3 CS 4 5 6 7 8 9 10 11 12 13 14 15 16 RESET DS1 DS0 OUT2 OUT1 AGND V+ AIN1 AIN2 AIN3 AIN4 AIN5 AIN6 17 CALGAIN- 18 CALGAIN+ ______________________________________________________________________________________ 11 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Pin Description (continued) PIN 19 20 NAME REFINREFIN+ FUNCTION Negative Differential Reference Input. Bias REFIN- between V+ and AGND, provided that REFIN+ is more positive than REFIN-. Positive Differential Reference Input. Bias REFIN+ between V+ and AGND, provided that REFIN+ is more positive than REFIN-. Negative Offset Calibration Input. Used for system offset calibration. It forms the negative input of a fully differential input pair with CALOFF+. Normally these inputs are connected to zero-reference voltages in the system. When system offset calibration is not required and the auto-sequence mode is used, the CALOFF+/CALOFF- input pair provides an additional fully differential input channel. Positive Offset Calibration Input. Used for system offset calibration. It forms the positive input of a fully differential input pair with CALOFF-. Normally these inputs are connected to zero-reference voltages in the system. When system offset calibration is not required and the auto-sequence mode is used, the CALOFF+/CALOFF- input pair provides an additional fully differential input channel. Digital Ground. Reference point for digital circuitry. Digital Supply Voltage (+2.7V to +3.6V) Interrupt Output. A logic low indicates that a new output word is available from the data register. INT returns high upon completion of a full output word read operation. INT also returns high for short periods (determined by the filter and clock control bits) if no data read has taken place. A logic high indicates internal activity, and a read operation should not be attempted under this condition. INT can also provide a strobe to indicate valid data at DOUT (MDOUT = 1). Serial Data Output. DOUT outputs data from the internal shift register containing information from the Communications Register, Global Setup Registers, Transfer Function Registers, or Data Register. DOUT can also provide the digital bit stream directly from the - modulator (MDOUT = 1). Serial Data Input. Data on DIN is written to the input shift register and later transferred to the Communications Register, Global Setup Registers, Special Function Register, or Transfer Function Registers, depending on the register selection bits in the Communications Register. Serial Clock Input. Apply an external serial clock to transfer data to and from the MAX1403. This serial clock can be continuous, with data transmitted in a train of pulses, or intermittently. If CS is used to frame the data transfer, then SCLK may idle high or low between conversions and CS determines the desired active clock edge (see Selecting Clock Polarity). If CS is tied permanently low, SCLK must idle high between data transfers. 21 CALOFF- 22 CALOFF+ 23 24 DGND VDD 25 INT 26 DOUT 27 DIN 28 SCLK 12 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC _______________Detailed Description Circuit Description The MAX1403 is a low-power, multichannel, serial-output, sigma-delta ADC designed for applications with a wide dynamic range, such as weigh scales and pressure transducers. The functional block diagram in Figure 2 contains a switching network, a modulator, a PGA, two buffers, an oscillator, an on-chip digital filter, two matched transducer excitation current sources, and a bidirectional serial communications port. Three fully differential input channels feed into the switching network. Each channel may be independently programmed with a gain between +1V/V and +128V/V. These three differential channels may also be configured to operate as five pseudo-differential input channels. Two additional, fully differential system-calibration channels allow system gain and offset error to be measured. These system-calibration channels can be used as additional differential signal channels when dedicated gain and offset error correction channels are not required. Two chopper-stabilized buffers are available to isolate the selected inputs from the capacitive loading of the PGA and modulator. Three independent DACs provide compensation for the DC component of the input signal on each of the differential input channels. The sigma-delta modulator converts the input signal into a digital pulse train whose average duty cycle represents the digitized signal information. The pulse train is then processed by a digital decimation filter, resulting in a conversion accuracy exceeding 16 bits. The digital filter's decimation factor is user-selectable, which allows the conversion result's resolution to be reduced to achieve a higher output data rate. When used with 2.4576MHz or 1.024MHz master clocks, the decimation filter can be programmed to produce zeros in its frequency response at the line frequency and associated harmonics. This ensures excellent line rejection without the need for further postfiltering. In addition, the modulator sampling frequency can be optimized for either lowest power dissipation or highest output data rate. MAX1403 V+ MAX1403 CLKIN CLKOUT OUT1 OUT2 CALOFF+ CALGAIN+ AIN1 AIN2 AIN3 AIN4 AIN5 AIN6 CALOFFCALGAINBUFFER SWITCHING NETWORK V+ PGA MODULATOR DIVIDER CLOCK GEN BUFFER VDD DIGITAL FILTER DGND V+ AGND SCLK DAC DIN DOUT INTERFACE AND CONTROL INT CS RESET REFIN+ REFINDS1 DS0 AGND Figure 2. Functional Diagram ______________________________________________________________________________________ 13 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 The MAX1403 can be configured to sequentially scan all signal inputs and to transmit the results through the serial interface with minimum communications overhead. The output word contains a channel identification tag to indicate the source of each conversion result. The MAX1403 features a mode where the raw modulator data output is accessible. In this mode, the DOUT and INT functions are reassigned (see the Modulator Data Output section). COMMUNICATIONS REGISTER DIN RS2 RS1 RS0 Serial Digital Interface The serial digital interface provides access to eight onchip registers (Figure 3). All serial-interface commands begin with a write to the communications register (COMM). On power-up, system reset, or interface reset, the part expects a write to its communications register. The COMM register access begins with a 0 start bit. The COMM register R/W bit selects a read or write operation, and the register select bits (RS2, RS1, RS0) select the register to be addressed. Hold DIN high when not writing to COMM or another register (Table 1). The serial interface consists of five signals: CS, SCLK, DIN, DOUT, and INT. Clock pulses on SCLK shift bits into DIN and out of DOUT. INT provides an indication that data is available. CS is a device chip-select input as well as a clock polarity select input (Figure 4). Using CS allows the SCLK, DIN, and DOUT signals to be shared among several SPI-compatible devices. When short on I/O pins, connect CS low and operate the serial digital interface in CPOL = 1, CPHA = 1 mode using SCLK, DIN, and DOUT. This 3-wire interface mode is ideal for opto-isolated applications. Furthermore, a microcontroller (such as a PIC16C54 or 80C51) can use a single bidirectional I/O pin for both sending to DIN and receiving from DOUT (see Applications Information), because the MAX1403 drives DOUT only during a read cycle. Additionally, connecting the INT signal to a hardware interrupt allows faster throughput and reliable, collisionfree data flow. GLOBAL SETUP REGISTER 1 GLOBAL SETUP REGISTER 2 SPECIAL FUNCTION REGISTER XFER FUNCTION REGISTER 1 XFER FUNCTION REGISTER 2 XFER FUNCTION REGISTER 3 DOUT DATA REGISTER D17-D10 DATA REGISTER D9-D2 DATA REGISTER D1-D0/CID REGISTER SELECT DECODER Figure 3. Register Summary t11 INT t1 t3 CS t12 t4 SCLK (CPOL = 1) t13 t5 t16 t7 t8 t17 t9 t18 Table 1. Control-Register Addressing RS2 0 0 0 0 1 1 1 1 RS1 0 0 1 1 0 0 1 1 RS0 0 1 0 1 0 1 0 1 TARGET REGISTER Communications Register Global Setup Register 1 Global Setup Register 2 Special Function Register Transfer Function Register 1 Transfer Function Register 2 Transfer Function Register 3 Data Register DIN (DURING WRITE)* DOUT (DURING READ)* DS1, DS0 SCLK (CPOL = 0) t14 t15 MSB D6 t6 MSB D6 t19 t20 D5 D5 D4 D3 D2 D1 D0 t10 D4 D3 D2 D1 D0 *DOUT IS HIGH IMPEDANCE DURING THE WRITE CYCLE; DIN IS IGNORED DURING THE READ CYCLE. Figure 4. Serial-Interface Timing 14 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC Selecting Clock Polarity The serial interface can be operated with the clock idling either high or low. This is compatible with Motorola's SPI interface operated in CPOL = 1, CPHA = 1 mode or CPOL = 0, CPHA = 1 mode. The clock polarity is determined by the state of SCLK at the falling edge of CS. Ensure that the setup times t4/t12 and t5/t13 are not violated. If CS is connected to ground, resulting in no falling edge on CS, SCLK must idle high (CPOL = 1, CPHA = 1). Data-Ready Signal (DRDY bit true or INT = low) The data-ready signal indicates that new data may be read from the 24-bit data register. After the end of a successful data register read, the data-ready signal becomes false. If a new measurement completes before the data is read, the data-ready signal becomes false. The data-ready signal becomes true again when new data is available in the data register. The MAX1403 provides two methods of monitoring the data-ready signal. INT provides a hardware solution (active low when data is ready to be accessed), while the DRDY bit in the COMM register provides a software solution (active high). Read data as soon as possible once data-ready becomes true. This becomes increasingly important for faster measurement rates. If the data read is delayed significantly, a collision may result. A collision occurs when a new measurement completes during a dataregister read operation. After a collision, information in the data register is invalid. The failed read operation must be completed even though the data is invalid. Resetting the Interface Reset the serial interface by clocking in 32 1s. Resetting the interface does not affect the internal registers. If continuous data output mode is in use, clock in eight 0s followed by 32 1s. More than 32 1s may be clocked in, since a leading 0 is used as the start bit for all operations. Continuous Data Output Mode When scanning the input channels (SCAN = 1), the serial interface allows the data register to be read repeatedly without requiring a write to the COMM register. Communications Register First Bit (MSB) FUNCTION Name Defaults DATA RDY 0/DRDY 0 REGISTER SELECT BITS RS2 0 RS1 0 RS0 0 R/W 0 RESET 0 STDBY 0 FSYNC 0 15 (LSB) The initial COMM write (01111000) is followed by 24 clocks (DIN = high) to read the 24-bit data register. Once the data register has been read, it can be read again after the next conversion by writing another 24 clocks (DIN = high). Terminate the continuous data output mode by writing to the COMM register with any valid access. MAX1403 Modulator Data Output (MDOUT = 1) Single-bit, raw modulator data is available at DOUT for custom filtering when MDOUT = 1. INT provides a modulator clock for data synchronization. Data is valid on the falling edge of INT. Write operations can still be performed; however, read operations are disabled. After MDOUT is returned to 0, valid data is accessed by the normal serial-interface read operation. On-Chip Registers Communications Register 0/DRDY: (Default = 0) Data Ready Bit. On a write, this bit must be reset to 0 to signal the start of the Communications Register data word. On a read, a 1 in this location (0/DRDY) signifies that valid data is available in the data register. This bit is reset after the data register is read or, if data is not read, 0/DRDY will go low at the end of the next measurement. RS2, RS1, RS0: (Default = 0, 0, 0) Register Select Bits. These bits select the register to be accessed (Table 1). R/W: (Default = 0) Read/Write Bit. When set high, the selected register is read; when R/W = 0, the selected register is written. RESET: (Default = 0) Software Reset Bit. Setting this bit high causes the part to be reset to its default powerup condition (RESET = 0). STDBY: (Default = 0) Standby Power-Down Bit. Setting the STDBY bit places the part in "standby" condition, shutting down everything except the serial interface and the CLK oscillator. FSYNC: (Default = 0) Filter Sync Bit. When FSYNC = 0, conversions are automatically performed at a data rate determined by CLK, FS1, FS0, MF1, and MF0 bits. When FSYNC = 1, the digital filter and analog modulator ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 are held in reset, inhibiting normal self-timed operation. This bit may be used to convert on command to minimize the settling time to valid output data, or to synchronize operation of a number of MAX1403s. FSYNC does not reset the serial interface or the 0/DRDY flag. To clear the 0/DRDY flag while FSYNC is active, simply read the data register. FAST: (Default 0) Fast Bit. FAST = 0 causes the digital filter to perform a SINC3 filter function on the modulator data stream. The output data rate will be determined by the values in the CLK, FS1, FS0, MF1, and MF0 bits (Table 2). The settling time for SINC3 function is 3 * [1 / (output data rate)]. In SINC 3 mode, the MAX1403 automatically holds the DRDY signal false (after any significant configuration change) until settled data is available. FAST = 1 causes the digital filter to perform a SINC1 filter function on the modulator data stream. The signal-to-noise ratio achieved with this filter function is less than that of the SINC3 filter; however, SINC1 settles in a single output sample period rather than a minimum of three output sample periods for SINC3. When switching from SINC1 to SINC3 mode, the DRDY flag will be deasserted and reasserted after the filter has fully settled. This mode change requires a minimum of three samples. Global Setup Register 1 A1, A0: (Default = 0, 0) Channel-Selection Control Bits. These bits (combined with the state of the DIFF, M1, and M0 bits) determine the channel selected for conversion according to Tables 8, 9, and 10. These bits are ignored if the SCAN bit is set. MF1, MF0: (Default = 0, 0) Modulator Frequency Bits. MF1 and MF0 determine the ratio of CLKIN oscillator frequency to modulator operating frequency. They affect the output data rate, the position of the digital filter notch frequencies, and the power dissipation of the device. Achieve lowest power dissipation with MF1 = 0 and MF0 = 0. Highest power dissipation and fastest output data rate occur with these bits set to 1, 1 (Table 2). CLK: (Default = 1) CLK Bit. The CLK bit is used in conjunction with X2CLK to tell the MAX1403 the frequency of the CLKIN input signal. If CLK = 0, a CLKIN input frequency of 1.024MHz (2.048MHz for X2CLK = 1) is expected. If CLK = 1, a CLKIN input frequency of 2.4576MHz (4.9152MHz for X2CLK = 1) is expected. This bit affects the decimation factor in the digital filter and thus the output data rate (Table 2). FS1, FS0: (Default = 0, 1) Filter Selection Bits. These bits (in conjunction with the CLK bit) control the decimation ratio of the digital filter. They determine the output data rate, the position of the digital filter frequency response notches, and the noise present in the output result (Table 2). Global Setup Register 2 SCAN: (Default = 0) Scan Bit. Setting this bit to a 1 causes sequential scanning of the input channels as determined by DIFF, M1, and M0 (see Scanning (ScanMode) section). When SCAN = 0, the MAX1403 repeatedly measures the unique channel selected by A1, A0, DIFF, M1, and M0 (Table 4). M1, M0: (Default 0, 0) Mode Control Bits. These bits control access to the calibration channels CALOFF and CALGAIN. When SCAN = 0, setting M1 = 0 and M0 = 1 selects the CALOFF input, and M1 = 1 and M0 = 0 selects the CALGAIN input (Table 3). When SCAN = 1 and M1 M0, the scanning sequence includes both CALOFF and CALGAIN inputs (Table 4). When SCAN is set to 1 and the device is scanning the available input channels, selection of either calibration mode (01 or 10) will cause the scanning sequence to be extended to include a conversion on both the CALGAIN+/CALGAINinput pair and the CALOFF+/CALOFF-input pair. The Global Setup Register 1 First Bit (MSB) FUNCTION Name Defaults CHANNEL SELECTION A1 0 A0 0 MODULATOR FREQUENCY MF1 0 MF0 0 CLK 1 FILTER SELECTION FS1 0 FS0 1 FAST 0 (LSB) Global Setup Register 2 First Bit (MSB) FUNCTION Name Defaults 16 SCAN 0 MODE CONTROL M1 0 M0 0 BUFF 0 DIFF 0 BOUT 0 IOUT 0 X2CLK 0 (LSB) ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC exact sequence depends on the state of the DIFF bit (Table 4). When scanning, the calibration channels use the PGA gain, format, and DAC settings defined by the contents of Transfer Function Register 3. BUFF: (Default = 0) The BUFF bit controls operation of the input buffer amplifiers. When this bit is 0, the internal buffers are bypassed and powered down. When this bit is set high, the buffers drive the input sampling capacitors and minimize the dynamic input load. DIFF: (Default = 0) Differential/Pseudo-Differential Bit. When DIFF = 0, the part is in pseudo-differential mode, and AIN1-AIN5 are measured respective to AIN6, the analog common. When DIFF = 1, the part is in differential mode with the analog inputs defined as AIN1/AIN2, AIN3/AIN4, and AIN5/AIN6. The available input channels for each mode are tabulated in Table 5. Note that DIFF also affects the scanning sequence when the part is placed in SCAN mode (Table 4). BOUT: (Default = 0) Burn-Out Current Bit. Setting BOUT = 1 connects 100nA current sources to the selected analog input channel. This mode is used to check that a transducer has not burned out or opened circuit. The burn-out current source must be turned off (BOUT = 0) before measurement to ensure best linearity. IOUT: (Default = 0) The IOUT bit controls the Transducer Excitation Currents. A 0 in this bit disables OUT1 and OUT2, effectively making these pins highimpedance. A 1 in this location activates both IOUT1 and IOUT2, causing each pin to source 200A. X2CLK: (Default = 0) Times-Two Clock Bit. Setting this bit to 1 selects a divide-by-2 prescaler in the clock signal path. This allows use of a higher frequency crystal or clock source and improves immunity to asymmetric clock sources. MAX1403 Table 2. Data Output Rate vs. CLK, Filter Select, and Modulator Frequency Bits CLKIN FREQUENCY, fCLKIN (MHz) CLK X2CLK = 0 1.024 1.024 1.024 1.024 2.4576 2.4576 2.4576 2.4576 X2CLK = 1 2.048 2.048 2.048 2.048 4.9152 4.9152 4.9152 4.9152 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 MF1 MF0 FS1, FS0* (0, 0) 20 40 80 160 50 100 200 400 FS1, FS0* (0, 1) 25 50 100 200 60 120 240 480 FS1, FS0 (1, 0) 100 200 400 800 300 600 1200 2400 FS1, FS0 (1, 1) 200 400 800 1600 600 1200 2400 4800 AVAILABLE OUTPUT DATA RATES (sps) * Data rates offering noise-free 16-bit resolution. Note: When FAST = 0, f-3dB = 0.262 * Data Rate. When FAST = 1, f-3dB = 0.443 * Data Rate. Note: Default condition is in bold print. Table 3. Special Modes Controlled by M1, M0 (SCAN = 0) M1 0 0 M0 0 1 DESCRIPTION Normal Mode: The device operates normally. Calibrate Offset: In this mode, the MAX1403 converts the voltage applied across CALOFF+ and CALOFF-. The PGA gain, DAC, and format settings of the selected channel (defined by DIFF, A1, A0) are used. Calibrate Gain: In this mode, the MAX1403 converts the voltage applied across CALGAIN+ and CALGAIN-. The PGA gain, DAC, and format settings of the selected channel (defined by DIFF, A1, A0) are used. Reserved: Do not use. 17 1 1 0 1 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC Table 4. SCAN Mode Scanning Sequences (SCAN = 1) DIFF 0 M1 0 M0 0 SEQUENCE AIN1-AIN6, AIN2-AIN6, AIN3-AIN6, AIN4-AIN6, AIN5-AIN6 AIN1-AIN6, AIN2-AIN6, AIN3-AIN6, AIN4-AIN6, AIN5-AIN6, CALOFF, CALGAIN AIN1-AIN6, AIN2-AIN6, AIN3-AIN6, AIN4-AIN6, AIN5-AIN6, CALOFF, CALGAIN AIN1-AIN2, AIN3-AIN4, AIN5-AIN6 AIN1-AIN2, AIN3-AIN4, AIN5-AIN6, CALOFF, CALGAIN AIN1-AIN2, AIN3-AIN4, AIN5-AIN6, CALOFF, CALGAIN 0 0 1 0 1 1 1 1 0 0 1 0 0 1 0 Note: All other combinations reserved. Table 5. Available Input Channels (SCAN = 0) DIFF 0 0 0 1 1 1 M1 0 0 1 0 0 1 M0 0 1 0 0 1 0 AVAILABLE CHANNELS AIN1-AIN6, AIN2-AIN6, AIN3-AIN6, AIN4-AIN6 CALOFF CALGAIN AIN1-AIN2, AIN3-AIN4, AIN5-AIN6 CALOFF CALGAIN Special Function Register (Write-Only) MDOUT: (Default = 0) Modulator Out Bit. MDOUT = 0 enables data readout on the DOUT pin, the normal condition for the serial interface. MDOUT = 1 changes the function of the DOUT and INT pins, providing raw, single-bit modulator output instead of the normal serialdata interface output. This allows custom filtering directly on the modulator output, without going through the on-chip digital filter. The INT pin provides a clock to indicate when the modulator data at DOUT should be sampled (falling edge of INT). Note that in this mode, the on-chip digital filter continues to operate normally. When MDOUT is returned to 0, valid data may be accessed through the normal serial-interface read operation. FULLPD: (Default = 0) Complete Power-Down Bit. FULLPD = 1 forces the part into a complete power-down condition, which includes the clock oscillator. The serial interface continues to operate. The part requires a hardware reset to recover correctly from this condition. Note: Changing the reserved bits in the special-function register from the default status of all 0s will select one of the reserved modes and the part will not operate as expected. This register is a write-only register. However, in the event that this register is mistakenly read, clock 24 bits of data out of the part to restore it to the normal interface-idle state. Transfer-Function Registers The three transfer-function registers control the method used to map the input voltage to the output codes. All of the registers have the same format. The mapping of control registers to associated channels depends on the mode of operation and is affected by the state of M1, M0, DIFF, and SCAN (Tables 8, 9, and 10). MAX1403 Special Function Register (Write-Only) First Bit (MSB) FUNCTION Name Defaults RESERVED BITS 0 0 0 0 MDOUT 0 0 0 RESERVED BITS 0 0 0 0 0 0 FULLPD 0 (LSB) Transfer-Function Register First Bit (MSB) FUNCTION Name Defaults 18 G2 0 PGA GAIN CONTROL G1 0 G0 0 PGA U/B 0 D3 0 OFFSET CORRECTION D2 0 D1 0 D0 0 (LSB) ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC Analog Inputs AIN1 to AIN6 Inputs AIN1 and AIN2 map to transfer-function register 1, regardless of scanning mode (SCAN = 1) or singleended vs. differential (DIFF) modes. Likewise, AIN3 and AIN4 inputs always map to transfer-function register 2. Finally, AIN5 always maps to transfer-function register 3 (input AIN6 is analog common). CALGAIN and CALOFF When not in scan mode (SCAN = 0), A1 and A0 select which transfer function applies to CALGAIN and CALOFF. In scan mode (SCAN = 1), CALGAIN and CALOFF are always mapped to transfer-function register 3. Note that when scanning while M1 M0, the scan sequence includes both CALGAIN and CALOFF channels (Table 4). CALOFF always precedes CALGAIN, even though both channels share the same channel ID tag (Table 11). Note that changing the status of any active channel control bits will cause INT to immediately transition high and the modulator/filter to be reset. INT will reassert after the appropriate digital-filter settling time. The control settings of the inactive channels may be changed freely without affecting the status of INT or causing the filter/modulator to be reset. PGA Gain Bits G2-G0 control the PGA gain according to Table 6. Unipolar/Bipolar Mode The U/B bit places the channel in either bipolar or unipolar mode. A 0 selects bipolar mode, and a 1 selects unipolar mode. This bit does not affect the analog-signal conditioning. The modulator always accepts bipolar inputs and produces a bitstream with 50% ones-density when the selected inputs are at the same potential. This bit controls the processing of the digitalfilter output, such that the available output bits are mapped to the correct output range. Note that U/B must be set before a conversion is performed; it will not affect any data already held in the output register. Selecting bipolar mode does not imply that any input may be taken below AGND. It simply changes the gain and offset of the part. All inputs must remain within their specified operating voltage range. MAX1403 Offset-Correction DACs Bits D3-D0 control the offset-correction DAC. The DAC range depends on the PGA gain setting and is expressed as a percentage of the available full-scale input range (Table 7). D3 is a sign bit, and D2-D0 represent the DAC magnitude. Note that when a DAC value of 0000 is programmed (the default), the DAC is disconnected from the modulator inputs. This prevents the DAC from degrading noise performance when offset correction is not required. Transfer-Function Register Mapping Tables 8, 9, and 10 show the channel-control register mapping in the various operating modes. Table 7. DAC Code vs. DAC Value D3 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 D2 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 D1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 D0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 BIPOLAR DAC VALUE (% of FSR) UNIPOLAR DAC VALUE (% of FSR) DAC not connected +8.3 +16.7 +25 +33.3 +41.6 +50 +58.3 -8.3 -16.7 -25 -33.3 -41.6 -50 -58.3 +16.7 +33.3 +50 +66.7 +83.3 +100 +116.7 -16.7 -33.3 -50 -66.7 -83.3 -100 -116.7 Table 6. PGA Gain Codes G2 0 0 0 0 1 1 1 1 G1 0 0 1 1 0 0 1 1 G0 0 1 0 1 0 1 0 1 PGA GAIN x1 x2 x4 x8 x16 x32 x64 x128 DAC not connected ______________________________________________________________________________________ 19 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Table 8. Transfer-Function Register Mapping--Normal Mode (M1 = 0, M0 = 0) SCAN 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 DIFF 0 0 0 0 1 1 1 1 0 0 0 0 0 1 1 1 1 A1 0 0 1 1 0 0 1 1 X X X X X X X X 1 A0 0 1 0 1 0 1 0 1 X X X X X X X X 1 CHANNEL AIN1-AIN6 AIN2-AIN6 AIN3-AIN6 AIN4-AIN6 AIN1-AIN2 AIN3-AIN4 AIN5-AIN6 Do Not Use AIN1-AIN6 AIN2-AIN6 AIN3-AIN6 AIN4-AIN6 AIN5-AIN6 AIN1-AIN2 AIN3-AIN4 AIN5-AIN6 Do Not Use 1 1 2 2 3 1 2 3 Do Not Use TRANSFERFUNCTION REGISTER 1 1 2 2 1 2 3 X = Don't care Table 9. Transfer-Function Register Mapping--Offset-Calibration Mode (M1 = 0, M0 = 1) SCAN 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 20 DIFF 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 1 A1 0 0 1 1 0 0 1 1 X X X X X X X X X X X X 1 A0 0 1 0 1 0 1 0 1 X X X X X X X X X X X X 1 CHANNEL CALOFF+-CALOFFCALOFF+-CALOFFCALOFF+-CALOFFCALOFF+-CALOFFCALOFF+-CALOFFCALOFF+-CALOFFCALOFF+-CALOFFDo Not Use AIN1-AIN6 AIN2-AIN6 AIN3-AIN6 AIN4-AIN6 AIN5-AIN6 CALOFF+-CALOFFCALGAIN+-CALGAINAIN1-AIN2 AIN3-AIN4 AIN5-AIN6 CALOFF+-CALOFFCALGAIN+-CALGAINDo Not Use 1 1 2 2 3 3 3 1 2 3 3 3 TRANSFERFUNCTION REGISTER 1 1 2 2 1 2 3 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Table 10. Transfer-Function Register Mapping--Gain-Calibration Mode (M1 = 1, M0 = 0) SCAN 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 DIFF 0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 1 1 1 1 1 1 A1 0 0 1 1 0 0 1 1 X X X X X X X X X X X X 1 A0 0 1 0 1 0 1 0 1 X X X X X X X X X X X X 1 CHANNEL CALGAIN+-CALGAINCALGAIN+-CALGAINCALGAIN+-CALGAINCALGAIN+-CALGAINCALGAIN+-CALGAINCALGAIN+-CALGAINCALGAIN+-CALGAINDo Not Use AIN1-AIN6 AIN2-AIN6 AIN3-AIN6 AIN4-AIN6 AIN5-AIN6 CALOFF+-CALOFFCALGAIN+-CALGAINAIN1-AIN2 AIN3-AIN4 AIN5-AIN6 CALOFF+-CALOFFCALGAIN+-CALGAINDo Not Use 1 1 2 2 3 3 3 1 2 3 3 3 TRANSFERFUNCTION REGISTER 1 1 2 2 1 2 3 X = Don't care Data Register (Read-Only) The data register is a 24-bit, read-only register. Any attempt to write data to this location will have no effect. If a write operation is attempted, 8 bits of data must be clocked into the part before it will return to its normal idle mode, expecting a write to the communications register. Data is output MSB first, followed by one reserved 0 bit, two auxiliary data bits, and a 3-bit channel ID tag indicating the channel from which the data originated. Data Register (Read-Only) Bits First Bit (Data MSB) D17-D0: The conversion result. D17 is the MSB. The result is in offset binary format. 00 0000 0000 0000 0000 represents the minimum value, and 11 1111 1111 1111 1111 represents the maximum value. Inputs exceeding the available input range are limited to the corresponding minimum or maximum output values. 0: This reserved bit will always be 0. DATA BITS D17 D16 D15 D14 DATA BITS D9 D8 (Data LSB) DATA BITS D1 D0 RESERVED `0' AUXILIARY DATA DS1 DS0 CID2 CHANNEL ID TAG CID1 CID0 21 D7 D6 D5 D4 D3 D2 (LSB) D13 D12 D11 D10 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Table 11. Channel ID Tag Codes CID2 0 0 0 0 1 1 1 1 CID1 0 0 1 1 0 0 1 1 CID0 0 1 0 1 0 1 0 1 CHANNEL AIN1-AIN6 AIN2-AIN6 AIN3-AIN6 AIN4-AIN6 AIN1-AIN2 AIN3-AIN4 AIN5-AIN6 Calibration channels. Table 12 shows the channel configurations available for both operating modes. DS1, DS0: The status of the auxiliary data input pins. These are latched on the first falling edge of the SCLK signal for the current data register read access. CID2-0: Channel ID tag (Table 11). Scanning (SCAN-Mode) To sample and convert the available input channels sequentially, set the SCAN control bit in the global setup register. The sequence is determined by DIFF (fully differential or pseudo-differential) and by the mode control bits M1 and M0 (Tables 8, 9, 10). With SCAN set, the device automatically sequences through each available channel, transmitting a single conversion result before proceeding to the next channel. The MAX1403 automatically allows sufficient time for each conversion to fully settle, to ensure optimum resolution before asserting the data-ready signal and moving to the next available channel. The scan rate is, therefore, dependent on the clock bit (CLK), the filter control bits (FS1, FS0), and the modulator frequency selection bits (MF1, MF0). Burn-Out Currents The input circuitry also provides two "burn-out" currents. These small currents may be used to test the integrity of the selected transducer. They can be selectively enabled or disabled by the BOUT bit in the global setup register. Switching Network A switching network provides selection between three fully differential input channels or five pseudo-differential channels, using AIN6 as a shared common. The switching network provides two additional fully differential input channels intended for system calibration, which may be used as extra fully differential signal Table 12. Input Channel Configuration in Fully Differential and Pseudo-Differential Mode (SCAN = 0) M1 0 0 0 0 0 0 1 0 0 0 0 1 M0 0 0 0 0 0 1 0 0 0 0 1 0 DIFF 0 0 0 0 X X X 1 1 1 X X A1 0 0 1 1 X X X 0 0 1 X X A0 0 1 0 1 X X X 0 1 0 X X Fully Differential PseudoDifferential MODE HIGH INPUT AIN1 AIN2 AIN3 AIN4 AIN5* CALOFF+** CALGAIN+** AIN1 AIN3 AIN5 CALOFF+** CALGAIN+** LOW INPUT AIN6 AIN6 AIN6 AIN6 AIN6* CALOFF-** CALGAIN-** AIN2 AIN4 AIN6 CALOFF-** CALGAIN-** X = Don't care * This combination is available only in pseudo-differential mode when using the internal scanning logic. ** These combinations are only available in the calibration modes. 22 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC Transducer Excitation Currents The MAX1403 provides two matched 200A transducer excitation currents at OUT1 and OUT2. These currents have low absolute temperature coefficients and tight TC matching. These characteristics enable accurate compensation of errors due to IR drops in long transducer cable runs. They may be enabled or disabled using a single register control bit (IOUT). Dynamic Input Impedance at the Channel Selection Network When used in unbuffered mode (BUFF = 0), the analog inputs present a dynamic load to the driving circuitry. The size of the sampling capacitor and the input sampling frequency (Figure 5) determine the dynamic load seen by the driving circuitry. The MAX1403 samples at a constant rate for all gain settings. This provides a maximum time for the input to settle at a given data rate. The dynamic load presented by the inputs varies with the gain setting. For gains of +2V/V, +4V/V, and +8V/V, the input sampling capacitor increases with the chosen gain. Gains of +16V/V, +32V/V, +64V/V, and +128V/V present the same input load as the x8 gain setting. When designing with the MAX1403, as with any other switched-capacitor ADC input, consider the advantages and disadvantages of series input resistance. A series resistor reduces the transient-current impulse to the external driving amplifier. This improves the amplifier MAX1403 REXT CEXT RMUX CPIN CST RSW CSAMPLE CC Figure 5. Analog Input, Unbuffered Mode (BUFF = 0) phase margin and reduces the possibility of ringing. The resistor spreads the transient-load current from the sampler over time due to the RC time constant of the circuit. However, an improperly chosen series resistance can hinder performance in fast 16-bit converters. The settling time of the RC network can limit the speed at which the converter can operate properly, or reduce the settling accuracy of the sampler. In practice, this means ensuring that the RC time constant--resulting from the product of the driving source impedance and the capacitance presented by both the MAX1403's input and any external capacitances--is sufficiently small to allow settling to the desired accuracy. Tables 13a-13d summarize the maximum allowable series resistance vs. external capacitance for each MAX1403 gain setting in order to ensure 16-bit performance in unbuffered mode. Table 13a. REXT, CEXT Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0) Mode--1x Modulator Sampling Frequency (MF1, MF0 = 00); X2CLK = 0; CLKIN = 2.4576MHz PGA GAIN 1 2 4 8, 16, 32, 64, 128 EXTERNAL RESISTANCE, REXT (k) CEXT = 0pF 34 34 25 17 CEXT = 50pF 15 15 13 10 CEXT = 100pF 9.8 9.8 8.7 7.3 CEXT = 500pF 2.9 2.9 2.7 2.4 CEXT = 1000pF 1.6 1.6 1.5 1.4 CEXT = 5000pF 0.43 0.43 0.40 0.37 Table 13b. REXT, CEXT Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0) Mode--2x Modulator Sampling Frequency (MF1, MF0 = 01); X2CLK = 0; CLKIN = 2.4576MHz PGA GAIN 1 2 4 8, 16, 32, 64, 128 EXTERNAL RESISTANCE, REXT (k) CEXT = 0pF 17 17 13 8.4 CEXT = 50pF 7.5 7.5 6.4 5.0 CEXT = 100pF 4.9 4.9 4.4 3.7 CEXT = 500pF 1.4 1.4 1.3 1.2 CEXT = 1000pF 0.81 0.81 0.76 0.70 CEXT = 5000pF 0.22 0.22 0.20 0.18 23 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Table 13c. REXT, CEXT Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0) Mode--4x Modulator Sampling Frequency (MF1, MF0 = 10 ); X2CLK = 0; CLKIN = 2.4576MHz PGA GAIN 1 2 4 8, 16, 32, 64, 128 EXTERNAL RESISTANCE, REXT (k) CEXT = 0pF 8.3 8.3 6.2 4.1 CEXT = 50pF 3.7 3.7 3.2 2.5 CEXT = 100pF 2.4 2.4 2.2 1.8 CEXT = 500pF 0.72 0.72 0.67 0.60 CEXT = 1000pF 0.40 0.40 0.38 0.35 CEXT = 5000pF 0.11 0.11 0.10 0.09 Table 13d. REXT, CEXT Values for Less than 16-Bit Gain Error in Unbuffered (BUFF = 0) Mode--8x Modulator Sampling Frequency (MF1, MF0 = 11); X2CLK = 0; CLKIN = 2.4576MHz PGA GAIN 1 2 4 8, 16, 32, 64, 128 EXTERNAL RESISTANCE, REXT (k) CEXT = 0pF 4.1 4.1 3.0 2.0 CEXT = 50pF 1.8 1.8 1.5 1.2 CEXT = 100pF 1.2 1.2 1.1 0.88 CEXT = 500pF 0.35 0.35 0.32 0.29 CEXT = 1000pF 0.20 0.20 0.18 0.17 CEXT = 5000pF 0.05 0.05 0.05 0.04 Input Buffers The MAX1403 provides a pair of input buffers to isolate the inputs from the capacitive load presented by the PGA/modulator (Figure 6). The buffers are chopper stabilized to reduce the effect of their DC offsets and lowfrequency noise. Since the buffers can represent more than 50% of the total analog power dissipation, they may be shut down in applications where minimum power dissipation is required and the capacitive input load is not a concern. Disable the buffers in applications where the inputs must operate close to AGND or V+. When used in buffered mode, the buffers isolate the inputs from the sampling capacitors. The samplingrelated gain error is dramatically reduced in this mode. A small dynamic load remains from the chopper stabilization. The multiplexer exhibits a small input leakage current of up to 10nA. With high source resistances, this leakage current may result in a DC offset. REXT CEXT RMUX CPIN CST RIN CAMP CSAMPLE CC Figure 6. Analog Input, Buffered Mode (BUFF = 1) 24 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Table 14. REXT, CEXT Values for Less than 16-Bit Gain Error in Buffered (BUFF = 1) Mode--All Modulator Sampling Frequencies (MF1, MF0 = XX); X2CLK = 0; CLKIN = 2.4576MHz PGA GAIN 1 2 4 8 16 32 64 128 EXTERNAL RESISTANCE, REXT (k) CEXT = 0pF 10 10 10 10 10 10 10 10 CEXT = 50pF 10 10 10 10 10 10 10 10 CEXT = 100pF 10 10 10 10 10 10 10 10 CEXT = 500pF 10 10 10 10 10 10 10 10 CEXT = 1000pF 10 10 10 10 10 10 10 10 CEXT = 5000pF 10 10 10 10 10 10 10 10 Reference Input The MAX1403 is optimized for ratiometric measurements and includes a fully differential reference input. Apply the reference voltage across REFIN+ and REFIN-, ensuring that REFIN+ is more positive than REFIN-. REFIN+ and REFIN- must be between AGND and V+. The MAX1403 is specified with a +1.25V reference. (Table 6) precedes the modulator. Figure 8 shows the default bipolar transfer function with the following illustrated codes: 1) PGA = 0, DAC = 0; 2) PGA = 3, DAC = 0; or 3) PGA = 3, DAC = 3. Output Noise Tables 16a and 16b show the rms noise for typical output frequencies (notches) and -3dB frequencies for the MAX1403 with f CLKIN = 2.4576MHz. The numbers given are for the bipolar input ranges with V REF = +1.25V, with no buffer (BUFF = 0), and with the buffer inserted (BUFF = 1). These numbers are typical and are generated at a differential analog input voltage of 0. Figure 7 shows graphs of Effective Resolution vs. Gain and Notch Frequency. The effective resolution values were derived from the following equation: Effective Resolution = (SNRdB - 1.76dB) / 6.02 The maximum possible signal divided by the noise of the device, SNRdB, is defined as the ratio of the input full-scale voltage (i.e., 2 * VREFIN / GAIN) to the output rms noise. Note that it is not calculated using peak-topeak output noise numbers. Peak-to-peak noise numbers can be up to 6.6 times the rms numbers, while effective resolution numbers based on peak-to-peak noise can be 2.5 bits below the effective resolution based on rms noise, as quoted in the tables. The noise shown in Tables 16a and 16b is composed of device noise and quantization noise. The device noise is relatively low, but becomes the limiting noise source for high gain settings. The quantization noise is dependent on the notch frequency and becomes the dominant noise source as the notch frequency is increased. Modulator The MAX1403 performs analog-to-digital conversion using a single-bit, second-order, switched-capacitor modulator. A single comparator within the modulator quantizes the input signal at a much higher sample rate than the bandwidth of the signal to be converted. The quantizer then presents a stream of 1s and 0s to the digital filter for processing, to remove the frequencyshaped quantization noise. The MAX1403 modulator provides 2nd-order frequency shaping of the quantization noise resulting from the single bit quantizer. The modulator is fully differential for maximum signal-to-noise ratio and minimum susceptibility to power-supply noise. The modulator operates at one of a total of eight different sampling rates (fM) determined by the master clock frequency (fCLKIN), the X2CLK bit, the CLK bit, and the modulator frequency control bits MF1 and MF0. Power dissipation is optimized for each of these modes by controlling the bias level of the modulator. Table 15 shows the input and reference sample rates. PGA A programmable gain amplifier (PGA) with a userselectable gain of x1, x2, x4, x8, x16, x32, x64, or x128 ______________________________________________________________________________________ 25 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Table 15. Modulator Operating Frequency, Sampling Frequency, and 16-Bit Data Output Rates CLKIN FREQUENCY, fCLKIN (MHz) CLK X2CLK = 0 DEFAULT 1.024 1.024 1.024 1.024 2.4576 2.4576 2.4576 2.4576 X2CLK = 1 2.048 2.048 2.048 2.048 4.9152 4.9152 4.9152 4.9152 0 0 0 0 1 1 1 1 0 0 1 1 0 0 1 1 0 1 0 1 0 1 0 1 MF1 MF0 AIN/REFIN SAMPLING FREQUENCY, fS (kHz) 16 32 64 128 38.4 76.8 153.6 307.2 MODULATOR FREQUENCY, fM (kHz) 8 16 32 64 19.2 38.4 76.8 153.6 AVAILABLE OUTPUT DATA RATES AT 16-BIT ACCURACY (sps) 20, 25 40, 50 80, 100 160, 200 50, 60 100, 120 200, 240 400, 480 Note: Default condition is in bold print. Table 16a. Noise vs. Gain and Output Data Rate--Unbuffered Mode, VREF = 1.25V, fCLKIN = 2.4576MHz OUTPUT DATA RATE (sps) 50 60 300 600 100 120 600 1200 200 240 1200 2400 400 480 2400 4800 -3dB FREQ. (Hz) x1 13.1 15.7 78.6 157.2 26.2 31.4 157.2 314.4 52.4 62.9 314.4 628.8 104.8 125.7 628.8 1258 5.42 5.91 80.5 441 5.53 6.06 81.5 450 5.39 6.27 77.8 431 5.36 5.88 79.7 441 x2 3.03 3.20 38.6 236 2.96 3.28 39.9 232 2.92 3.28 40.1 232 3.00 3.25 39.6 227 TYPICAL OUTPUT NOISE (VRMS) FOR VARIOUS PROGRAMMABLE GAINS x4 1.70 1.90 20.6 112 1.73 1.90 19.6 115 1.70 1.89 20.1 109 1.82 1.94 20.2 111 x8 1.11 1.25 10.3 54.8 1.13 1.25 10.2 53.4 1.09 1.20 10.0 54.9 1.17 1.28 10.5 55.5 x16 1.06 1.13 5.73 29.2 1.06 1.17 5.45 27.8 1.06 1.18 5.53 28.2 1.10 1.26 5.74 29.7 x32 1.05 1.18 3.62 14.5 1.06 1.11 3.49 14.7 1.02 1.14 3.56 14.1 1.06 1.16 3.63 14.6 x64 1.05 1.15 2.84 7.61 1.08 1.12 2.72 8.00 1.02 1.17 2.74 8.08 1.10 1.17 3.02 7.73 x128 1.04 1.15 2.67 5.13 1.05 1.11 2.59 5.08 1.03 1.11 2.59 4.99 1.11 1.15 2.76 5.43 BIT STATUS MF1:MF0 = 0 FS1:FS0 = 0 FS1:FS0 = 1 FS1:FS0 = 2 FS1:FS0 = 3 MF1:MF0 = 1 FS1:FS0 = 0 FS1:FS0 = 1 FS1:FS0 = 2 FS1:FS0 = 3 MF1:MF0 = 2 FS1:FS0 = 0 FS1:FS0 = 1 FS1:FS0 = 2 FS1:FS0 = 3 MF1:MF0 = 3 FS1:FS0 = 0 FS1:FS0 = 1 FS1:FS0 = 2 FS1:FS0 = 3 26 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Table 16b. Noise vs. Gain and Output Data Rate--Buffered Mode, VREF = 1.25V, fCLKIN = 2.4576MHz OUTPUT DATA RATE (sps) 50 60 300 600 100 120 600 1200 200 240 1200 2400 400 480 2400 4800 -3dB FREQ. (Hz) x1 13.1 15.7 78.6 157.2 26.2 31.4 157.2 314.4 52.4 62.9 314.4 628.8 104.8 125.7 628.8 1258 5.72 6.29 80.6 436 5.82 6.01 77.7 434 5.82 6.17 79.0 439 5.60 6.18 76.3 455 x2 3.21 3.57 39.8 225 3.35 3.65 40.1 222 3.07 3.54 41.1 226 3.10 3.47 39.3 225 TYPICAL OUTPUT NOISE (VRMS) FOR VARIOUS PROGRAMMABLE GAINS x4 2.10 2.30 19.3 116 2.08 2.27 20.2 111 1.87 2.09 19.8 111 1.85 2.02 20.8 114 x8 1.41 1.55 10.2 57.1 1.43 1.51 10.6 57.0 1.26 1.45 10.5 57.9 1.32 1.38 9.83 57.1 x16 1.42 1.61 6.14 28.8 1.37 1.51 5.93 28.3 1.20 1.30 5.68 28.7 1.24 1.37 5.92 29.9 x32 1.44 1.56 4.25 15.0 1.36 1.50 4.19 14.8 1.18 1.27 3.68 15.4 1.25 1.29 3.92 14.5 x64 1.38 1.49 3.03 8.70 1.35 1.50 3.54 8.37 1.15 1.31 3.14 8.26 1.19 1.33 3.92 8.13 x128 1.34 1.56 3.52 5.99 1.31 1.47 3.23 5.81 1.17 1.29 2.99 5.32 1.21 1.33 3.07 5.55 BIT STATUS MF1:MF0 = 0 FS1:FS0 = 0 FS1:FS0 = 1 FS1:FS0 = 2 FS1:FS0 = 3 MF1:MF0 = 1 FS1:FS0 = 0 FS1:FS0 = 1 FS1:FS0 = 2 FS1:FS0 = 3 MF1:MF0 = 2 FS1:FS0 = 0 FS1:FS0 = 1 FS1:FS0 = 2 FS1:FS0 = 3 MF1:MF0 = 3 FS1:FS0 = 0 FS1:FS0 = 1 FS1:FS0 = 2 FS1:FS0 = 3 20 19 EFFECTIVE RESOLUTION (BITS) 18 17 16 15 14 13 12 11 10 1 a) BUFF = 0 2 4 8 16 32 GAIN (V/V) 64 128 256 CLK = 1 BUFF = 0 FS1: FS0 = 3 FS1: FS0 = 2 CLK = 1 EFFECTIVE RESOLUTION (BITS) FS1: FS0 = 0 OR 1 20 19 18 17 16 15 14 13 12 11 10 1 b) BUFF = 1 2 4 8 16 32 64 128 256 GAIN (V/V) CLK = 1 BUFF = 1 FS1: FS0 = 3 FS1: FS0 = 2 FS1: FS0 = 0 OR 1 Figure 7. Effective Resolution vs. Gain and Notch Frequency ______________________________________________________________________________________ 27 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Offset-Correction DAC The MAX1403 provides a coarse (3-bit plus sign) offsetcorrection DAC at the modulator input. Use this DAC to remove the offset component in the input signal, allowing the ADC to operate on a more sensitive range. The DAC offsets up to 116.7% of the selected range in 16.7% increments for unipolar mode, and up to 58.3% of the selected range in 8.3% increments for bipolar mode. When a DAC value of 0 is selected, the DAC is completely disconnected from the modulator inputs and does not contribute any noise. Figures 8 and 9 show the effect of the DAC codes on the input range and transfer function. In standby mode (STBY = 1) all circuitry, with the exception of the serial interface and the clock oscillator, is powered down. The interface consumes minimal power with a static SCLK. Enter power-down mode (including the oscillator) by setting the FULLPD bit in the special-function register. When exiting a full-power shutdown, perform a hardware reset or a software reset after the master clock signal is established (typically 10ms when using the on-board oscillator with an external crystal) to ensure that any potentially corrupted registers are cleared. It is often helpful to use higher-frequency crystals or resonators, especially for surface-mount applications where the result may be reduced PC board area for the oscillator component and a lower price or better component availability. Also, it may be necessary to operate the part with a clock source whose duty cycle is not close to 50%. In either case, the MAX1403 can operate with a master clock frequency of up to 5MHz, and includes an internal divide-by-2 prescaler to restore the internal clock frequency to a range of up to 2.5MHz with a 50% duty cycle. To activate this prescaler, set the X2CLK bit in the control registers. Note that using CLKIN frequencies above 2.5MHz in combination with the X2CLK mode will result in a small increase in digital supply current. (VREF = 1.25V PGA = 000) 2.708V 2.50V 2.292V 2.083V 1.875V 1.667V 1.458V 1.25V 1.042V 0.833V 0.625V 0.416V 0.208V 0V -0.208V -0.416V -0.625V -0.833V -1.042V -1.25V -1.458V -1.667V -1.875V -2.083V -2.292V -2.50V -2.708V -7 -6 D3: 1 1 D2: 1 1 D1: 1 1 D0: 1 0 Clock Oscillator The clock oscillator may be used with an external crystal (or resonator) connected between CLKIN and CLKOUT, or may be driven directly by an external oscillator at CLKIN with CLKOUT left unconnected. In normal operating mode, the MAX1403 is specified for operation with CLKIN at either 1.024MHz (CLK = 0) or 2.4576MHz (CLK = 1, default). When operated at these frequencies, the device may be programmed to produce frequency response nulls at the local line frequency (either 60Hz or 50Hz) and the associated line harmonics. MAX CODE 262144 FULL-SCALE 259522 PGA = 3 DAC = 0 PGA = 3 DAC = +3 MIDSCALE 131072 NEGATIVE DAC STEP SHIFTS THE TRANSFER FUNCTION TOWARD THE POSITIVE RAIL. ZERO-SCALE 2621 AGND (VAIN-)-VREF (VAIN-) - VREF/8 - VREF/16 V+ (VAIN-) + VREF (VAIN-) (VAIN-) - VREF/8 - VREF/16 (VAIN-) + VREF/8 (VAIN-) - VREF/8 PGA = 0 DAC = 0 MA XIM CODE UM MI UT NIM UM INP UT MI (U NIM /B =1 UM ) INP UT (U /B =0 ) INPUT VOLTAGE RANGE -5 1 1 0 1 -4 1 1 0 0 -3 1 0 1 1 -2 1 0 1 0 -1 1 0 1 0 0 0 0 0 0 +1 0 0 0 1 +2 0 0 1 0 +3 0 0 1 1 +4 0 1 0 0 +5 0 1 0 1 +6 0 1 1 0 +7 0 1 1 1 DAC CODE Figure 8. Effect of PGA and DAC Codes on the Bipolar Transfer Function 28 Figure 9. Input Voltage Range vs. DAC Code ______________________________________________________________________________________ INPUT VOLTAGE RANGE INP 13/6 VREF/2PGA 2 VREF/2PGA 11/6 VREF/2PGA 10/6 VREF/2PGA 9/6 VREF/2PGA 8/6 VREF/2PGA 7/6 VREF/2PGA VREF/2PGA 5/6 VREF/2PGA 4/6 VREF/2PGA 3/6 VREF/2PGA 2/6 VREF/2PGA 1/6 VREF/2PGA 0 -1/6 VREF/2PGA -2/6 VREF/2PGA -3/6 VREF/2PGA -4/6 VREF/2PGA -5/6 VREF/2PGA -VREF/2PGA -7/6 VREF/2PGA -8/6 VREF/2PGA -9/6 VREF/2PGA -10/6 VREF/2PGA -11/6 VREF/2PGA -2 VREF/2PGA -13/6 VREF/2PGA +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Digital Filter The on-chip digital filter processes the 1-bit data stream from the modulator using a SINC3 or SINC1 filter. The SINC filters are conceptually simple, efficient, and extremely flexible, especially where variable resolution and data rates are required. Also, the filter notch positions are easily controlled since they are directly related to the output data rate (1 / data word period). The SINC1 function results in a faster settling response while retaining the same frequency response notches as the default SINC3 filter. This allows the filter to settle faster at the expense of resolution and quantization noise. The SINC1 filter settles in one data word period. With 60Hz notches (60Hz data rate), the settling time would be 1 / 60Hz or 16.7ms, whereas the SINC3 filter would settle in 3 / 60Hz or 50ms. Toggle between these filter responses using the FAST bit in the global setup register. Use SINC1 mode for faster settling and switch to SINC3 mode when full accuracy is required. Switch from the SINC1 to SINC3 mode by resetting the FAST bit low. The DRDY signal will go false and will be reasserted when valid data is available, a minimum of three dataword periods later. The digital filter can be bypassed by setting the MDOUT bit in the global setup register. When MDOUT = 1, the raw output of the modulator is directly available at DOUT. of 15.72Hz for a first filter notch frequency of 60Hz. The response shown in Figure 10 is repeated at either side of the digital filter's sample frequency (fM) and at either side of the related harmonics (2fM, 3fM, . . .). The response of the SINC3 filter is similar to that of a SINC1 (averaging filter) filter but with a sharper rolloff. The output data rate for the digital filter corresponds with the positioning of the first notch of the filter's frequency response. Therefore, for the plot of Figure 10 where the first notch of the filter is at 60Hz, the output data rate is 60Hz. The notches of this (sinx/x)3 filter are repeated at multiples of the first notch frequency. The SINC 3 filter provides an attenuation of better than 100dB at these notches. Determine the cutoff frequency of the digital filter by the value loaded into CLK, X2CLK, MF1, MF0, FS1, and FS0 in the global setup register. Programming a different cutoff frequency with FS0 and FS1 does not alter the profile of the filter response; it changes the frequency of the notches. For example, Figure 11 shows a cutoff frequency of 13.1Hz and a first notch frequency of 50Hz. For step changes at the input, a settling time must be allowed before valid data can be read. The settling time depends upon the output data rate chosen for the filter. The settling time of the SINC3 filter to a full-scale step input can be up to four-times the output data period. For a synchronized step input (using the FSYNC function or the internal scanning logic), the settling time is three-times the output data period. Filter Characteristics The MAX1403 digital filter implements both a SINC1 (sinx/x) and SINC3 (sinx/x)3 lowpass filter function. The transfer function for the SINC3 function is that of three cascaded SINC1 filters described in the z-domain by: 1 H(z) = N 1 - z -N 1 - z -1 3 0 -20 -40 GAIN (dB) -60 -80 -100 -120 -140 -160 0 20 40 60 80 100 120 140 160 180 200 FREQUENCY (Hz) fCLKIN = 2.4576MHz MF1, 0 = 0 FS1, 0 = 1 fN = 60Hz and in the frequency domain by: 1 H(f) = N f sin N fM f sin fM 3 where N, the decimation factor, is the ratio of the modulator frequency fM to the output frequency fN. Figure 10 shows the filter frequency response. The SINC3 characteristic cutoff frequency is 0.262 times the first notch frequency. This results in a cutoff frequency Figure 10. Frequency Response of the SINC3 Filter (Notch at 60Hz) ______________________________________________________________________________________ 29 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Analog Filtering The digital filter does not provide any rejection close to the harmonics of the modulator sample frequency. However, due to the high oversampling ratio of the MAX1403, these bands occupy only a small fraction of the spectrum and most broadband noise is filtered. Therefore, the analog filtering requirements in front of the MAX1403 are considerably reduced compared to a conventional converter with no on-chip filtering. In addition, because the part's common-mode rejection of 90dB extends out to several kHz, common-mode noise susceptibility in this frequency range is substantially reduced. Depending on the application, it may be necessary to provide filtering prior to the MAX1403 to eliminate unwanted frequencies the digital filter does not reject. It may also be necessary in some applications to provide additional filtering to ensure that differential noise signals outside the frequency band of interest do not saturate the analog modulator. If passive components are placed in front of the MAX1403, when the part is used in unbuffered mode, ensure that the source impedance is low enough not to introduce gain errors in the system (Tables 13a-13d). This can significantly limit the amount of passive antialiasing filtering that can be applied in front of the MAX1403 in unbuffered mode. However, when the part is used in buffered mode, large source impedances will simply result in a small DC offset error (a 1k source resistance will cause an offset error of less than 10V). Therefore, where any significant source impedances are required, Maxim recommends operating the part in buffered mode. 0 -20 -40 GAIN (dB) -60 -80 -100 -120 MOSI -140 -160 0 20 40 60 80 100 120 140 160 180 200 FREQUENCY (Hz) CS DIN Calibration Channels Two fully differential calibration channels allow measurement of the system gain and offset errors. Connect the CALOFF channel to 0V and the CALGAIN channel to the reference voltage. Average several measurements on both CALOFF and CALGAIN. Subtract the average offset code and scale to correct for the gain error. This linear calibration technique can be used to remove errors due to source impedances on the analog input (e.g., when using a simple RC anti-aliasing filter on the front end). Applications Information SPI Interface (68HC11, PIC16C73) Microprocessors with a hardware SPI (serial peripheral interface) can use a 3-wire interface to the MAX1403 (Figure 12). The SPI hardware generates groups of eight pulses on SCLK, shifting data in on one pin and out on the other pin. For best results, use a hardware interrupt to monitor the INT pin and acquire new data as soon as it is available. If hardware interrupts are not available, or if interrupt latency is longer than the selected conversion rate, use the FSYNC bit to prevent automatic measurement while reading the data output register. The example code in Listing 1 shows how to interface with the MAX1403 using a 68HC11. System-dependent initialization code is not shown. VDD fCLKIN = 2.4576MHz MF1, 0 = 0 FS1, 0 = 0 fN = 50Hz VDD SS INTERRUPT 68HC11 SCK MISO RESET INT SCLK DOUT MAX1403 Figure 11. Frequency Response of the SINC3 Filter (Notch at 50Hz) 30 Figure 12. MAX1403 to 68HC11 Interface ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 /* Assumptions: ** The MAX140X's CS pin is tied to ground ** The MAX140X's INT pin drives a falling-edge-triggered interrupt ** MAX140X's DIN is driven by MOSI, DOUT drives MISO, and SCLK drives SCLK */ /* Low-level function to write 8 bits using 68HC11 SPI */ void WriteByte (BYTE x) { /* System-dependent: write to SPI hardware and wait until it is finished */ HC11_SPDR = x; while (HC11_SPSR & HC11_SPSR_SPIF) { /* idle loop */ } } /* Low-level function to read 8 bits using 68HC11 SPI */ BYTE ReadByte (void) { /* System-dependent: use SPI hardware to clock in 8 bits */ HC11_SPDR = 0xFF; while (HC11_SPSR & HC11_SPSR_SPIF) { /* idle loop */ } return HC11_SPDR; } /* Low-level interrupt handler called whenever the MAX140X's INT pin goes low. ** This function reads new data from the MAX140X and feeds it into a ** user-defined function Process_Data(). */ void HandleDRDY (void) { BYTE data_H_bits, data_M_bits, data_L_bits; /* storage for data register */ WriteByte(0x78); /* read the latest data regsiter value */ data_H_bits = ReadByte(); data_M_bits = ReadByte(); data_L_bits = ReadByte(); Process_Data(data_H_bits, data_M_bits, data_L_bits); /* System-dependent: re-enable the interrupt service routine */ } /* High-level function to configure the MAX140X's registers ** Refer to data sheet for custom setup values. */ void Initialize (void) { /* System-dependent: configure the SPI hardware (CPOL=1,CPHA=1) */ /* write to all of configuration registers */ MY_GS1 = 0x0A; MY_GS2 = 0x00; MY_GS3 = 0x00; MY_TF1 = 0x00; MY_TF2 = 0x00; MY_TF3 = 0x00; WriteByte(0x10); WriteByte(MY_GS1); /* write Global Setup 1 */ WriteByte(0x20); WriteByte(MY_GS2); /* write Global Setup 2 */ WriteByte(0x30); WriteByte(MY_GS3); /* write Global Setup 3 */ WriteByte(0x40); WriteByte(MY_TF1); /* write Transfer Function 1 */ WriteByte(0x50); WriteByte(MY_TF2); /* write Transfer Function 2 */ WriteByte(0x60); WriteByte(MY_TF3); /* write Transfer Function 3 */ /* System-dependent: enable the data-ready (DRDY) interrupt handler */ } Listing 1. Example SPI Interface ______________________________________________________________________________________ 31 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Bit-Banging Interface (80C51, PIC16C54) VDD RESET 8051 P3.0 DOUT DIN MAX1403 Any microcontroller can use general-purpose I/O pins to interface to the MAX1403. If a bidirectional or opendrain I/O pin is available, reduce the interface pin count by connecting DIN to DOUT (Figure 13). Listing 2 shows how to emulate the SPI in software. Use the same initialization routine shown in Listing 1. For best results, use a hardware interrupt to monitor the INT pin and acquire new data as soon as it is available. If hardware interrupts are not available, or if interrupt latency is longer than the selected conversion rate, use the FSYNC bit to prevent automatic measurement while reading the data output register. P3.1 SCLK CS Figure 13. MAX1403 to 8051 Interface /* Low-level function to write 8 bits ** The example shown here is for a bit-banging system with (CPOL=1, CPHA=1) */ void WriteByte (BYTE x) { drive SCK pin high count = 0; while (cout <= 7) { if (bit 7 of x is 1) drive DIN pin high else drive DIN pin low drive SCK pin low x = x * 2; drive SCK pin high count = count + 1; } } /* Low-level function to read 8 bits ** The example shown here is for a bit-banging system with (CPOL=1, CPHA=1) */ BYTE ReadByte (void) { x = 0; drive SCK pin high count = 0; while (cout <= 7) { x = x * 2; drive SCK pin low if (DOUT pin is high) x = x + 1; drive SCK pin high count = count + 1; } } return x; Listing 2. Bit-Banging SPI Replacement 32 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC Strain-Gauge Operation Connect the differential inputs of the MAX1403 to the bridge network of the strain gauge. In Figure 14, the analog positive supply voltage powers the bridge network and the MAX1403 along with its reference voltage. The on-chip PGA allows the MAX1403 to handle an analog input voltage range as low as 10mV full scale. The differential inputs of the device allow this analog input range to have an absolute value anywhere between AGND and V+. Temperature Measurement Figure 15 shows a connection from a thermocouple to the MAX1403. In this application, the MAX1403 is operated in its buffered mode to allow large decoupling capacitors on the front end. These decoupling capacitors eliminate any noise pickup from the thermocouple leads. When the MAX1403 is operated in buffered mode, it has a reduced common-mode range. In order to place the differential voltage from the thermocouple on a suitable common-mode voltage, the AIN2 input of the MAX1403 is biased at the reference voltage, +1.25V. MAX1403 ANALOG SUPPLY REFIN+ RREF REFINACTIVE GAUGE V+ V+ VDD CLOCK GEN CLKIN CLKOUT MAX1403 BUFFER BUFFER R AIN1 AIN2 SWITCHING NETWORK PGA DIVIDER MODULATOR DIGITAL FILTER DUMMY GAUGE R ADDITIONAL ANALOG AND CALIBRATION CHANNELS BUFFER BUFFER DAC INTERFACE AND CONTROL OUT1 OUT2 AGND SCLK DIN DOUT INT CS RESET DS1 DS0 AGND DGND Figure 14. Strain-Gauge Application with MAX1403 ______________________________________________________________________________________ 33 +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Loop-Powered, 4-20mA Transmitters Low-power, single-supply operation, and easy interfacing with optocouplers make the MAX1403 ideal for loop-powered 4-20mA transmitters. Loop-powered transmitters draw their power from the 4-20mA loop, limiting the transmitter circuitry to a current budget of 4mA. Tolerances in the loop further limit this current budget to 3.5mA. Since the MAX1403 consumes only 250A, a total of 3.25mA remains to power the remaining transmitter circuitry. Figure 16 shows a block diagram for a loop-powered 4-20mA transmitter. MAX1403 THERMOCOUPLE JUNCTION R AIN1 SWITCHING NETWORK AIN2 C C BUFFER R PGA +3V +1.25V REFIN+ REFINAGND DGND 3-Wire and 4-Wire RTD Configurations Tightly matched 200A current sources compensate for errors in 3-wire and 4-wire RTD configurations. In the 3wire configuration (Figure 17), the lead resistances result in errors if only one current source is used. The 200A will flow through RL1 developing a voltage error between AIN1 and AIN2. An additional current source compensates for this error by developing an equivalent voltage across RL2, ensuring the differential voltage at AIN1 and AIN2 is not affected by lead resistance. This assumes both leads are of the same material and of equal length (R L1 = R L2 ), and assumes OUT1 and OUT2 have matching tempcos (5ppm/C). Both current sources will flow through RL3, developing a commonmode voltage that will not affect the differential voltage at AIN1 and AIN2. Using one of the current sources to supply the reference voltage ensures a more accurate ratiometric result. Figure 15. Thermocouple Application with MAX1403 V+ ISOLATION BARRIER V+ ROFST RGAIN VOLTAGE REGULATOR RX VIN+ MAX1403 SENSOR 4 SPI 4 SPI P/C 3 SPI RY DAC 4-20mA LOOP INTERFACE CC GND GND RFDBK RSENSE VIN- Figure 16. 4-20mA Transmitter 34 ______________________________________________________________________________________ +3V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 +3V V+ OUT1 12.5k AIN1 RL1 RTD AIN2 OUT2 RL2 VDD 200A REFIN- REFIN+ OUT2 V+ 200A VDD MAX1403 REFIN+ MODULATOR PGA GAIN = 1 TO 128 OUT1 RREF REFIN200A MODULATOR 200A AGND RL3 MAX1403 AIN1 DGND RTD PGA AIN2 GAIN = 1 TO 128 Figure 17. 3-Wire RTD Application Unlike the 3-wire configuration, the 4-wire configuration (Figure 18) has no error associated with lead resistances, as no current flows in the measurement leads connected to AIN1 and AIN2. Current source OUT1 provides the excitation current for the RTD, and current source OUT2 provides current to generate the reference voltage. This reference voltage, developed across R REF , ensures that the analog input voltage span remains ratiometric to the reference voltage. RTD tempco errors in the analog input voltage are due to the temperature drift of the RTD current source and are compensated for by the variation in the reference voltage. A common resistance value for the RTD is 100, generating a 20mV signal directly handled at the analog input of the MAX1403. The voltage at OUT1 and OUT2 can go to within 1.0V of the V+ supply. AGND DGND Figure 18. 4-Wire RTD Application Grounding and Layout For best performance, use printed circuit boards with separate analog and digital ground planes. Wire-wrap boards are not recommended. Design the printed circuit board so that the analog and digital sections are separated and confined to different areas of the board. Join the digital and analog ground planes at only one point. If the MAX1403 is the only device requiring an AGND to DGND connection, then the ground planes should be connected at the AGND and DGND pins of the MAX1403. In systems where multiple devices require AGND to DGND connections, the connection should still be made at only one point. Make the star ground as close to the MAX1403 as possible. Avoid running digital lines under the device because these may couple noise onto the die. Run the analog ground plane under the MAX1403 to minimize coupling of digital noise. Make the power-supply lines to the MAX1403 as wide as possible to provide low-impedance paths and reduce the effects of glitches on the power-supply line. Power Supplies No specific power sequence is required for the MAX1403; either the V+ or the VDD supply can come up first. While the latchup performance of the MAX1403 is good, to avoid latchup it is important that power be applied to the MAX1403 before the analog input signals (AIN_) or the CLKIN inputs. If this is not possible, then the current flow into any of these pins should be limited to 50mA. If separate supplies are used for the MAX1403 and the system digital circuitry, then the MAX1403 should be powered up first. ______________________________________________________________________________________ 35 +3V, +5V, 18-Bit, Low-Power, Multichannel, Oversampling (Sigma-Delta) ADC MAX1403 Shield fast switching signals, such as clocks, with digital ground to avoid radiating noise to other sections of the board. Avoid running clock signals near the analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This will reduce the effects of feedthrough on the board. A microstrip technique is best but is not always possible with double-sided boards. In this technique, the component side of the board is dedicated to ground planes while signals are placed on the solder side. Good decoupling is important when using high-resolution ADCs. Decouple all analog supplies with 10F tantalum capacitors in parallel with 0.1F HF ceramic capacitors to AGND. Place these components as close to the device as possible to achieve the best decoupling. See the MAX1403 evaluation kit manual for the recommended layout. The evaluation board package includes a fully assembled and tested evaluation board. Chip Information TRANSISTOR COUNT: 34,648 SUBSTRATE CONNECTED TO AGND Package Information SSOP.EPS Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 36 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. |
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