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Preliminary Technical Data
FEATURES
Dual 14-bit SAR ADC Simultaneous Sampling Throughput rate: 4.25 MSPS Per Channel Specified for VDD of 2.5 V Power dissipation: 35 mW at 4.25 MSPS On-chip reference: 2.048 V 0.5% max @ 25C, 10ppm/C Dual conversion with read High speed serial interface: SPI(R)-/QSPITM-/MICROWIRETM-/DSP-compatible -40C to +125C operation Shutdown mode: 10 A max 16-lead TSSOP package
Differential Input,Dual,Simultaneous Sampling, 4.25 MSPS, 14-Bit, SAR ADC AD7357
FUNCTIONAL BLOCK DIAGRAM
Vdd Vdrive
AD7357
VINA+ VINAT/H 14-BIT SUCCESSIVE APPROXIMATION ADC SDATAA
BUF REF BUF CONTROL LOGIC SCLK CS
VINB+ VINBT/H
14-BIT SUCCESSIVE APPROXIMATION ADC
SDATAB
AGND
AGND REFGND DGND
Figure 1.
GENERAL DESCRIPTION
The AD73571 is a dual, 14-bit, high speed, low power, successive approximation ADC that operates from a single 2.5 V power supply and features throughput rates up to 4.25 MSPS. The part contains two ADCs, each preceded by a low noise, wide bandwidth trackand-hold circuit that can handle input frequencies in excess of 200 MHz. The conversion process and data acquisition use standard control inputs allowing for easy interfacing to microprocessors or DSPs. The input signal is sampled on the falling edge of CS; conversion is also initiated at this point. The conversion time is determined by the SCLK frequency. The AD7357 uses advanced design techniques to achieve very low power dissipation at high throughput rates. With 2.5 V supply and a 4.25 MSPS throughput rate, the part consumes 14 mA typically. The part also offers flexible power/throughput rate management when operating in normal mode as the quiescent current consumption is so low. The analog input range for the part is the differential common mode +/- Vref/2. The AD7357 has an on-chip 2.048 V reference that can be overdriven when an external reference is preferred. The AD7357 is available in a 16-lead thin shrink small outline package (TSSOP).
PRODUCT HIGHLIGHTS
1. Two Complete ADC Functions Allow Simultaneous Sampling and Conversion of Two Channels. The conversion result of both channels is simultaneously available on separate data lines or in succession on one data line if only one serial port is available. High Throughput with Low Power Consumption. The AD7357 offers a 4.25 MSPS throughput rate with 35 mW power consumption.
2.
3.
The part features two standard successive approximation ADCs with accurate control of the sampling instant via a CS input and once off conversion control.
1
Protected by U.S. Patent No. 6,681,332
Table 1: Related Devices.
Generic AD7356 AD7352 Resolution 12 12 Throughput 5MSPS 3MSPS Analog Input Differential Differential
Rev. PrD
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 (c)2007 Analog Devices, Inc. All rights reserved.
AD7357 TABLE OF CONTENTS
Revision Historyx ............................................................................. 2 Specifications..................................................................................... 3 AD7357 Specifications................................................................. 3 Timing Specifications .................................................................. 5 Absolute Maximum Ratings............................................................ 6 ESD Caution.................................................................................. 6 Pin Configuration and Function Descriptions............................. 7 Typical Performance Characteristics ............................................. 8 Terminology ...................................................................................... 9 Theory of Operation ...................................................................... 11 Circuit Information.................................................................... 11 Converter Operation.................................................................. 11
Preliminary Technical Data
ADC Transfer Function............................................................. 11 Analog Input Structure.............................................................. 11 Analog Inputs ............................................................................. 12 Modes of Operation ....................................................................... 13 Normal Mode.............................................................................. 13 Partial Power-Down Mode ....................................................... 13 Full Power-Down Mode ............................................................ 13 Power-Up Times......................................................................... 15 Serial Interface ................................................................................ 16 Outline Dimensions ....................................................................... 17 Ordering Guide............................................................................... 17
REVISION HISTORYX
09/07--Revision PrD
Rev. PrD | Page 2 of 17
Preliminary Technical Data SPECIFICATIONS
AD7357 SPECIFICATIONS
AD7357
VDD = 2.5 +/-10% V, VDRIVE = 2.5 V to 3.3 +10% V, internal VREF = 2.048 V, unless otherwise noted, FCLKIN = 80 MHz, FSAMPLE = 4.25 MSPS; TA = TMIN to TMAX1, unless otherwise noted. Table 1.
Parameter DYNAMIC PERFORMANCE Signal-to-Noise Ratio (SNR) Signal-to-Noise and Distortion (SINAD) Total Harmonic Distortion (THD) Spurious Free Dynamic Range (SFDR) Intermodulation Distortion (IMD) Second Order Terms Third Order Terms Channel-to-Channel Isolation SAMPLE AND HOLD Aperture Delay Aperture Delay Matching Aperture Jitter Full Power Bandwidth DC ACCURACY Resolution Integral Nonlinearity Differential Nonlinearity Offset Error Offset Error Match Gain Error Gain Error Match ANALOG INPUT Fully Differential Input Range: Vin+ and Vin- DC Leakage Current Input Capacitance REFERENCE INPUT/OUTPUT VREF Input Voltage Range DC Leakage Current VREF Output Voltage VREF Temperature Coefficient VREF Long Term Stability VREF Output Voltage Hysteresis2 VREF Noise VREF Output Impedance VREF Input Capacitance LOGIC INPUTS Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN Specification 78 77 TBD TBD TBD TBD -85 5 40 15 200 30 14 2 0.99 10 1 6 10 1 6 VCM VREF2 1 35 10 2.048+100mV / Vdd 1 2.048 10 100 50 TBD TBD TBD 0.6 x Vdrive 0.3 x Vdrive 1 10 Unit dB min dB min dB max dB max fa = TBD Hz, fb = TBD Hz dB typ dB typ dB typ ns max ps max ps typ MHz typ MHz typ Bits LSB max LSB max LSB max LSB typ LSB max LSB max LSB typ LSB max V A max pF typ pF typ V min / V max A max V ppmC typ ppm typ ppm typ V Typ Typ pF typ V min V max A max pF typ Test Conditions/Comments fIN = 1 MHz sine wave
fIN = TBD kHz, fNOISE = TBD kHz
@ 3 dB @ 0.1 dB
Guaranteed no missed codes to 14 bits
VCM = common-mode voltage , Vin+ and Vin- must remain within GNDVDD When in track When in hold
0.5% max @ 25C For 1000 hours
When in track
VIN = 0 V or VDRIVE
Rev. PrD | Page 3 of 17
AD7357
Parameter LOGIC OUTPUTS Output High Voltage, VOH Output Low Voltage, VOL Floating-State Leakage Current Floating-State Output Capacitance Output Coding CONVERSION RATE Conversion Time Track-and-Hold Acquisition Time Throughput Rate POWER REQUIREMENTS VDD VDRIVE IDD Normal Mode (Operational) Normal Mode (Static) Partial Power-Down Mode Full Power-Down Mode Power Dissipation Normal Mode (Operational) Normal Mode (Static) Partial Power-Down Mode Full Power-Down Mode
1 2
Preliminary Technical Data
Specification Vdrive-0.2 0.2 1 TBD Unit Test Conditions/Comments
V min V max A max pF typ Straight Binary ns ns max MSPS max V V min/max Digital IPS = 0 V or VDRIVE mA typ mA typ mA typ A typ mW typ mW typ mW typ W typ
t2 + 15.5 x tSCLK 30 4.25 2.5 2.5/3.3 14 7 5 10 35 17.5 12.5 2.5
Full-scale step input
SCLK off SCLK on or off
SCLK off SCLK on or off
Temperature ranges are as follows: Y Grade: -40C to +125C, B Grade: -40C to +85C. See theTerminology section.
Rev. PrD | Page 4 of 17
Preliminary Technical Data
TIMING SPECIFICATIONS
VDD = 2.5 +/-10%V, VDRIVE = 2.5 V to 3.3 +10% V, internal reference = 2.048 V, TA = TMAX to TMIN1, unless otherwise noted. Table 2.
Parameter
fSCLK tCONVERT tQUIET t2 t3 t42 t5 t6 t7 t8 t9 t10 Latency
1 2
AD7357
Limit at TMIN, TMAX
50 80 t2 +15.5 x tSCLK 5 5 TBD TBD 0.40 tSCLK 0.40 tSCLK TBD TBD TBD TBD TBD
Unit
kHz min MHz max ns max ns min
Description
14 bit resolution, tSCLK = 1/fSCLK Minimum time between end of serial read and next falling edge of CS CS to SCLK setup time Delay from CS until DOUTA and DOUTB are three-state disabled Data access time after SCLK falling edge SCLK low pulse width SCLK high pulse width SCLK to data valid hold time CS rising edge to DOUTA, DOUTB, high impedance CS rising edge to falling edge pulse width SCLK falling edge to DOUTA, DOUTB, high impedance SCLK falling edge to DOUTA, DOUTB, high impedance
ns min ns max ns max ns min ns min ns min ns max ns min ns min ns max 1 Conversion Latency
Temperature ranges are as follows: Y Grade: -40C to +125C, B Grade: -40C to +85C. The time required for the output to cross 0.4 V or 2.4 V.
Rev. PrD | Page 5 of 17
AD7357 ABSOLUTE MAXIMUM RATINGS
Table 3.
Parameter VDD to AGND, DGND, REFGND VDRIVE to AGND, DGND, REFGND VDD to VDRIVE AGND to DGND to REFGND Analog Input Voltages1 to AGND Digital Input Voltages2 to DGND Digital Output Voltages3 to DGND Input Current to Any Pin Except Supplies4 Operating Temperature Range Y Grade B Grade Storage Temperature Range Junction Temperature TSSOP Package JA Thermal Impedance JC Thermal Impedance Lead Temperature, Soldering Reflow Temperature (10 to 30 sec) ESD
1 2
Preliminary Technical Data
Rating -0.3 V to +2.8V -0.3 V to +3.8V +2.8V to -3.8V -0.3 V to +0.3 V -0.3 V to VDD + 0.3 V -0.3V to VDRIVE + 0.3V -0.3 V to VDRIVE + 0.3 V 10 mA
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
-40C to +125C -40C to +85C -65C to +150C 150C 143C/W 45C/W 255C TBD kV
Analog input voltages are VINA+, VINA-, VINB+, VINB-, REFA and REFB. Digital input voltages are CS and SCLK. 3 Digital output voltages are SDATAA and SDATAB. 4 Transient currents of up to 100 mA will not cause SCR latch up.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
Rev. PrD | Page 6 of 17
Preliminary Technical Data PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
VINA+ 1 VINA2 16 15 14
AD7357
VDRIVE SCLK SDATAA SDATAB DGND AGND CS VDD
REFA 3 REFGND 4 AGND 5 REFB VINB6 7
AD7357 TOP VIEW (Not to Scale)
13 12 11 10 9
VINB+ 8
Figure 2. Pin Configuration
Table 4. Pin Function Descriptions Pin No.
9 16 10 15 14,13
Mnemonic
VDD VDRIVE CS SCLK SDATAA, SDATAB
Description
Power Supply Input. The VDD range for the AD7357 is 2.5V +/- 5%. The supply should be decoupled to AGND with a 0.1 F capacitor and a 10 F tantalum capacitor. Logic Power Supply Input. The voltage supplied at this pin determines at what voltage the interface will operate. This pin should be decoupled to DGND. The voltage at this pin may be different to that at VDD. Chip Select. Active low logic input. This input provides the dual function of initiating conversions on the AD7357 and framing the serial data transfer. Serial Clock. Logic input. A serial clock input provides the SCLK for accessing the data from the AD7357. This clock is also used as the clock source for the conversion process. Serial Data Outputs. The data output is supplied to each pin as a serial data stream. The bits are clocked out on the falling edge of the SCLK input. 16 SCLK falling edges are required to access the 14 bits of data from the AD7357. The data simultaneously appears on both data output pins from the simultaneous conversions of both ADCs. The data stream consists of one leading zero followed by the 14 bits of conversion data followed by a trailing zero. The data is provided MSB first. If CS is held low for 18 SCLK cycles rather than 16, then two further trailing zeros will appear after the 14 bits of data. If CS is held low for a further 18 SCLK cycles on either SDATAA or SDATAB , the data from the other ADC follows on the SDATA pin. This allows data from a simultaneous conversion on both ADCs to be gathered in serial format on either SDATAA or SDATAB. Digital Ground. This is the ground reference point for all digital circuitry on the AD7357. This pin should connect to the DGND plane of a system. The DGND and AGND voltages should ideally be at the same potential and must not be more than 0.3 V apart, even on a transient basis. Analog Ground. This is the ground reference point for all analog circuitry on the AD7357. All analog input signals and should be referred to this AGND voltage. The AGND and DGND voltages should ideally be at the same potential and must not be more than 0.3 V apart, even on a transient basis. Reference Ground. This is the ground reference point for the reference circuitry on the AD7357. Any external reference signal should be referred to this REFGND voltage. Decoupling capacitors must be placed between this pin and the REFA and REFB pins. Reference decoupling capacitor pins. Decoupling capacitors are connected between these pins and the REFGND pin to decouple the reference buffer for each respective ADC. It is recommended to decouple the each reference pin with a 10F capacitor. Provided the output is buffered, the on-chip reference can be taken from these pins and applied externally to the rest of the system. The nominal internal reference voltage is 2.048V and this appears at these pins. These pins can also be overdriven by an external reference. The input voltage range for the external reference is 2.048+100mV to Vdd. Analog Inputs of ADC A. These analog inputs form a fully differential pair. Analog Inputs of ADC B. These analog inputs form a fully differential pair.
12
DGND
5, 11
AGND
4
REFGND
3, 6
REFA, REFB
1, 2 8, 7
VINA-, VINA+ VINB-, VINB+
Rev. PrD | Page 7 of 17
AD7357 TYPICAL PERFORMANCE CHARACTERISTICS
Preliminary Technical Data
Figure 3. Typical FFT
Figure 6. Channel to Channel Isolation
Figure 4. Typical DNL
Figure 7. Histogram of Codes
Figure 5. Typical INL
Rev. PrD | Page 8 of 17
Preliminary Technical Data TERMINOLOGY
Integral Nonlinearity (INL) The maximum deviation from a straight line passing through the endpoints of the ADC transfer function. The endpoints of the transfer function are zero scale, 1 LSB below the first code transition, and full scale, 1 LSB above the last code transition. Differential Nonlinearity (DNL) The difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Offset Error The deviation of the first code transition (00 . . .000) to (00 . . . 001) from the ideal (that is, -VREF + 0.5 LSB). Offset Error Match This is the difference in offset error between the two ADCs. Gain Error The deviation of the last code transition (111 . . .110) to (111 . . . 111) from the ideal (that is, VREF - 1.5 LSB) after the offset error has been adjusted out. Gain Error Match The difference in gain error between the two ADCs. Channel-to-Channel Isolation Channel-to-channel isolation is a measure of the level of crosstalk between channels. It is measured by applying a fullscale sine wave signal to one of the two channels and applying a 50 kHz signal to the other channel. The channel-to-channel isolation is defined as the ratio of the power of the 50 kHz signal on the converted channel to the power of the noise signal on the other channel that appears in the FFT of this channel. The noise frequency on the unselected channel varies from 40 kHz to 740 kHz. The noise amplitude is at 2 x VREF, while the signal amplitude is at 1 x VREF. See Figure 6. Power Supply Rejection Ratio (PSRR) PSRR is defined as the ratio of the power in the ADC output at full-scale frequency, f, to the power of a 100 mV p-p sine wave applied to the ADC VDD supply of frequency fS. The frequency of the input varies from 1 kHz to 1 MHz. PSRR (dB) = 10log(Pf/PfS) where: Pf is the power at frequency f in the ADC output. PfS is the power at frequency fS in the ADC output.
AD7357
Common-Mode Rejection Ratio (CMRR) CMRR is defined as the ratio of the power in the ADC output at full-scale frequency, f, to the power of a 100 mV p-p sine wave applied to the common-mode voltage of VIN+ and VIN- of frequency fS as CMRR (dB) = 10log (Pf/PfS) where: Pf is the power at frequency f in the ADC output. PfS is the power at frequency fS in the ADC output. Track-and-Hold Acquisition Time The track-and-hold amplifier returns to track mode at the end of conversion. The track-and-hold acquisition time is the time required for the output of the track-and-hold amplifier to reach its final value, within 1/2 LSB, after the end of conversion. Signal-to-(Noise + Distortion) Ratio (SINAD) This is the measured ratio of signal-to-noise and distortion at the output of the ADC. The signal is the rms amplitude of the fundamental. Noise is the sum of all nonfundamental signals up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent on the number of quantization levels in the digitization process; the more levels, the smaller the quantization noise. The theoretical signal-to-noise and distortion ratio for an ideal N-bit converter with a sine wave input is given by SINAD = (6.02 N + 1.76) dB Thus, for a 12-bit converter, this is 74 dB and for a 14 bit converter, this is 86dB. Total Harmonic Distortion (THD) THD is the ratio of the rms sum of harmonics to the fundamental. For the AD7356, it is defined as
THD (dB ) = -20log where:
V2 2 + V3 2 + V4 2 + V5 2 + V6 2 V1
V1 is the rms amplitude of the fundamental. V2, V3, V4, V5, and V6 are the rms amplitudes of the second through the sixth harmonics.
Rev. PrD | Page 9 of 17
AD7357
Peak Harmonic or Spurious Noise Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the ADC output spectrum (up to fS/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification is determined by the largest harmonic in the spectrum, but for ADCs where the harmonics are buried in the noise floor, it is a noise peak Intermodulation Distortion With inputs consisting of sine waves at two frequencies, fa and fb, any active device with nonlinearities will create distortion products at sum and difference frequencies of mfa nfb where m, n = 0, 1, 2, 3, and so on. Intermodulation distortion terms are those for which neither m nor n are equal to zero. For example, the second-order terms include (fa + fb) and (fa - fb), while the third-order terms include (2fa + fb), (2fa - fb), (fa + 2fb), and (fa - 2fb).
Preliminary Technical Data
sum of the individual distortion products to the rms amplitude of the sum of the fundamentals expressed in dBs.
Thermal Hysteresis
Thermal Hysteresis is defined as the absolute maximum change of reference output voltage after the device is cycled through temperature from either T_HYS+ = +25C to TMAX to +25C T_HYS- = +25C to TMIN to +25C It is expressed in ppm using the following equation:
VHYS ( ppm) = VREF (25C ) - VREF (T _ HYS) x 10 6 VREF (25C )
The AD7356 is tested using the CCIF standard where two input frequencies near the top end of the input bandwidth are used. In this case, the second-order terms are usually distanced in frequency from the original sine waves, while the third-order terms are usually at a frequency close to the input frequencies. As a result, the second- and third-order terms are specified separately. The calculation of the intermodulation distortion is as per the THD specification, where it is the ratio of the rms
where: VREF(25C) = VREF at 25C VREF(T_HYS) = Maximum change of VREF at T_HYS+ or T_HYS-.
Rev. PrD | Page 10 of 17
Preliminary Technical Data THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7357 is a fast, dual, 14-bit, single-supply, successive approximation analog-to-digital converter. The part operates from a 2.5 V power supply and features throughput rates up to 4.25 MSPS. The AD7357 contains two on-chip differential track-and-hold amplifiers, two successive approximation analog-to-digital converters and a serial interface with two separate data output pins. They part is housed in a 16-lead TSSOP package, offering the user considerable space-saving advantages over alternative solutions. The serial clock input accesses data from the part, but also provides the clock source for each successive approximation ADC. The AD7357 has an on-chip 2.048V reference. If an external reference is desired the internal reference can be overdriven with a reference of value ranging from (2.048V + 100mV) to Vdd. If the internal reference is to be used elsewhere in the system, then the reference output needs to be buffered first. The differential analog input range for the AD7357 is VCM VREF2. The AD7357 features power-down options to allow power saving between conversions. The power-down feature is implemented via the standard serial interface, as described in the Modes of Operation section.
AD7357
become unbalanced. Both inputs are disconnected once the conversion begins. The control logic and charge redistribution DACs are used to add and subtract fixed amounts of charge from the sampling capacitor arrays to bring the comparator back into a balanced condition. When the comparator is rebalanced, the conversion is complete. The control logic generates the ADC output code. The output impedances of the sources driving the VIN+ and VIN- pins must be matched, otherwise, the two inputs will have different settling times, resulting in errors.
CAPACITIVE DAC B VIN+ A SW1 A B CAPACITIVE DAC
04603-014
CS
COMPARATOR
SW2
CS
SW3
CONTROL LOGIC
VIN-
VREF
Figure 9. ADC Conversion Phase
ADC TRANSFER FUNCTION
The output coding for the AD7357 is straight binary. The designed code transitions occur at successive LSB values (1 LSB, 2 LSBs and so on). The LSB size is (2 xVREF)/16384 for the AD7357. The ideal transfer characteristic of the AD7357 is shown in Figure 10.
CONVERTER OPERATION
The AD7357 has two successive approximation analog-todigital converters, each based around two capacitive DACs. Figure 8 and Figure 9 show simplified schematics of one of these ADCs in acquisition and conversion phase, respectively. The ADC is comprised of control logic, a SAR, and two capacitive DACs. In Figure 8 (the acquisition phase), SW3 is closed, SW1 and SW2 are in position A, the comparator is held in a balanced condition, and the sampling capacitor arrays may acquire the differential signal on the input.
CAPACITIVE DAC B VIN+ A SW1 A B CAPACITIVE DAC
04603-013
111...111 111...110 111...101
ADC CODE
000...010 000...001 000...000 -VREF+1 LSB -VREF+0.5 LSB +VREF-1 LSB +VREF-1.5 LSB ANALOG INPUT
CS
COMPARATOR
SW2
CS
SW3
CONTROL LOGIC
VIN-
Figure 10. AD7356 Ideal Transfer Characteristic
ANALOG INPUT STRUCTURE
Figure 11 shows the equivalent circuit of the analog input structure of the AD7357. The four diodes provide ESD protection for the analog inputs. Care must be taken to ensure that the analog input signals never exceed the supply rails by more than 300mV. This causes these diodes to become forwardbiased and start conducting into the substrate. These diodes can
VREF
Figure 8. ADC Acquisition Phase
When the ADC starts a conversion (Figure 9), SW3 opens and SW1 and SW2 move to Position B, causing the comparator to
Rev. PrD | Page 11 of 17
AD7357
conduct up to 10mA without causing irreversible damage to the part. The C1 capacitors in Figure 11 are typically TBD pF and can primarily be attributed to pin capacitance. The resistors are lumped components made up of the on resistance of the switches. The value of these resistors is typically about TBD . The C2 capacitors are the ADC's sampling capacitors with a capacitance of TBD pF typically.
VDD D VIN+ C1 D
Preliminary Technical Data
be tolerated. The THD increases as the source impedance increases and performance degrades. Figure 12 shows a graph of the THD vs. the analog input signal frequency for different source impedances. Figure 13 shows a graph of the THD vs. the analog input frequency while sampling at 4.25 MSPS. In this case the source impedance is TBD .
R1 C2
VDD D VIN- C1 D
04603-015
R1 C2
Figure 13.THD vs. Analog Input Frequency
Figure 11.Equivalent Analog Input Circuit, Conversion Phase - Switches Open, Track Phase - Switches Closed
ANALOG INPUTS
Differential signals have some benefits over single-ended signals, including noise immunity based on the devices common-mode rejection and improvements in distortion performance. The amplitude of the differential signal is the difference between the signals applied to the VIN+ and VIN- pins in each differential pair (VIN+ - VIN-). VIN+ and VIN- should be simultaneously driven by two signals each of amplitude VREF that are 180 out of phase. This amplitude of the differential signal is, therefore -VREF to +VREF peak-to -peak regardless of the common mode (CM). The common mode is the average of the two signals and is therefore the voltage on which the two inputs are centered. CM = (VIN+ + VIN-)/2 This results in the span of each input being CM VREF/2. This voltage has to be set up externally. When a conversion takes place, the common mode is rejected resulting in a virtually noise free signal of amplitude -VREF to +VREF corresponding to the digital codes of 0 to 16383 for the AD7357.
For ac applications, removing high frequency components from the analog input signal is recommended by the use of an RC low-pass filter on the analog input pins. In applications where harmonic distortion and signal-to-noise ratio are critical, the analog input should be driven from a low impedance source. Large source impedances significantly affect the ac performance of the ADC and may necessitate the use of an input buffer amplifier. The choice of the op amp will be a function of the particular application.
Figure 12.THD vs. Analog Input Frequency for Various Source Impedances
When no amplifier is used to drive the analog input, the source impedance should be limited to low values. The maximum source impedance will depend on the amount of THD that can
Rev. PrD | Page 12 of 17
Preliminary Technical Data MODES OF OPERATION
The mode of operation of the AD7357 is selected by controlling the (logic) state of the CS signal during a conversion. There are three possible modes of operation: normal mode, partial powerdown mode and full power-down mode. After a conversion has been initiated, the point at which CS is pulled high determines which power-down mode, if any, the device enters. Similarly, if already in a power-down mode, CS can control whether the device returns to normal operation or remains in power-down. These modes of operation are designed to provide flexible power management options. These options can be chosen to optimize the power dissipation/throughput rate ratio for the differing application requirements.
AD7357
Once a data transfer is complete and SDATAA and SDATAB have returned to three-state, another conversion can be initiated after the quiet time, tQUIET, has elapsed by bringing CS low again (assuming the required acquisition time has been allowed).
PARTIAL POWER-DOWN MODE
This mode is intended for use in applications where slower throughput rates are required. Either the ADC is powered down between each conversion, or a series of conversions may be performed at a high throughput rate and the ADC is then powered down for a relatively long duration between these bursts of several conversions. When the AD7357 is in partial power-down, all analog circuitry is powered down except for the on-chip reference and reference buffers. To enter partial power, the conversion process must be interrupted by bringing CS high anywhere after the 2nd falling edge of SCLK and before the 10th falling edge of SCLK, as shown in Figure 15. Once CS has been brought high in this window of SCLKs, the part enters partial power-down, the conversion that was initiated by the falling edge of CS is terminated, and SDATAA and SDATAB go back into three-state. If CS is brought high before the 2nd SCLK falling edge, the part remains in normal mode and does not power down. This avoids accidental power-down due to glitches on the CS line.
CS
1 2 10 14
NORMAL MODE
This mode is intended for applications needing fastest throughput rates since the user does not have to worry about any power-up times with the AD7357 remaining fully powered at all times. Figure 14 shows the general diagram of the operation of the AD7357 in this mode.
CS
1 10 14
SCLK
04603-029
DOUTA DOUTB
LEADING ZEROS + CONVERSION RESULT
Figure 14. Normal Mode Operation
SCLK
04603-030
The conversion is initiated on the falling edge of CS, as described in the Serial Interface section. To ensure that the part remains fully powered up at all times, CS must remain low until at least 10 SCLK falling edges have elapsed after the falling edge of CS. If CS is brought high any time after the 10th SCLK falling edge but before the 16th SCLK falling edge, the part remains powered up, but the conversion is terminated and SDATAA and SDATAB go back into three-state. 16 serial clock cycles are required to complete the conversion and access the conversion result for the AD7357. The SDATA lines do not return to threestate after 16 SCLK cycles have elapsed, but instead do so when CS is brought high again. If CS is left low for another 2 SCLK cycles, two trailing zeros are clocked out after the data. If CS is left low for a further 16 SCLK cycles, the result for the other ADC on board is also accessed on the same SDATA line as shown in Figure 20 (see the Serial Interface section). Once 32 SCLK cycles have elapsed, the SDATA line returns to three-state on the 32nd SCLK falling edge. If CS is brought high prior to this, the SDATA line returns to three-state at that point. Thus, CS may idle low after 32 SCLK cycles until it is brought high again sometime prior to the next conversion if so desired, since the bus still returns to three-state upon completion of the dual result read.
DOUTA DOUTB
THREE-STATE
Figure 15. Entering Partial Power-Down Mode
To exit this mode of operation and power up the AD7357 again, a dummy conversion is performed. On the falling of CS, the device begins to power up, and continues to power up as long as CS is held low until after the falling edge of the 10th SCLK. The device is fully powered up after approximately TBD s has elapsed, and valid data results from the next conversion, as shown in Figure 16. If CS is brought high before the 2nd falling edge of SCLK, the AD7357 again goes into partial power-down. This avoids accidental power-up due to glitches on the CS line. Although the device may begin to power up on the falling edge of CS, it powers down again on the rising edge of CS. If the AD7357 is already in partial power-down mode and CS is brought high between the 2nd and 10th falling edges of SCLK, the device enters full power-down mode.
FULL POWER-DOWN MODE
This mode is intended for use in applications where throughput rates slower than those in the partial power-down mode are required, as power-up from a full power-down takes substantially longer than that from a partial power-down. This
Rev. PrD | Page 13 of 17
AD7357
mode is more suited to applications where a series of conversions performed at a relatively high throughput rate are followed by a long period of inactivity and thus, power-down. When the AD7357 is in full power-down, all analog circuitry is powered down. Full power-down is entered in a similar way as partial power-down, except the timing sequence shown in Figure 15 must be executed twice. The conversion process must be interrupted in a similar fashion by bringing CS high anywhere after the 2nd falling edge of SCLK and before the 10th falling edge of SCLK. The device enters partial power down at this point. To reach full power-down, the next conversion cycle must be interrupted in the same way, as shown in Figure 17. Once CS
THE PART BEGINS TO POWER UP.
Preliminary Technical Data
has been brought high in this window of SCLKs, the part completely powers down. Note that it is not necessary to complete the 16 SCLKs once CS has been brought high to enter a power-down mode. To exit full power-down mode and power-up the AD7357, a dummy conversion is performed, as when powering up from partial power-down. One the falling edge of CS , the device begins to power up, as long as CS is held low until after the falling edge of the 10th SCLK. The required power-up time must elapse before a conversion can be initiated, as shown in Figure 18.
THE PART IS FULLY POWERED UP; SEE POWER-UP TIMES SECTION.
tPOWER-UP1
CS
1
10
14
1
14
SCLK
DOUTA DOUTB
INVALID DATA
VALID DATA
Figure 16. Exiting Partial Power-Down Mode
THE PART ENTERS PARTIAL POWER DOWN. THE PART BEGINS TO POWER UP. THE PART ENTERS FULL POWER DOWN.
CS
SCLK
1
2
10
14
1
2
10
14
04603-031
DOUTA DOUTB
THREE-STATE INVALID DATA INVALID DATA
THREE-STATE
Figure 17. Entering Full Power-Down Mode
THE PART BEGINS TO POWER UP. CS THE PART IS FULLY POWERED UP, SEE POWER-UP TIMES SECTION.
tPOWER-UP2
SCLK
1
10
14
1
14
DOUTA DOUTB
INVALID DATA
VALID DATA
Figure 18. Exiting Full Power-Down Mode
Rev. PrD | Page 14 of 17
04603-033
04603-032
Preliminary Technical Data
POWER-UP TIMES
The AD7357 has two power-down modes, partial power-down and full power-down, which are described in detail in the previous sections. This section deals with the power-up time required when coming out of either of these modes. To power up from partial power-down mode, one dummy cycle is required. The device is fully powered up after approximately TBD s from the falling edge of CS has elapsed. Once the partial power-up time has elapsed, the ADC is fully powered up and the input signal is acquired properly. The quiet time, tQUIET, must still be allowed from the point where the bus goes back into three-state after the dummy conversion to the next falling edge of CS. To power up from full power-down, approximately TBD s should be allowed from the falling edge of CS, shown in Figure 18 as tPOWER-UP2. Note that during power-up from partial power-down mode, the track-and-hold, which is in hold mode while the part is powered down, returns to track mode after the first SCLK edge that the part receives after the falling edge of CS.
AD7357
When power supplies are first applied to the AD7357, the ADC can power up in either of the power-down modes or in normal mode. Because of this, it is best to allow a dummy cycle to elapse to ensure that the part is fully powered up before attempting a valid conversion. Likewise, if the part is to be kept in partial power-down mode immediately after the supplies are applied, then two dummy cycles must be initiated. The first dummy cycle must hold CS low until after the 10th SCLK falling edge; in the second cycle, CS must be brought high between the second and 10th SCLK falling edges (see Figure 15). Alternatively, if the part is to be placed into full power-down mode when the supplies are applied, three dummy cycles must be initiated. The first dummy cycle must hold CS low until after the 10th SCLK falling edge; the second and third dummy cycles place the part into full power-down mode (see Figure 17). See the Modes of Operation section.
Rev. PrD | Page 15 of 17
AD7357 SERIAL INTERFACE
Figure 19 shows the detailed timing diagram for serial interfacing to the AD7357. The serial clock provides the conversion clock and controls the transfer of information from the AD7357 during conversion. The CS signal initiates the data transfer and conversion process. The falling edge of CS puts the track and hold into hold mode at which point the analog input is sampled and the bus is taken out of three-state. The conversion is also initiated at this point and requires a minimum of 16 SCLKs to complete. Once 15 SCLK falling edges have elapsed, the track and hold will go back into track on the next SCLK rising edge, as shown in Figure 19 at point B. On the rising edge of CS, the conversion will be terminated and SDATAA and SDATAB will go back into threestate. If CS is not brought high, but is instead held low for a further 16 SCLK cycles on SDATAA, the data from the conversion on ADCB will be output on SDATAA. Likewise, if CS is held low for a further 16 SCLK cycles on SDATAA, the data from the conversion on ADCA will be output on SDATAB. This is illustrated in Figure 20 where the case for SDATAA is shown. In this case, the SDATA line in use will go back into three-state on the 32nd SCLK falling edge or the rising edge of CS, which ever occurs first.
Preliminary Technical Data
A minimum of 16 serial clock cycles are required to perform the conversion process and to access data from one conversion on either data line of the AD7357. CS going low provides the leading zero to be read in by the microcontroller or DSP. The remaining data is then clocked out by subsequent SCLK falling edges, beginning with a second leading zero. Thus, the first falling clock edge on the serial clock has the leading zero provided and also clocks out the second leading zero. The 14-bit result then follows with the final bit in the data transfer valid on the 16th falling edge, having being clocked out on the previous (15th) falling edge. In applications with a slower SCLK, it may be possible to read in data on each SCLK rising edge depending on the SCLK frequency. The first rising edge of SCLK after the CS falling edge would have the second leading zero provided, and the 15th rising SCLK edge would have DB0 provided.
t ACQUISITION
CS
t CONVERT t2
SCLK
1 2 3 4
t9
B
t6
5 15 16
DOUTA 0 DB13 0 DOUTB THREESTATE 2 LEADING ZEROS
t3
t4
DB12 DB11
t7
DB10 DB2
t5
DB1 DB0 0
t8 tQUIET
THREE-STATE
Figure 19. Serial Interface Timing Diagram
CS
t2
SCLK
1 2 3 4
t6
5 15 16 17 18 31 32
t5 t3
DOUTA THREESTATE 2 LEADING ZEROS 0 0 DB13A DB12 A
t4
DB11 A
t7
DB0 A 0 0 DB13B DB12B DB1 B DB0 B 0 THREESTATE 2 ZEROS
Figure 20. Reading Data from Both ADCs on One SDATA Line with 32 SCLKs
Rev. PrD | Page 16 of 17
Preliminary Technical Data OUTLINE DIMENSIONS
5.10 5.00 4.90
AD7357
16
9
4.50 4.40 4.30
1 8
6.40 BSC
PIN 1 1.20 MAX 0.20 0.09 0.65 BSC 0.30 0.19 COPLANARITY 0.10 SEATING PLANE 8 0 0.75 0.60 0.45
0.15 0.05
COMPLIANT TO JEDEC STANDARDS MO-153-AB
Figure 21. 16-LeadThin Shrink Small Outline Package Dimensions shown in millimeters
ORDERING GUIDE
Model AD7357BRUZ AD7357BRUZ-500RL7 AD7357BRUZ-RL AD7357YRUZ AD7357YRUZ-500RL7 AD7357YRUZ-RL Temperature Range -40C to +85C -40C to +85C -40C to +85C -40C to +125C -40C to +125C -40C to +125C Package Description 16-Lead TSSOP 16-Lead TSSOP 16-Lead TSSOP 16-Lead TSSOP 16-Lead TSSOP 16-Lead TSSOP Package Option RU-16 RU-16 RU-16 RU-16 RU-16 RU-16
(c)2007 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. PR06003-0-9/07(PrD)
Rev. PrD | Page 17 of 17

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