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ADC12L066 12-Bit, 66 MSPS, 395 mW A/D Converter with Internal Sample-and-Hold
ADVANCE INFORMATION
November 2001
ADC12L066 12-Bit, 66 MSPS, 395 mW A/D Converter with Internal Sample-and-Hold
General Description
The ADC12L066 is a monolithic CMOS analog-to-digital converter capable of converting analog input signals into 12-bit digital words at 66 Megasamples per second (MSPS), minimum. This converter uses a differential, pipelined architecture with digital error correction and an on-chip sample-andhold circuit to minimize die size and power consumption while providing excellent dynamic performance. A unique sample-and-hold stage yields a full-power bandwidth of 450 MHz. Operating on a single 3.3V power supply, this device consumes just 395 mW at 66 MSPS, including the reference current. The Power Down feature reduces power consumption to just 50 mW. The differential inputs provide a full scale input swing equal to VREF with the possibility of a single-ended input. Full use of the differential input is recommended for optimum performance. For ease of use, the buffered, high impedance, single-ended reference input is converted on-chip to a differential reference for use by the processing circuitry. Output data format is 12-bit offset binary. This device is available in the 32-lead LQFP package and will operate over the industrial temperature range of -40C to +85C.
Features
n n n n Single supply operation Low power consumption Power down mode On-chip reference buffer
Key Specifications
n n n n n n n n n n Resolution Conversion Rate Full Power Bandwidth DNL INL SNR SFDR Data Latency Supply Voltage Power Consumption, 40 MHz 12 Bits 66 MSPS(min) 450 MHz 0.5 LSB(typ) 1.0 LSB(typ) 66 dB(typ) 78 dB(typ) 6 Clock Cycles +3.3V 300 mV 395 mW(typ)
Applications
n n n n n n n n Ultrasound and Imaging Instrumentation Cellular Base Stations/Communications Receivers Sonar/Radar xDSL Wireless Local Loops Data Acquisition Systems DSP Front Ends
Connection Diagram
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(c) 2001 National Semiconductor Corporation
DS200328
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ADC12L066
Ordering Information
Industrial (-40C TA +85C) ADC12L066CIVY ADC12L066CIVYX ADC12L066EVAL Package 32 Pin LQFP 32 Pin LQFP Tape and Reel Evaluation Board
Block Diagram
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ADC12L066
Pin Descriptions and Equivalent Circuits
Pin No. ANALOG I/O 2 VIN+ Non-Inverting analog signal Input. With a 1.0V reference voltage the input signal level is 1.0 VP-P. Inverting analog signal Input. With a 1.0V reference voltage the input signal level is 1.0 VP-P. This pin may be connected to VCM for single-ended operation, but a differential input signal is required for best performance. Symbol Equivalent Circuit Description
3
VIN-
1
VREF
Reference input. This pin should be bypassed to AGND with a 0.1 F monolithic capacitor. VREF is 1.0V nominal and should be between 0.8V and 1.2V.
31
VRP
32
VRM
These pins are high impedance reference bypass pins only. Connect a 0.1 F capacitor from each of these pins to AGND. DO NOT connect anything else to these pins.
30
VRN
DIGITAL I/O 10 CLK Digital clock input. The range of frequencies for this input is 1 MHz to 75 MHz (typical) with guaranteed performance at 66 MHz. The input is sampled on the rising edge of this input. OE is the output enable pin that, when low, enables the TRI-STATE data output pins. When this pin is high, the outputs are in a high impedance state. PD is the Power Down input pin. When high, this input puts the converter into the power down mode. When this pin is low, the converter is in the active mode.
11
OE
8
PD
3
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ADC12L066
Pin Descriptions and Equivalent Circuits
Pin No. Symbol Equivalent Circuit
(Continued) Description
14-19, 22-27
D0-D11
Digital data output pins that make up the 12-bit conversion results. D0 is the LSB, while D11 is the MSB of the offset binary output word.
ANALOG POWER Positive analog supply pins. These pins should be connected to a quiet +3.3V source and bypassed to AGND with 0.1 F monolithic capacitors located within 1 cm of these power pins, and with a 10 F capacitor. The ground return for the analog supply. Positive digital supply pin. This pin should be connected to the same quiet +3.3V source as is VA and bypassed to DGND with a 0.1 F monolithic capacitor in parallel with a 10 F capacitor, both located within 1 cm of the power pin. Decouple this pin from the VA pins. The ground return for the digital supply. Positive digital supply pin for the ADC12L066's output drivers. This pin should be connected to a voltage source of +2.5V to VD and bypassed to DR GND with a 0.1 F monolithic capacitor. If the supply for this pin is different from the supply used for VA and VD, it should also be bypassed with a 10 F tantalum capacitor. The voltage at this pin should never exceed the voltage on VD. All bypass capacitors should be located within 1 cm of the supply pin. The ground return for the digital supply for the ADC12L066's output drivers. This pin should be connected to the system digital ground, but not be connected in close proximity to the ADC12L066's DGND or AGND pins. See Section 5.0 (Layout and Grounding) for more details.
5, 6, 29
VA
4, 7, 28 DIGITAL POWER
AGND
13
VD
9, 12
DGND
21
VDR
20
DR GND
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ADC12L066
Absolute Maximum Ratings
2)
(Notes 1,
Machine Model (Note 5) Soldering Temperature, Infrared, 10 sec. (Note 6) Storage Temperature
250V 235C -65C to +150C
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VA, VD, VDR |VA-VD| Voltage on Any Input or Output Pin Input Current at Any Pin (Note 3) Package Input Current (Note 3) Package Dissipation at TA = 25C ESD Susceptibility Human Body Model (Note 5) 2500V 4.2V 100 mV -0.3V to VA or VD +0.3V
Operating Ratings (Notes 1, 2)
Operating Temperature Supply Voltage (VA, VD) Output Driver Supply (VDR) VREF Input CLK, PD, OE VIN Input |AGND-DGND| -40C TA +85C +3.0V to +3.60V +2.5V to VD 0.8V to 1.2V -0.05V to VD + 0.05V -0V to (VA - 0.5V) 100mV
25 mA 50 mA
See (Note 4)
Converter Electrical Characteristics
Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, VA = VD = VDR = +3.3V, PD = 0V, VREF = +1.0V, fCLK = 66 MHz, tr = tf = 2 ns, CL = 20 pF/pin. Boldface limits apply for TA = TJ = TMIN to TMAX: all other limits TA = TJ = 25C (Notes 7, 8, 9) Symbol Parameter Conditions Typical (Note 10) Limits (Note 10) 12 Units (Limits) Bits LSB(max) LSB(max) %FS(max) %FS(max) %FS(max)
STATIC CONVERTER CHARACTERISTICS Resolution with No Missing Codes INL DNL GE Integral Non Linearity (Note 11) Differential Non Linearity Gain Error Offset Error (VIN+ = VIN-) Under Range Output Code Over Range Output Code REFERENCE AND ANALOG INPUT CHARACTERISTICS VCM CIN VREF Common Mode Input Voltage VIN Input Capacitance (each pin to GND) Reference Voltage (Note 13) Reference Input Resistance VIN = 1.0 Vdc + 1 VP-P (CLK LOW) (CLK HIGH) 1.0 8 7 1.00 100 0.8 1.2 V pF pF V(min) V(max) M(min) Positive Error Negative Error
1.0 0.5
-0.8 +0.1 +0.1 0 4095
2.4 3 3 0.9
0 4095
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ADC12L066
DC and Logic Electrical Characteristics
Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, VA = VD = VDR = +3.3V, PD = 0V, VREF = +1.0V, fCLK = 66 MHz, tr = tf = 2 ns, CL = 20 pF/pin. Boldface limits apply for TA = TJ = TMIN to TMAX: all other limits TA = TJ = 25C (Notes 7, 8, 9) Symbol Parameter Conditions Typical (Note 10) Limits (Note 10) 2.0 0.8 10 -10 5 IOUT = -0.5 mA IOUT = 1.6 mA VOUT = 3.3V VOUT = 0V VOUT = 0V VOUT = VDR PD Pin = DGND, VREF = 1.0V PD Pin = VDR PD Pin = DGND PD Pin = VDR, fCLK = 0 PD Pin = DGND, (Note 14) PD Pin = VDR, fCLK = 0 PD Pin = DGND, CL = 0 pF (Note 15) PD Pin = VDR, fCLK = 0 Rejection of Full-Scale Error with VA = 3.0V vs 3.6V SNR Degradation w/10 MHz, 250 mVP-P riding on VA 0 dBFS Input, Output at -3 dB fIN = 25 MHz, Differential VIN = -0.5 dBFS SNR Signal-to-Noise Ratio fIN = 150 MHz, Differential VIN = -0.5 dBFS fIN = 250 MHz, Differential VIN = -0.5 dBFS fIN = 25 MHz, Differential VIN = -0.5 dBFS SINAD Signal-to-Noise & Distortion fIN = 150 MHz, Differential VIN = -0.5 dBFS fIN = 250 MHz, Differential VIN = -0.5 dBFS fIN = 25 MHz, Differential VIN = -0.5 dBFS ENOB Effective Number of Bits fIN = 150 MHz, Differential VIN = -0.5 dBFS fIN = 250 MHz, Differential VIN = -0.5 dBFS 100 -100 -20 20 93 4 4.8 2 128 7 2.7 0.4 Units (Limits) V(min) V(max) A A pF V(min) V(max) nA nA mA(min) mA(min) mA(max) mA mA(max) mA mA(max) mA 405 mW mW dB dB
CLK, PD, OE DIGITAL INPUT CHARACTERISTICS VIN(1) VIN(0) IIN(1) IIN(0) CIN VOUT(1) VOUT(0) IOZ +ISC -ISC Logical "1" Input Voltage Logical "0" Input Voltage Logical "1" Input Current Logical "0" Input Current Digital Input Capacitance Logical "1" Output Voltage Logical "0" Output Voltage TRI-STATE Output Current Output Short Circuit Source Current Output Short Circuit Sink Current VD = 3.3V VD = 3.0V VIN+, VIN- = 3.3V VIN+, VIN- = 0V
D0-D11 DIGITAL OUTPUT CHARACTERISTICS
POWER SUPPLY CHARACTERISTICS IA ID IDR Analog Supply Current Digital Supply Current Digital Output Supply Current Total Power Consumption PSRR1 PSRR2 Power Supply Rejection Power Supply Rejection
<1
0 395 50 58 -53
DYNAMIC CONVERTER CHARACTERISTICS BW Full Power Bandwidth 450 67 66 65 66.7 66.2 64.2 10.8 10.7 10.3 10.0 9.8 62 61 63 64 MHz dB (min) dB (min) dB dB (min) dB (min) dB Bits (min) Bits (min) Bits
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ADC12L066
DC and Logic Electrical Characteristics
(Continued)
Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, VA = VD = VDR = +3.3V, PD = 0V, VREF = +1.0V, fCLK = 66 MHz, tr = tf = 2 ns, CL = 20 pF/pin. Boldface limits apply for TA = TJ = TMIN to TMAX: all other limits TA = TJ = 25C (Notes 7, 8, 9) Symbol Parameter Conditions fIN = 25 MHz, Differential VIN = -0.5 dBFS 2nd Harm Second Harmonic Distortion fIN = 150 MHz, Differential VIN = -0.5 dBFS fIN = 250 MHz, Differential VIN = -0.5 dBFS fIN = 25 MHz, Differential VIN = -0.5 dBFS 3rd Harm Third Harmonic Distortion fIN = 150 MHz, Differential VIN = -0.5 dBFS fIN = 250 MHz, Differential VIN = -0.5 dBFS fIN = 25 MHz, Differential VIN = -0.5 dBFS THD Total Harmonic Distortion fIN = 150 MHz, Differential VIN = -0.5 dBFS fIN = 250 MHz, Differential VIN = -0.5 dBFS fIN = 25 MHz, Differential VIN = -0.5 dBFS SFDR Spurious Free Dynamic Range fIN = 150 MHz, Differential VIN = -0.5 dBFS fIN = 250 MHz, Differential VIN = -0.5 dBFS IMD Intermodulation Distortion fIN = 9.5 MHz and 10.5 MHz, each = -7 dBFS Typical (Note 10) -85 -81 -79 -83 -79 -77 -78 -74 -72 80 76 74 -70 -65 -61 -70 -66 Limits (Note 10) -72 -68 Units (Limits) dB (max) dB (max) dB dB (max) dB (max) dB dB (max) dB (max) dB dB (min) dB (min) dB dBFS
AC Electrical Characteristics
Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, VA = VD = +3.3V, VDR = +2.5V, PD = 0V, VREF = +1.0V, fCLK = 66 MHz, tr = tf = 2 ns, CL = 20 pF/pin. Boldface limits apply for TA = TJ = TMIN to TMAX: all other limits TA = TJ = 25C (Notes 7, 8, 9, 12) Symbol fCLK1 fCLK2 Parameter Maximum Clock Frequency Minimum Clock Frequency Recommended Clock Duty Cycle tCH tCL tCONV tOD tAD tAJ tDIS Clock High Time Clock Low Time Conversion Latency Data Output Delay after Rising CLK Edge Aperture Delay Aperture Jitter Data outputs into TRI-STATE Mode VDR = 2.5V VDR = 3.3V 12 9 2 1.2 10 13 Conditions Typical (Note 10) 75 1 50 40 60 6.5 6.5 6 Limits (Note 10) 66 Units (Limits) MHz(min) MHz %(min) %(max) ns(min) ns(min) Clock Cycles ns ns(max) ns ps rms ns
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ADC12L066
AC Electrical Characteristics
(Continued)
Unless otherwise specified, the following specifications apply for AGND = DGND = DR GND = 0V, VA = VD = +3.3V, VDR = +2.5V, PD = 0V, VREF = +1.0V, fCLK = 66 MHz, tr = tf = 2 ns, CL = 20 pF/pin. Boldface limits apply for TA = TJ = TMIN to TMAX: all other limits TA = TJ = 25C (Notes 7, 8, 9, 12) Symbol tEN tPD Parameter Data Outputs Active after TRI-STATE Power Down Mode Exit Cycle Conditions Typical (Note 10) 10 20 Limits (Note 10) Units (Limits) ns tCLK
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test conditions. Note 2: All voltages are measured with respect to GND = AGND = DGND = 0V, unless otherwise specified. Note 3: When the input voltage at any pin exceeds the power supplies (that is, VIN < AGND, or VIN > VA, VD or VDR), the current at that pin should be limited to 25 mA. The 50 mA maximum package input current rating limits the number of pins that can safely exceed the power supplies with an input current of 25 mA to two. Note 4: The absolute maximum junction temperature (TJmax) for this device is 150C. The maximum allowable power dissipation is dictated by TJmax, the junction-to-ambient thermal resistance (JA), and the ambient temperature, (TA), and can be calculated using the formula PDMAX = (TJmax - TA )/JA. In the 32-pin LQFP, JA is 79C/W, so PDMAX = 1,582 mW at 25C and 823 mW at the maximum operating ambient temperature of 85C. Note that the power consumption of this device under normal operation will typically be about 650 mW (395 typical power consumption + 255 mW output loading with 250 MHz input). The values for maximum power dissipation listed above will be reached only when the device is operated in a severe fault condition (e.g. when input or output pins are driven beyond the power supply voltages, or the power supply polarity is reversed). Obviously, such conditions should always be avoided. Note 5: Human body model is 100 pF capacitor discharged through a 1.5 k resistor. Machine model is 220 pF discharged through 0. Note 6: The 235C reflow temperature refers to infrared reflow. For Vapor Phase Reflow (VPR), the following Conditions apply: Maintain the temperature at the top of the package body above 183C for a minimum 60 seconds. The temperature measured on the package body must not exceed 220C. Only one excursion above 183C is allowed per reflow cycle. Note 7: The inputs are protected as shown below. Input voltages above VA or below GND will not damage this device, provided current is limited per (Note 3). However, errors in the A/D conversion can occur if the input goes above VA or below GND by more than 100 mV. As an example, if VA is 3.3V, the full-scale input voltage must be 3.4V to ensure accurate conversions.
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Note 8: To guarantee accuracy, it is required that |VA-VD| 100 mV and separate bypass capacitors are used at each power supply pin. Note 9: With the test condition for VREF = +1.0V (2VP-P differential input), the 12-bit LSB is 488 V. Note 10: Typical figures are at TA = TJ = 25C, and represent most likely parametric norms. Test limits are guaranteed to National's AOQL (Average Outgoing Quality Level). Note 11: Integral Non Linearity is defined as the deviation of the analog value, expressed in LSBs, from the straight line that passes through positive and negative full-scale. Note 12: Timing specifications are tested at TTL logic levels, VIL = 0.4V for a falling edge and VIH = 2.4V for a rising edge. Note 13: Optimum dynamic performance will be obtained by keeping the reference input in the 0.8V to 1.2V range. The LM4051CIM3-ADJ or the LM4051CIM3-1.2 bandgap voltage reference is recommended for this application. Note 14: IDR is the current consumed by the switching of the output drivers and is primarily determined by load capacitance on the output pins, the supply voltage, VDR, and the rate at which the outputs are switching (which is signal dependent). IDR =VDR(C0 x f0 + C1 x f1 +....C11 x f11) where VDR is the output driver power supply voltage, Cn is total capacitance on the output pin, and fn is the average frequency at which that pin is toggling. Note 15: Excludes IDR. See note 14.
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ADC12L066
Specification Definitions
APERTURE DELAY is the time after the rising edge of the clock to when the input signal is acquired or held for conversion. APERTURE JITTER (APERTURE UNCERTAINTY) is the variation in aperture delay from sample to sample. Aperture jitter manifests itself as noise in the output. COMMON MODE VOLTAGE (VCM) is the d.c. potential present at both signal inputs to the ADC. CONVERSION LATENCY See PIPELINE DELAY. DIFFERENTIAL NON-LINEARITY (DNL) is the measure of the maximum deviation from the ideal step size of 1 LSB. DUTY CYCLE is the ratio of the time that a repetitive digital waveform is high to the total time of one period. The specification here refers to the ADC clock input signal. EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE BITS) is another method of specifying Signal-to-Noise and Distortion or SINAD. ENOB is defined as (SINAD - 1.76) / 6.02 and says that the converter is equivalent to a perfect ADC of this (ENOB) number of bits. FULL POWER BANDWIDTH is a measure of the frequency at which the reconstructed output fundamental drops 3 dB below its low frequency value for a full scale input. GAIN ERROR is the deviation from the ideal slope of the transfer function. It can be calculated as: Gain Error = Positive Full Scale Error - Offset Error INTEGRAL NON LINEARITY (INL) is a measure of the deviation of each individual code from a line drawn from negative full scale (12 LSB below the first code transition) through positive full scale (12 LSB above the last code transition). The deviation of any given code from this straight line is measured from the center of that code value. INTERMODULATION DISTORTION (IMD) is the creation of additional spectral components as a result of two sinusoidal frequencies being applied to the ADC input at the same time. It is defined as the ratio of the power in the intermodulation products to the total power in the original frequencies. IMD is usually expressed in dBFS. MISSING CODES are those output codes that will never appear at the ADC outputs. The ADC12L066 is guaranteed not to have any missing codes. NEGATIVE FULL SCALE ERROR is the difference between the input voltage (VIN+ - VIN-) just causing a transition from negative full scale to the next code and its ideal value of 0.5 LSB.
OFFSET ERROR is the difference between the ideal differential input voltage (VIN+ - VIN- = 0V) and the actual input voltage required to cause a transition from an output code 2047 to 2048. OUTPUT DELAY is the time delay after the rising edge of the clock before the data update is presented at the output pins. PIPELINE DELAY (LATENCY) is the number of clock cycles between initiation of conversion and when that data is presented to the output driver stage. Data for any given sample is available at the output pins the Pipeline Delay plus the Output Delay after the sample is taken. New data is available at every clock cycle, but the data lags the conversion by the pipeline delay. POSITIVE FULL SCALE ERROR is the difference between the actual last code transition and its ideal value of 112 LSB below positive full scale. POWER SUPPLY REJECTION RATIO (PSRR) is a measure of how well the ADC rejects a change in the power supply voltage. For the ADC12L066, PSRR1 is the ratio of the change in Full-Scale Error that results from a change in the dc power supply voltage, expressed in dB. PSRR2 is a measure of how well an a. c. signal riding upon the power supply is rejected at the output. SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in dB, of the rms value of the input signal to the rms value of the sum of all other spectral components below one-half the sampling frequency, not including harmonics or dc. SIGNAL TO NOISE PLUS DISTORTION (S/N+D or SINAD) Is the ratio, expressed in dB, of the rms value of the input signal to the rms value of all of the other spectral components below half the clock frequency, including harmonics but excluding dc. SPURIOUS FREE DYNAMIC RANGE (SFDR) is the difference, expressed in dB, between the rms values of the input signal and the peak spurious signal, where a spurious signal is any signal present in the output spectrum that is not present at the input. TOTAL HARMONIC DISTORTION (THD) is the ratio, expressed in dB or dBc, of the rms total of the first six harmonic components to the rms value of the input signal.
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ADC12L066
Timing Diagram
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Output Timing
Transfer Characteristic
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FIGURE 1. Transfer Characteristic
Functional Description
Operating on a single +3.3V supply, the ADC12L066 uses a pipelined architecture and has error correction circuitry to help ensure maximum performance. Differential analog input signals are digitized to 12 bits. Each analog input signal should have a peak-to-peak voltage equal to the input reference voltage, VREF, and be centered around VREF/2. Table 1 and Table 2 indicate the input to the
output relationship of the ADC12L066. As indicated in Table 2, biasing one input to VREF/2 and driving the other input with its full range signal results in a 6 dB reduction of the output range, limiting it to the range of 14 to 34 of the minimum output range obtainable if both inputs were driven with complimentary signals. Section 1.3 explains how to avoid this signal reduction.
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ADC12L066
Functional Description
(Continued)
1.3 SIGNAL INPUTS The signal inputs are VIN+ and VIN-. The input signal, VIN, is defined as VIN = (VIN+) - (VIN-) Figure 2 shows the expected input signal range. Note that the nominal input common mode voltage is 0.5V and the nominal input signals each run between the limits of AGND and 1.0V with VREF = 1.0V. If the differential input signal increases above 2 VP-O, the input common mode range should become VREF/2. The Peaks of the input signals should never exceed the voltage described as Peak Input Voltaged = VA - 1.0 to maintain dynamic performance.
TABLE 1. Input to Output Relationship -- Differential Input VIN+ VCM -0.5* VREF VCM -0.25* VREF VCM VCM +0.25* VREF VCM +0.5* VREF VIN- VCM +0.5* VREF VCM +0.25* VREF VCM VCM -0.25* VREF VCM -0.5* VREF Output 0000 0000 0000 0100 0000 0000 1000 0000 0000 1100 0000 0000 1111 1111 1111
TABLE 2. Input to Output Relationship -- Single-Ended Input VIN+ VCM -VREF VCM -0.5* VREF VCM VCM +0.5* VREF VCM +VREF VIN- VCM VCM VCM VCM VCM Output 0000 0000 0000 0100 0000 0000 1000 0000 0000 1100 0000 0000 1111 1111 1111
The output word rate is the same as the clock frequency, which can be between 1 MSPS and 75 MSPS (typical). The analog input voltage is acquired at the rising edge of the clock and the digital data for that sample is delayed by the pipeline for 6 clock cycles. A logic high on the power down (PD) pin reduces the converter power consumption to 50 mW.
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FIGURE 2. Expected Input Signal Range The ADC12L066 performs best with a differential input with each input centered around VCM (minimum of 0.5V). The peak-to-peak voltage swing at both VIN+ and VIN- should not exceed the value of the reference voltage or the output data will be clipped. The two input signals should be exactly 180 out of phase from each other and of the same amplitude. For single frequency inputs, angular errors result in a reduction of the effective full scale input. For a complex waveform, however, angular errors will result in distortion. For angular deviations of up to 10 degrees from these two signals being 180 out of phase, the full scale error in LSB can be described as approximately EFS = dev1.79 Where dev is the angular difference between the two signals having a 180 relative phase relationship to each other (see Figure 3). Drive the analog inputs with a source impedance less than 100.
Applications Information
1.0 OPERATING CONDITIONS We recommend that the following conditions be observed for operation of the ADC12L066: 3.0 V VA 3.6V VD = VA 1.5V VDR VD 1 MHz fCLK 75 MHz 0.8V VREF 1.2V 1.1 ANALOG INPUTS The ADC12L066 has two analog signal inputs, VIN+ and VIN-. These two pins form a differential input pair. There is one reference input pin, VREF. 1.2 REFERENCE PINS The ADC12L066 is designed to operate with a 1.0V reference, but performs well with reference voltages in the range of 0.8V to 1.2V. Lower reference voltages will decrease the signal-to-noise ratio (SNR) of the ADC12L066. Increasing the reference voltage (and the input signal swing) beyond 1.2V will degrade THD for a full-scale input. It is very important that all grounds associated with the reference voltage and the input signal make connection to the analog ground plane at a single point to minimize the effects of noise currents in the ground path. The three Reference Bypass Pins (VRP, VRM and VRN) are made available for bypass purposes only. These pins should each be bypassed to ground with a 0.1 F capacitor. DO NOT LOAD these pins.
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FIGURE 3. Angular Errors Between the Two Input Signals Will Reduce the Output Level or Cause Distortion For differential operation, each analog input signal should have a peak-to-peak voltage equal to the input reference voltage, VREF, and be centered around VCM. For single ended operation, one of the analog inputs should be con11 www.national.com
ADC12L066
Applications Information
(Continued)
nected to the d.c. common mode voltage of the driven input. The peak-to-peak differential input signal should be twice the reference voltage to maximize SNR and SINAD performance (Figure 2b). For example, set VREF to 1.0V, bias VIN- to 1.0V and drive VIN+ with a signal range of 0V to 2.0V. Because very large input signal swings can degrade distortion performance, better performance with a single-ended input can be obtained by reducing the reference voltage when maintaining a full-range output. Tables 1, 2 indicate the input to output relationship of the ADC12L066. The VIN+ and the VIN- inputs of the ADC12L066 consist of an analog switch followed by a switched-capacitor amplifier. The capacitance seen at the analog input pins changes with the clock level, appearing as 8 pF when the clock is low, and 7 pF when the clock is high. Although this difference is small, a dynamic capacitance is more difficult to drive than is a fixed capacitance, so choose the driving amplifier carefully. The CLC409 and CLC428 are good amplifiers for driving the ADC12L066. The internal switching action at the analog inputs causes energy to be output from the input pins. As the driving source tries to compensate for this, it adds noise to the signal. To prevent this, use 18 series resistors at each of the signal inputs with a 10 pF capacitor across the inputs, as can be seen in Figure 5 and Figure 6. These components should be placed close to the ADC because the input pins of the ADC is the most sensitive part of the system and this is the last opportunity to filter the input. The 10 pF capacitor value is for undersampling application and should be replaced with a 120 pF capacitor for Nyquist application. 2.0 DIGITAL INPUTS Digital inputs consist of CLK, OE and PD. 2.1 CLK The CLK signal controls the timing of the sampling process. Drive the clock input with a stable, low jitter clock signal in the range of 1 MHz to 75 MHz with rise and fall times of less than 2 ns. The trace carrying the clock signal should be as short as possible and should not cross any other signal line, analog or digital, not even at 90. The CLK signal also drives an internal state machine. If the CLK is interrupted, or its frequency is too low, the charge on internal capacitors can dissipate to the point where the accuracy of the output data will degrade. This is what limits the lowest sample rate to 1 MSPS. The CLOCK signal also drives an internal state machine. If the clock is interrupted, or its frequency is too low, the charge on internal capacitors can dissipate to the point where the accuracy of the output data will degrade. The duty cycle of the clock signal can affect the performance of any A/D Converter. Because achieving a precise duty cycle is difficult, the ADC12L066 is designed to maintain performance over a range of duty cycles. While it is specified and performance is guaranteed with a 50% clock duty cycle, performance is typically maintained over a clock duty cycle range of 35% to 65%. The clock line should be series terminated in the characteristic impedance of that line at the clock source. If the clock line is longer than
where tr is the clock rise time and tprop is the propagation rate of the signal along the trace, the CLOCK pin should be a.c. terminated with a series RC such that the resistor value is equal to the characteristic impedance of the clock line and the capacitor value is
where 'I' is the line length in inches and Zo is the characteric impedance of the clock line. This termination should be located as close as possible to, but within one centimeter of, the ADC12L066 clock pin as shown in Figure 4. A typical propagation rate on FR4 material is about 150ps/inch, or about 60ps/cm. 2.2 OE The OE pin, when high, puts the output pins into a high impedance state. When this pin is low the outputs are in the active state. The ADC12L066 will continue to convert whether this pin is high or low, but the output can not be read while the OE pin is high. 2.3 PD The PD pin, when high, holds the ADC12L066 in a powerdown mode to conserve power when the converter is not being used. The power consumption in this state is 50 mW. The output data pins are undefined in this mode. Power consumption during power-down is not affected by the clock frequency, or by whether there is a clock signal present. The data in the pipeline is corrupted while in the power down mode. 3.0 OUTPUTS The ADC12L066 has 12 TTL/CMOS compatible Data Output pins. The offset binary data is present at these outputs while the OE and PD pins are low. While the tOD time provides information about output timing, a simple way to capture a valid output is to latch the data on the rising edge of the conversion clock (pin 10). Be very careful when driving a high capacitance bus. The more capacitance the output drivers must charge for each conversion, the more instantaneous digital current flows through VDR and DR GND. These large charging current spikes can cause on-chip ground noise and couple into the analog circuitry, degrading dynamic performance. Adequate bypassing and careful attention to the ground plane will reduce this problem. Additionally, bus capacitance beyond the specified 20 pF/pin will cause tOD to increase, making it difficult to properly latch the ADC output data. The result could be an apparent reduction in dynamic performance. To minimize noise due to output switching, minimize the load currents at the digital outputs. This can be done by connecting buffers between the ADC outputs and any other circuitry (74ACQ541, for example). Only one input should be connected to each output pin. Additionally, inserting series resistors of 47 to 56 at the digital outputs, close to the ADC pins, will isolate the outputs from trace and other circuit capacitances and limit the output currents, which could otherwise result in performance degradation. See Figure 4.
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While the ADC12L066 will operate with VDR voltages down to 2.5V, tOD increases with reduced VDR. Be careful of external timing when using reduced VDR.
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FIGURE 4. Simple Application Circuit with Single-Ended to Differential Buffer
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FIGURE 5. Differential Drive Circuit of Figure 4
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FIGURE 6. Driving the Signal Inputs with a Transformer 4.0 POWER SUPPLY CONSIDERATIONS The power supply pins should be bypassed with a 10 F capacitor and with a 0.1 F ceramic chip capacitor within a centimeter of each power pin. Leadless chip capacitors are preferred because they have low series inductance. As is the case with all high-speed converters, the ADC12L066 is sensitive to power supply noise. Accordingly, the noise on the analog supply pin should be kept below 100 mVP-P. No pin should ever have a voltage on it that is in excess of the supply voltages, not even on a transient basis. Be especially careful of this during turn on and turn off of power. The VDR pin provides power for the output drivers and may be operated from a supply in the range of 2.5V to VD (nominal 3.3V). This can simplify interfacing to 3V devices and systems. DO NOT operate the VDR pin at a voltage higher than VD. 5.0 LAYOUT AND GROUNDING Proper grounding and proper routing of all signals are essential to ensure accurate conversion. Maintaining separate analog and digital areas of the board, with the ADC12L066 between these areas, is required to achieve specified performance. The ground return for the data outputs (DR GND) carries the ground current for the output drivers. The output current can exhibit high transients that could add noise to the conversion
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process. To prevent this from happening, the DR GND pins should NOT be connected to system ground in close proximity to any of the ADC12L066's other ground pins. Capacitive coupling between the typically noisy digital circuitry and the sensitive analog circuitry can lead to poor performance. The solution is to keep the analog circuitry separated from the digital circuitry, and to keep the clock line as short as possible. Digital circuits create substantial supply and ground current transients. The logic noise thus generated could have significant impact upon system noise performance. The best logic family to use in systems with A/D converters is one which employs non-saturating transistor designs, or has low noise characteristics, such as the 74LS, 74HC(T) and 74AC(T)Q families. The worst noise generators are logic families that draw the largest supply current transients during clock or signal edges, like the 74F and the 74AC(T) families. The effects of the noise generated from the ADC output switching can be minimized through the use of 47 to 56 resistors in series with each data output line. Locate these resistors as close to the ADC output pins as possible. Since digital switching transients are composed largely of high frequency components, total ground plane copper weight will have little effect upon the logic-generated noise. This is because of the skin effect. Total surface area is more important than is total ground plane volume.
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FIGURE 7. Example of a Suitable Layout Generally, analog and digital lines should cross each other at 90 to avoid crosstalk. To maximize accuracy in high speed, high resolution systems, however, avoid crossing analog and digital lines altogether. It is important to keep clock lines as short as possible and isolated from ALL other lines, including other digital lines. Even the generally accepted 90 crossing should be avoided with the clock line as even a little coupling can cause problems at high frequencies. This is because other lines can introduce jitter into the clock line, which can lead to degradation of SNR. Also, the high speed clock can introduce noise into the analog chain. Best performance at high frequencies and at high resolution is obtained with a straight signal path. That is, the signal path through all components should form a straight line wherever possible. Be especially careful with the layout of inductors. Mutual inductance can change the characteristics of the circuit in which they are used. Inductors should not be placed side by side, even with just a small part of their bodies beside each other. The analog input should be isolated from noisy signal traces to avoid coupling of spurious signals into the input. Any external component (e.g., a filter capacitor) connected between the converter's input pins and ground or to the reference input pin and ground should be connected to a very clean point in the analog ground plane. 6.0 DYNAMIC PERFORMANCE To achieve the best dynamic performance, the clock source driving the CLK input must be free of jitter. Isolate the ADC clock from any digital circuitry with buffers, as with the clock tree shown in Figure 8. As mentioned in Section 5.0, it is good practice to keep the ADC clock line as short as possible and to keep it well away from any other signals. Other signals can introduce jitter into the clock signal, which can lead to reduced SNR performance, and the clock can introduce noise into other lines. Even lines with 90 crossings have capacitive coupling, so try to avoid even these 90 crossings of the clock line.
Figure 7 gives an example of a suitable layout. All analog circuitry (input amplifiers, filters, reference components, etc.) should be placed over the analog ground plane. All digital circuitry and I/O lines should be placed over the digital ground plane. Furthermore, all components in the reference circuitry and the input signal chain that are connected to ground should be connected together with short traces and enter the analog ground plane at a single point. All ground connections should have a low inductance path to ground.
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FIGURE 8. Isolating the ADC Clock from other Circuitry with a Clock Tree 7.0 COMMON APPLICATION PITFALLS Driving the inputs (analog or digital) beyond the power supply rails. For proper operation, all inputs should not go more than 100 mV beyond the supply rails (more than 100 mV below the ground pins or 100 mV above the supply pins). Exceeding these limits on even a transient basis may cause faulty or erratic operation. It is not uncommon for high speed digital components (e.g., 74F and 74AC devices) to
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ADC12L066
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exhibit overshoot or undershoot that goes above the power supply or below ground. A resistor of about 50 to 100 in series with any offending digital input, close to the signal source, will eliminate the problem. Do not allow input voltages to exceed the supply voltage, even on a transient basis. Not even during power up or power down. Be careful not to overdrive the inputs of the ADC12L066 with a device that is powered from supplies outside the range of the ADC12L066 supply. Such practice may lead to conversion inaccuracies and even to device damage. Attempting to drive a high capacitance digital data bus. The more capacitance the output drivers must charge for each conversion, the more instantaneous digital current flows through VDR and DR GND. These large charging current spikes can couple into the analog circuitry, degrading dynamic performance. Adequate bypassing and maintaining separate analog and digital areas on the pc board will reduce this problem. Additionally, bus capacitance beyond the specified 20 pF/pin will cause tOD to increase, making it difficult to properly latch the ADC output data. The result could, again, be an apparent reduction in dynamic performance. The digital data outputs should be buffered (with 74ACQ541, for example). Dynamic performance can also be improved by adding series resistors at each digital output, close to the ADC12L066, which reduces the energy coupled back into the converter output pins by limiting the output current. A reasonable value for these resistors is 47 to 56. Using an inadequate amplifier to drive the analog input. As explained in Section 1.3, the capacitance seen at the
input alternates between 8 pF and 7 pF, depending upon the phase of the clock. This dynamic load is more difficult to drive than is a fixed capacitance. If the amplifier exhibits overshoot, ringing, or any evidence of instability, even at a very low level, it will degrade performance. A small series resistor at each amplifier output and a capacitor across the analog inputs (as shown in Figures 5, 6) will improve performance. The CLC409 and CLC428 have been successfully used to drive the analog inputs of the ADC12L066. Also, it is important that the signals at the two inputs have exactly the same amplitude and be exactly 180o out of phase with each other. Board layout, especially equality of the length of the two traces to the input pins, will affect the effective phase between these two signals. Remember that an operational amplifier operated in the non-inverting configuration will exhibit more time delay than will the same device operating in the inverting configuration. Operating with the reference pins outside of the specified range. As mentioned in Section 1.2, VREF should be in the range of 0.8V VREF 1.2V Operating outside of these limits could lead to performance degradation. Using a clock source with excessive jitter, using excessively long clock signal trace, or having other signals coupled to the clock signal trace. This will cause the sampling interval to vary, causing excessive output noise and a reduction in SNR and SINAD performance.
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ADC12L066 12-Bit, 66 MSPS, 395 mW A/D Converter with Internal Sample-and-Hold
Physical Dimensions
unless otherwise noted
inches (millimeters)
32-Lead LQFP Package Ordering Number ADC12L066CIVY NS Package Number VBE32A
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