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AD4000 Datasheet(PDF) 21 Page - Analog Devices |
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AD4000 Datasheet(HTML) 21 Page - Analog Devices |
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21 / 36 page  Data Sheet AD4000/AD4004/AD4008 Rev. C | Page 21 of 36 DRIVER AMPLIFIER CHOICE Although the AD4000/AD4004/AD4008 are easy to drive, the driver amplifier must meet the following requirements: • The noise generated by the driver amplifier must be kept low enough to preserve the SNR and transition noise performance of the AD4000/AD4004/AD4008. The noise from the driver is filtered by the single-pole, low-pass filter of the analog input circuit made by RIN and CIN, or by the external filter, if one is used. Because the typical noise of the AD4000/AD4004/AD4008 is 37 µV rms, the SNR degradation due to the amplifier is ( ) + = − 2 2 ) ( 2 π μV 37 μV 37 log 20 N dB 3 LOSS Ne f SNR where: f−3 dB is the input bandwidth, in megahertz, of the AD4000/ AD4004/AD4008 (10 MHz) or the cutoff frequency of the input filter, if one is used. N is the noise gain of the amplifier (for example, 1 in buffer configuration). eN is the equivalent input noise voltage of the op amp, in nV/√Hz. • For ac applications, the driver must have a THD performance commensurate with the AD4000/ AD4004/AD4008. • For multichannel multiplexed applications, the driver amplifier and the analog input circuit of the AD4000/ AD4004/AD4008 must settle for a full-scale step onto the capacitor array at a 16-bit level (0.0001525%, 15.25 ppm). In the data sheet of the amplifier, settling at 0.1% to 0.01% is more commonly specified. This settling may differ significantly from the settling time at a 16-bit level and must be verified prior to driver selection. High Frequency Input Signals The AD4000/AD4004/AD4008 ac performance over a wide input frequency range using a 5 V reference voltage is shown in Figure 36 and Figure 37. Unlike other traditional SAR ADCs, the AD4000/AD4004/AD4008 maintain exceptional ac performance for input frequencies up to the Nyquist frequency with minimal performance degradation. Note that the input frequency is limited to the Nyquist frequency of the sample rate in use. 94 80 1k 10k 100k 1M INPUT FREQUENCY (Hz) 15.2 13.2 82 84 86 88 90 92 13.4 13.6 13.8 14.0 14.2 14.4 14.6 14.8 15.0 ENOB SINAD SNR Figure 36. SNR, SINAD, and ENOB vs. Input Frequency, VDD = 1.8 V, VIO = 3.3 V, VREF = 5 V, 25°C –80 –120 –110 –115 –105 –95 –85 –100 –90 1k 10k 100k 1M INPUT FREQUENCY (Hz) 80 120 85 90 95 100 105 110 115 THD SFDR Figure 37. THD and SFDR vs. Input Frequency, VDD = 1.8 V, VIO = 3.3 V, VREF = 5 V, 25°C Multiplexed Applications The AD4000/AD4004/AD4008 significantly reduce system complexity and cost for multiplexed applications that require superior performance in terms of noise, power, and throughput. Figure 38 shows a simplified block diagram of a multiplexed data acquisition system including a multiplexer, an ADC driver, and the precision SAR ADC. SAR ADC ADC DRIVER MULTIPLEXER R R R C C C C Figure 38. Multiplexed Data Acquisition Signal Chain Using the AD4000/AD4004/AD4008 |
Similar Part No. - AD4000_17 |
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Similar Description - AD4000_17 |
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