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EDNE JAN 2017

Analog Tips The noise from the feedback resistors would be; eR2 = eR4 = 4.07 nV/√Hz The noise from the R1 would be; eR1 × (1 – β1) × NG = 4.07 nV/√Hz and from R3 would be; eR3 × (1 - β2) × NG = 4.07 nV/√Hz. The ADA4940-1 current noise is 0.81 pA/√Hz from the data sheet. Inverting input voltage noise: iIN– × R1|| R2 × NG = 0.81 nV/√Hz. Noninverting input voltage noise: iIN+ × R3|| R4 × NG = 0.81 nV/√Hz. So, the equivalent output noise contribution from the ADA4940 would be: = √(7.9e-9)² + (0)² + 4 × (4.07e-9)² + 2 × (0.81e- 9)² = 11.33 nV/√Hz. The total integrated noise at the ADC input (after RC filter) would be; 11.33 nV/√Hz × √(2.7e6 × π/2) = 23.26 μV rms. The rms noise of AD7982 can be calculated from its typical signal-to-noise ratio (SNR) of 98 dB for a 5V reference. Using these numbers, the total noise contribution from the ADC driver and ADC would be Vnoise-rms = √(23.26e-6)² + (44.50e-6)² = 50.22 μV rms Note that the noise contribution from the ADR435 reference is ignored in this case as it’s negligible. So, the theoretical SNR of the data acquisition system can be estimated as shown below. The AD7982 achieves typically 96.67 dB of SNR and –111.03 dB of THD for a 1 kHz input signal as shown in its FFT performance of Figure 3. The measured SNR of 96.67 dB in this case is pretty close to the theoretical estimated SNR of 96.95 dB above. The actual loss from the target SNR of data sheet specified 98 dB is attributed to the equivalent output noise contribution from the ADA4940-1 differential amplifier circuit. The noise is one of the important specifications besides scrutinizing the bandwidth, settling time, input and output headroom/footroom, and power requirements when selecting an ADC driver for driving the SAR ADC for a given application. For more information on recommended amplifiers for driving the high resolution precision PulSAR ADCs, refer to the Analog Devices ADC driver Selection Guide. For designing differential amplifier circuits, download free and easy to use intuitive Analog Devices DiffAmpCalc tool. Figure 3 FFT plot, fIN = 1 kHz, FS = 1 MSPS (ADA4940-1 configured as fully differential driver). 29 EDN Europe January 2017 www.edn-europe.com


EDNE JAN 2017
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