What is the Noise requirement on ADC buffer?
The noise generated by an ADC driver amplifier needs to be kept as low as possible in order to avoid worsening the signal to noise ratio (SNR) and the transition noise performance of the 16-bit ADC.
The net SNR degradation (in dB) due to the amplifier will be:
where:NADC is the rms noise of the ADC in microvolts
f–3dB is the –3-dB input bandwidth of the ADC in MHz (or the cutoff frequency of the ADC input filter, if used)
N is the noise gain of the amplifier (1 if in unity-gain buffer configuration).
eN is the equivalent input noise voltage spectral density of the op amp in nV/rt-Hz.
FSR is the full-scale input span of the ADC (e.g., 5 V for a ±2.5-V range).
For example, assume that the 16-bit AD7671 is being driven by an AD8021 op amp. The ADC has 28-µV rms noise, 9.6-MHz bandwidth, and a 0-to-5-V input range. The op amp has an equivalent input noise of 2nV/rt-Hz, and a noise gain of +1, when configured as a unity gain buffer, so the SNR of the ADC will be degraded by only 0.08 dB.
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