[FAQ] ADC Noise: If you add an external amplifier to the input of an ADC, how does this affect system noise performance?

You can make your noise analysis easier by separating both the ADC and amplifier from their respective noise sources. In this case, you can model your system as a noiseless amplifier and noiseless ADC preceded by a voltage source equal to the input-referred noise of both, as in Figure 5.

Figure 5: “Noiseless” ADC and amplifier preceded by total noise, referred-to-input

Unfortunately, the measured output noise must refer back to the input, since input-referred noise is the specification used in most ADC data sheets. Assuming that the amplifier and ADC noise are uncorrelated, take the root-sum-square (RSS) of both values to determine the total output-referred noise. You also need to scale the amplifier noise by the amplifier’s gain, G_AMP. Equation 1 shows the resulting output-referred noise:

How do you translate this to input-referred noise? And what are the ramifications of the gain scaling factor, G_AMP? Read part 6 to find out.