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

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If you add an external amplifier to the input of a Delta-Sigma ADC, how does this affect system noise performance?

Regards,
Collin Wells
Precision ADC Applications

  • 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, GAMP. 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, GAMP? Read part 6 to find out.

    Regards,
    Collin Wells
    Precision ADC Applications

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