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The DAC is the 20 bit precision AD5791 which provides an output span across the +/- Vref inputs (power supply being greater than Vref). However, I want an output almost 3x that span. I can do this by putting an opamp on the DAC o/p, or I can feed the precision +/-5V reference voltages I am using through a couple of opamps.

Naturally, I am trying to keep the noise as low as possible. Which is the best approach?

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Both approaches you mentioned should have similar noise figures; I'd recommend choosing the approach that leads to the smallest circuit added (lower number of "noise sources").

If amplifying the reference voltages would need two op amps (one for negative, another for positive reference) you should choose to amplify the output instead, as it would need half the overall circuitry.

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20 bit resolution on 3V is 3uV for example, so I suggest you increase DAC Vref. (up to +/-16V)

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