[sdiy] Differential amps and noise cancellation in dsPIC 33FJ128GP802 uP's DAC

[Tom Wiltshire]

Hi all,

The dsPIC 33FJ128GP802 uP (and others in the family) includes a 16-bit DAC on the chip. The outputs from the DAC are provided as a differential output to help cancel any noise generated by the uP internally. NOt a bad idea, seeing as that is probably inevitable. Despite this precaution, the DAC is reasonably noisy and not suitable for all applications.

The example circuit for combining the differential outputs is shown attached - a simple diff amp with 10K resistors everywhere.

Now, my question is this:

If I change the gain of the differential amp by increasing R6/R8 (to 47K, say) does this affect the noise cancellation properties of the diff amp?

It seems to me that it might, because the positive input now has a different proportion of signal going to it than the negative input.

Thanks,
Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================


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