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

Guy McCusker guy.mccusker at gmail.com
Wed May 5 11:42:31 CEST 2021


Yes, it will have an impact, though the circuit is already not perfect
because the input impedances to the + and - terminals are different anyway.

There's a bit of discussion on this in the Analog Devices Linear Circuit
Design Handbook, chapter 2.3 where they are advocating/ trying to sell you
an instrumentation amp...
https://www.analog.com/media/en/training-seminars/design-handbooks/Basic-Linear-Design/Chapter2.pdf




On Wed, May 5, 2021 at 10:18 AM Tom Wiltshire <tom at electricdruid.net> wrote:

> 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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