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

[Tom Wiltshire]

Thanks Guy.

It seems to me like given the noisy environment the DAC works in (on-chip with a processor) anything that can improve the common-mode noise rejection of the DAC outputs would be an improvement, and might be a significant improvement.

I’ll have a look at that Analog link.

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



> On 5 May 2021, at 10:42, Guy McCusker <guy.mccusker at gmail.com> wrote:
> 
> 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 <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 <mailto: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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> 
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