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

[Neil Johnson]

It's still chapter 26 in the third edition.  Also read through the first
half of chapter 18 on line inputs.

Neil


On Wed, 5 May 2021 at 11:34, Mike Bryant <mbryant at futurehorizons.com> wrote:

> Small Signal Audio Design (I have Second Edition so it may have moved
> chapter)
>
>
>
> *From:* Tom Wiltshire [mailto:tom at electricdruid.net]
> *Sent:* 05 May 2021 11:17
> *To:* Mike Bryant
> *Cc:* synth-diy francesco mulassano via
> *Subject:* Re: [sdiy] Differential amps and noise cancellation in dsPIC
> 33FJ128GP802 uP's DAC
>
>
>
> The circuit is straight out of Microchip’s datasheet and is what they
> recommend. But that’s not to say that it can’t be improved!
>
>
>
> Which Douglas Self book did you mean? He’s written a few.
>
>
>
>
>
> On 5 May 2021, at 11:11, Mike Bryant <mbryant at futurehorizons.com> wrote:
>
>
>
> You probably want to use something a little more complicated for the best
> performance.  Douglas Self covers this well in Chapter 26 of his book.
>
>
>
>
>
>
>
> *From:* Synth-diy [mailto:synth-diy-bounces at synth-diy.org
> <synth-diy-bounces at synth-diy.org>] *On Behalf Of *Tom Wiltshire
> *Sent:* 05 May 2021 10:17
> *To:* synth-diy francesco mulassano via
> *Subject:* [sdiy] Differential amps and noise cancellation in dsPIC
> 33FJ128GP802 uP's DAC
>
>
>
> 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
>
>
>
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