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

[Mike Bryant]

Can’t find that in the second edition, but as he points out at the start of the chapter a single FET isn’t the best of things to attenuate a signal with.


From: ulfur hansson [mailto:ulfurh at gmail.com]
Sent: 05 May 2021 13:24
To: Neil Johnson
Cc: Mike Bryant; synth-diy francesco mulassano via
Subject: Re: [sdiy] Differential amps and noise cancellation in dsPIC 33FJ128GP802 uP's DAC

slightly OT, has anyone successfully built the FET compressor douglas describes?
sent from outer space


On 5 May 2021, at 11:32, Neil Johnson via Synth-diy <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>> wrote:

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<mailto: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<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<mailto: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] 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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