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

[ulfur hansson]

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> 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> wrote:
>> Small Signal Audio Design (I have Second Edition so it may have moved chapter)
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>> 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!
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>> Which Douglas Self book did you mean? He’s written a few.
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>> On 5 May 2021, at 11:11, Mike Bryant <mbryant at futurehorizons.com> wrote:
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>> 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.
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>> 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,
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>>  
>> 
>> 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.
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>> The example circuit for combining the differential outputs is shown attached - a simple diff amp with 10K resistors everywhere.
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>> Now, my question is this:
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>> 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?
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>> 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.
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>> Thanks,
>> 
>> Tom
>> 
>>  
>> 
>> ==================
>>        Electric Druid
>> Synth & Stompbox DIY
>> ==================
>> 
>> <image002.png>
>> 
>>  
>> 
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