[sdiy] Differential 12dB/oct filter

[Richie Burnett]

I'm also working with the dsPIC audio DAC here too, so interested in what 
you're doing.  Are you finding it's output excessively noisy above 20kHz?

You could try filtering each of the DAC+ and DAC- outputs from the dsPIC 
with first-order RC filters first before they are fed into the inputs of you 
"differential-amplifilter".  That would make a 2nd order filter.

Provided you design the first-order RC sections to take into account the 
loading due to the input impedance of your diff amp that follows you should 
get poles where you want them.  You won't be able to get complex poles 
though using this schemes, only real placement.  So, for instance you won't 
be able to make a perfect 2nd order Butterworth response, but you can 
probably get close enough.

The thing to watch out for is common-mode rejection.  If you do something to 
one of the balanced outputs from the DAC, you need to do exactly the same 
thing to the other output, otherwise common-mode noise coming out of the 
dsPIC won't cancel completely in the diff amp.  This has implications for 
resistance tolerance, and particularly capacitor tolerances!

If your ultimate aim is a single-ended audio output, I might be tempted to 
just follow the diff amp with a Sallen-Key or MFB filter though.  You have 
more control over the pole placements, no loading effects, less R's and C's, 
no common-mode degradation due to capacitor tolerances, but the extra cost 
of another op-amp :-(

Hope this helps,

-Richie,

-----Original Message----- 
From: Tom Wiltshire
Sent: Wednesday, April 27, 2016 2:41 PM
To: synthdiy diy
Subject: [sdiy] Differential 12dB/oct filter

Hi all,

I know about the differential op-amp configuration:

https://upload.wikimedia.org/wikipedia/commons/thumb/a/a2/Op-Amp_Differential_Amplifier.svg/300px-Op-Amp_Differential_Amplifier.svg.png

I've also used this with capacitors across Rf and Rg to give a 6dB roll-off 
above some cutoff frequency - the top of audio usually, in my case. This is 
the most basic differential op-amp filter.

Does anyone know a good resource for designing 12dB/oct filters with this 
same differential layout? Or some useful schematics?

I'm playing with the dsPIC's audio DAC again, and it has a differential 
output, so being able to combine the post-DAC filter and the differential 
amp into a single op-amp would save me an op-amp. Otherwise, it's a 
differential amp followed by a conventional 12dB filter.

Thanks,
Tom

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