Sallen-key filter and SNR

The worst thing is that you directly connect the 5V supply to the input of the V/I converter. you will need a precision reference here, maybe with

3.3V, then you probably have much less noise to begin with. Also the filter stages should have some gain, otherwise you weaken the signal and not the noise. You could also use a cap across the feedback resistor Rf for a first order stage. A Tschebycheff filter is *not* a good idea for audio either, much better will be a Bessel filter with a much lower corner frequency, if your signals are really 18kHz max. And with your knowledge I would engage a good analog engineer for this task, which will make the vastest improvement.

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ciao Ban
Bordighera, Italy

DO !

Not in SMD I reckon ( at least readily available ). Leaded yes.

I don't think the 33079 is designed for 5V single supply IIRC.

Graham

thats a nice site :)

Colin =^.^=

1% SM caps are readily available in higher values than 1000pF.

As someone else said, don't use the 5V rail to provide the mid-scale current. If the DAC has a differential current output, use a current mirror to double the output swing (also helps with even order distortion).

Also, you need capacitors to ground directly on the DAC output to protect the current to voltage converter from the _extremely_ fast slew rates the DAC output produces.

Regards Ian

Yep, you're right. Thanks for pointing that out. That still puts me way ahead of my goal of 68 dB down though. It seems that the limiting factor in this design is the thermal noise from the resistors.

Stephen

Is it supply noise that is most at issue? I'm not sure that I'd need a precision reference to get the bias right, since every channel, left+, left-, right+, right- gets the same 5V +- 2% supply and so will have equal DC offset.

The final output voltage will be 1.75V DC +- 0.8V.

Also the filter

Sorry I wasn't clear; it's 6 multiplexed 18 kHz channels plus stopband between each, so about 75 kHz total.

That's what happens when an RF engineer works at baseband. Let's say that I have a heightened appreciation that each frequency band has it's own design issues.

Stephen

[snip]

Actually once I got a good model for the parasitics I've been happy with the simulation results. I know what you mean about FR-4.

Are you suggesting multifeedback?

Stephen

AVX theoretically has 1000pF 0603 @ 1%. If they have them, others probably have them.

2% is film caps are apparently available upon request.