Sallen-key filter and SNR

[John Popelish]

Stephen Boulet wrote:

I'll look into scaling the design.

The reason I started with 10 pF and 100 pF capacitors is that those
parts were available in 1% values. I don't think that 1% 1000pF caps are
avaible (surface mount, 0402 or 0603, but I might be mistaken there).

Keep in mind that stray trace and pad capacitances are probably way
more than 1% of your 10pF capacitors.

Trying to design a low ripple Chebyshev filter (several tenths of a dB
at most) with 5% caps is challenging.

I'll first try a lower noise op amp (maybe on semi's MC33079). Thanks so
much to eveyone for their suggestions.

Stephen

 

[colin]

I've just been mucking around in
http://beis.de/Elektronik/Filter/ActiveHPFilter.html and was wondering
what the midband gain of the SK filler is.

thats a nice site

Colin =^.^=

 

[Ian]

"Stephen Boulet" <stephen_at@nospam.theboulets.net> wrote in message
news:7ICdnWkYrYL-reHfRVn-2w@speakeasy.net...

Well, it's cheap. I'm not sure it's the weakest link though.

I posted a link to the first two stages earlier:

http://www.theboulets.net/Sallen-key.pdf

It's a band-limited audio signal (2-74 kHz) representing several voice
channels destined to be upconverted to an RF band. I *think* that the
noise power in one 18 kHz channel is much less than the signal power at
0.8 Vpp.

Stephen
snip
The reason I started with 10 pF and 100 pF capacitors is that those
parts were available in 1% values. I don't think that 1% 1000pF caps are
avaible (surface mount, 0402 or 0603, but I might be mistaken there).

Trying to design a low ripple Chebyshev filter (several tenths of a dB
at most) with 5% caps is challenging.

I'll first try a lower noise op amp (maybe on semi's MC33079). Thanks so
much to eveyone for their suggestions.

Stephen

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

 

[Stephen Boulet]

Pooh Bear wrote:

My signal is 0.8 Vpp over an 18 kHz bandwidth, so:


Well... that's 283 mV rms.



49nV * sqrt(2) * sqrt(18kHz) would be the input noise voltage in
peak-to-peak terms. The input signal voltage is 0.8Vpp. So:


There seems to be a term of sqrt(2) in your calcualtion that gives you *peak*
voltage but not peak to peak voltage.

I make the rms input noise of the op-amp over that bandwidth 6.57 uV



20*log10(9.3uV/800mV) = -98.7 dB


Which gives me -92.7 dB ! You made the error of comparing peak and peak to peak
voltages. Hence the 6 dB difference. For simplicity I always normalise to rms -
it's less trouble !

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

 

[Stephen Boulet]

Ban wrote:

Stephen Boulet wrote:

Pooh Bear wrote:

Stephen Boulet wrote:


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.

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

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.

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

And with your knowledge I would engage a good analog engineer for this task,
which will make the vastest improvement.

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

 

[Stephen Boulet]

John Larkin wrote:

On Fri, 06 May 2005 22:05:16 -0500, Stephen Boulet
[snip]
A 10 pF 1% cap won't be anything like that once you put it on a board
and connect it to opamps. An opamp input runs several pF, typically.
The PCB pads and traces alone will add a good hunk of a pF, and FR-4
makes just about the worst capacitors on the planet.

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.

There are filter configs that have lower component sensitivities than
Sallen-Key.

John

Are you suggesting multifeedback?

Stephen

 

[gwhite]

Stephen Boulet wrote:

I don't think that 1% 1000pF caps are
avaible (surface mount, 0402 or 0603, but I might be mistaken there).

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

http://www.avx.com/docs/Catalogs/ccog.pdf

Trying to design a low ripple Chebyshev filter (several tenths of a dB
at most) with 5% caps is challenging.

2% is film caps are apparently available upon request.

http://www.avx.com/docs/Catalogs/pps.pdf