Re: Differential mode analog active filter?
- From: miso@xxxxxxxxx
- Date: Sun, 01 Jul 2007 05:56:01 -0000
On Jun 30, 4:14 pm, Tim Wescott <t...@xxxxxxxxxxxxxxxx> wrote:
On Sat, 30 Jun 2007 22:58:27 +0000, Vladimir Vassilevsky wrote:
Hello All,
There is a high precision ADC with the differential input (SNR ~ CMRR ~
100dB). The incoming analog signal is differential also. However there
should be the antialiasing filter in front of the ADC. The filter is
Bessel 4th order. This is the high performance design, and the cost of
the components is not a big issue.
What would be the best way to do this circuit without sacrificing the
SNR and CMRR performance?
If we make the two identical analog filters for /+/ and /-/ paths, then
the CMRR will suffer because of the limited tolerance of the components.
I would expect problems mainly due to the accuracy of the capacitors.
Making a filter with fully differential opamps does not seem to be very
different from the two separate filters. There is the same limitation
because of the tolerances.
It is possible to collapse the input differential signal into the single
ended using an inamp, then filter it and convert it back to the
differential in front of the ADC. The CMRR will be limited to that of
the inamp, and the SNR is in danger because of the conversion from the
differential to ground referenced and back.
I tried to design a schematic similar to the conventional filter but
with the floating ground. A simulation shows problems with the CMRR and
the frequency response; the bootstrapping of the power rails of the
opamps towards the floating ground helps somewhat, however it rather
complex and doesn't look as a very reliable solution.
What topology should be the good approach to the problem? Can you
suggest a book, an appnote or any other information?
VLV
The only concrete suggestions I have are negative -- if it's a
sampling converter, watch out that whatever solution you adopt is low
impedance on the output, and damps transient currents well. The
high-performance, differential input SARs that I have worked around (but
not on) inject a bit of charge back into their input pins when they
sample; this charge is more or less random, and if it's effects persist
until the next sample then it's just another thing that degrades accuracy.
I'd help you more, but on the projects that I've done using high
performance ADCs I've had analog circuit designers who were eager to
interpose themselves between me and the signal acquisition chain, so I
haven't had the opportunity to design one yet.
--
Tim Wescott
Control systems and communications consultinghttp://www.wescottdesign.com
Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes,http://www.wescottdesign.com/actfes/actfes.html
When I was at IDT, we had a flash converter with performance very
dependent on the op amp driving it. My recollection is on the DUT
board we used some damn expensive Comlinear part which cost multiples
of the DUT itself.
I'd design the filter single ended using leapfrog design. Such designs
tend to have the lowest THD due to the op amp being used with one
input at ground.
I've done fully differential leapfrog designs in switchcap
implementation, where I have full control over the op amp. Obvious to
most but just to be sure everyone is on the same page, the fully
differential designs need to set the output common mode of each op
amp.
.
- References:
- Differential mode analog active filter?
- From: Vladimir Vassilevsky
- Re: Differential mode analog active filter?
- From: Tim Wescott
- Differential mode analog active filter?
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