Re: Pspice Filter Analysis

Noway2 wrote: 
This post contains two related questions.

A few months back, I implemented a prototype ADC (analog) interace
circuit and used a 2 pole sallen-key filter using a Burr Brown opa237
opamp.  Based on my ADC sample freq relative to the cutoff freq a 2
pole filter is more than sufficient attenuation.  The input to filter
circuit was driven by an analog mux, which could be inhibited.  I
noticed that with the mux inhibited, the output of the opamp would
slowly rise up to the rail.  I believe this is the result of falling
into the trap of neglecting to create a dc bias path to ground for the
offset currents (this is an example of why I protoyped it).

My first question is, where in the circuit would you put the resistors
to ground in this type of circuit.  Wouldn't the resistors affect the
filter response as they would be in parallel with the input resistance
and also in parallel with one of the capacitors.  Or is it a case of
where these resistors are so much larger than the input resistors (>
10x) that they wouldn't have an effect?

The ideal place to carry the bias current might be through a separate MUX switch that grounds the input, when all other MUX inputs are off, so there is no bump or change in the bias resistance when the first MUX comes on. But, since the bias current is very small, a resistor to ground at the filter input would have the least effect on the filter response, and would only slightly lower the input impedance the MUX must drive.

In an attempt to answer this question myself, I generated a pspice
simulation of the circuit using the opamp spice model that I downloaded
from Ti.  The spice output plot shows something that is extremely
unexepcted.  It shows that the fitler behaves as expected (unity gain
until a 100Hz cutoff freq) which continues downward until about 1.1Khz
where it turns about and starts to climb until about 100Khz where it
stabilizes at about -13db.

All low pass filters based on negative feedback have a sneak path around the opamp through the feedback network. So when the frequency is high enough that the opamp gain is low and the output impedance is high, the response may rise because of the signal through this path. But 1.1kHz is way to low for this to be the explanation. I suspect there is an error in your simulation.

As I have a working prototype of this filter, I hooked the circuit back
up with a function generator and ran various frequencies into the
filter and watched the output.  The real circuit does NOT show this
type of behavior.  The amplitude values are as pspice predicts upto
about 800 Hz where the signal gets lost in the noise on my scope and
stays lost in the noise as far up as my generator can go (20Mhz).

My second question is, has anybody had this experience with pspice that
understands what is going on with it?  I have triple checked the
circuit file and I don't see where things are going wrong.  I also
substituted a ua741 for the opa237 in pspice and the result didn't
change.  The result appears to be dominated by analysis equations that
pspice is working with.  Any suggestions?

I suspect that pspice is less likely to be the cause of the error than you are. Nothing personal, but it has been much more thoroughly tested than you have.
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