Low Pass Filter + Buffer

Hello, I'm trying to make a low pass filter (time const = 1 sec.) I don't need a steep skirt etc. The catch is that the output, when measured with a multimeter, needs to be within about .25mV of the input (for a normal range of temperatures around room temp.)

Initially, I just made a lowpass filter using a precision resister and a ceramic capacitor. Then I put a LM741, used it as a buffer and then tried to tune out any voltage offset using the circuit that is recommended on the datasheet (basically just a variable resistor.)

Well to make a long story short, the circuit didn't work out very well. There is a small voltage offset that is hard to get rid of, that is different at different inputs.

Does anyone have any ideas for a good circuit that serves the purpose that I'd like it to. Thanks!


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How close to DC do you need to pass?

You could add another stage of OpAmp gain with an outer loop to get rid of the offset.

Or use a chopper.

...Jim Thompson

|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Jim Thompson

For a passive filter that is buffered, the offset should just be the offset of the op amp. Your only choice is a better op amp, i.e. one with lower offset.

There is the Max 281 if you don't need a low impedance output. You can also steal the idea and roll your own filter. When I designed the Max280/281, I used a root solver and did a root locus to optimize the dynamic range. The root locus is designed to minimize the peaking at the capacitor drive op amp. It is a good idea, though not a requirement. In fact, LTC didn't bother to do this, nor did they even match the true poles for the Butterworth.

Incidentally, this zero offset filter scheme was stolen from Fluke, though I think the patent had expired.

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This will tune out the voltage offest at the input, the remaining error is possibly cuased by input bias current, this can be reduced by matching the input resistance to each input, but the lm741 is old there are much better ICs with lower bias current/offset voltage, better CMRR.

As suggested already, use a chopper stabilised op amp if you want extreme precision eg ad8551.

Colin =^.^=

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You need a better opamp. One with less offset voltage and less bias current that drops voltage across the filter resistor.

Perhaps something like this:

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John Popelish

You don't say what values of R and C you use in the filter, but I wouldn't be surprised if your R is comparable to the 741's input impedance of several M-ohm. That would mean the 741 is NOT acting like a buffer!

You could:

  1. Use a smaller resistor, say 10k. In which case you'd need a 100 uF cap. The 741 would behave better as a buffer, and the offset adjustment should work better.

or 2. Use an op-amp with a larger impedance, a JFET or CMOS type, where the impedance can be typically 1e12 ohms or more.


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