Re: Summing integrator circuit?

From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 03/12/05 

Date: Fri, 11 Mar 2005 16:18:17 -0800

"Len Thomas" <lenthomas102@aol.com> wrote in message
 news:b5a431l9qqec0dfchdplhlrrct8r9djqtl@4ax.com...
>I know what a summing amp and integrator are as separate circuits, but
> can someone please explain the design criteria for a "summing
> integrator". IOW one op amp doing both tasks.

The summing could be of input currents into a
virtual ground, with a feedback capacitor balancing
the summed current and converting it into a voltage
output that would be -1/C times the integral of the
input currents. If you want a non-inverting integrator,
it gets a little trickier (or a lot if it has to be accurate).

> The integrators I have used in the past have a feedback resistor in
> the MegOhms and cap in parallel.

With a resistor across the integrating capacitor, it
cannot be considered an integrator except as a
crude approximation. Integrators do not need
that resistor. (I have had somebody tell me they
put one in to limit the gain for stability reasons. It
took me 15 minutes to educate that out of him.)

> If used with 10K series for each of
> the several summed inputs, this would affect the gain and seemingly
> limit options in this regard.

Unless you are running out of loop gain, the
inputs act independently and the integrator
gain is -1/(s*(R*C)) for each input R.

> The frequencies involved are 10-40Hz, subaudio and I am looking for
> unity gain. Input is 6V.

The phrase "unity gain" in reference to an
integrator is confusing to me. What response
do you want?

> Any suggestions please?

Use the above gain formula, picking a common
C value (since there is but one feedback C) and
weight your R values according to the relative
gains need for each input. 

-- 
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

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