Re: gEDA-user: opamp slew rate limiting

[asomers@xxxxxxxxx]

Two thoughts:

First, if you are saturating your op-amp, then why not using a
comparator instead?  Even the humble LM339 has a large signal response
time of 300ns, which at your voltage is equivalent to a slew rate of
50V/us

Secondly, the opamp's datasheet specified the slew rate using an
inverting amplifier for the test circuit.  The inverting configuration
guarantees that the input common-mode voltage is always very close to
0.  Not sure if this affects the slew rate, but it might.

-Alan

On Mon, Nov 9, 2009 at 6:58 PM,  <carzrgr8@xxxxxxxxxxxxx> wrote:
> I have a problem with an opamp at work and was hoping someone may have some insight.
>
> This particular opamp is opa2132 from TI, and has slew rate of about 20 V/us.  It's driven by a sine wave at 5 kHz.  The gain is sufficiently large that the opamp is driven into saturation by the sine wave and the slew rate limited edges are very obvious.  The rails are +/- 7.5V and it is able to drive to about +/- 6V.  It is connected to an old CD40106 schmitt trigger inverter.
>
> Here's the rub.  The opa2132 doesn't run anywhere near the advertised 20V/uS spec.  I see, at best, around 15V / 12 uS, or about 1 or 2 V/ uS.  Not even close to spec.  I am fairly certain that the schmitt trigger responds poorly to the very slow edges from the opamp.  I happen to use 2 channels of this, and the phase relationship is very important - but it gets destroyed through the schmitt trigger.
>
> I've gone as far as removing all loads from the opa2132, and it doesn't change the slew rate.  The data sheet has a drawing with 'large signal step response', and it shows the part slewing around 15 or 20V in 1 uS.  Not bad.  So what have I done wrong?  They do claim it's with a gain of -1, but I don't see how that has anything to do with it.
>
> Any help?
>
> gene
>
>
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