Re: gEDA-user: opamp slew rate limiting

[Andy Fierman <andyfierman@xxxxxxxxxxxxxxx>]

Hi Gene,

Some more things to think about as sanity checks.

Following on from Peter's question, what is the gain of your amplifier
(including any effects of the source resistance if it's an inverting
amplifier configuration)?

You say that "The gain is sufficiently large that the opamp is driven
into saturation by the sine wave" but by how much is it driven into
saturation?

Just near the peaks and troughs or very close to the zero crossings?

As has been alluded to already, the slew rate of the opamp is usually
specified for the device operating in the linear region. If it is
being driven hard into saturation then there will be some recovery
time before the output can drag itself. out of saturation as the
signal changes direction.

Don't be misled by the Large signal step response diagram in the TI
datasheet (top right p6 June 2004). The output is only swinging +/-10V
from a +/-15V supply so it is not in saturation.

Another potential gotcha is how much stray capacitance is there
between the output and the inverting input of the device? Have you got
an accidental integrator?

Cheers,

         Andy.

http://signality.co.uk



2009/11/10 Peter TB Brett <peter@xxxxxxxxxxxxx>:
> On Tue, 10 Nov 2009 01:58:26 +0000 (GMT), 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.
>
> What's the op-amp's gain-bandwidth product?
>
> Peter
>
> --
> Peter Brett <peter@xxxxxxxxxxxxx>
> Remote Sensing Research Group
> Surrey Space Centre
>
>
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