Re: gEDA-user: opamp slew rate limiting

[Andy Fierman <andyfierman@xxxxxxxxxxxxxxx>]

For the opa2132, it's not clear what the slew rate would do as it
comes out of saturation but it may well only really add some delay
rather than reduce the slew rate.

Using opamps as comparators can be tricky. Alan's suggestion of using
something from the LM339 / LM2907 / LM3302 comparator family might be
a better idea. Then you don't have to worry so much about common mode
or differential voltage ranges as you do with an opamp.

Anyway, hopefully we've set you on the right track.

:)

         Andy.

http://signality.co.uk



2009/11/10 gene glick <carzrgr8@xxxxxxxxxxxxx>:
> The gain is set at -10.  The prior stage has gain, and off-hand I don't
> recall how large the signal is, I'll check.
>
> GBW for the part is 8MHz, I run it at 5kHz*10= 50 kHz GBW - plenty of
> headroom there.
>
> SR definition is SR = 2 * pi * f * Vpk
> So I need SR > 6.28 * 5000 cycles/sec * 15 Volts, or 471,000
> Volts/Second.  If my math is right, that works out to 0.471 V/uS. Lots
> of margin there too, the part can do 20.
>
> I agree about the linear/saturation description, but couldn't find any
> hard literature on the subject.
>
> I removed all external capacitance, so all that remains is stray -
> hopefully small.
>
>> 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?
>>
> great point.  Maybe I should increase the gain to force the saturation
> earlier into the cycle.  I was trying just the opposite.
>
> Ozzy Lash wrote:
>  >    Are you sure your not just tracking the slow edge of the sine wave?
>  >    Your 12 microseconds is about 20 degrees for your 5 kHz sinusoid.  If
>  >    you increase the frequency does the edge steepen?  Does the slew rate
>  >    go up and down as you increase and decrease the amplitude of the
>  >    sinusoid?  If so, I think that is your problem
>  >    Bill
> Yeah, I now think this is the case.
>
>> 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.
> Yeah, I thought the same thing.  Wouldn't that just add delay, but not
> change the slew rate once it comes out of saturation?
>>
>
>
> _______________________________________________
> geda-user mailing list
> geda-user@xxxxxxxxxxxxxx
> http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
>


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user