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Re: gEDA-user: opamp slew rate limiting



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?
> 


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