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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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