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From: Bret Cahill on 5 Aug 2010 19:39
To get to a higher frequency, is it possible to just use a smaller cap
and/or resistor on op amp derivative taking circuits?


Bret Cahill


From: Tim Wescott on 6 Aug 2010 00:59
On 08/05/2010 04:39 PM, Bret Cahill wrote:
> To get to a higher frequency, is it possible to just use a smaller cap
> and/or resistor on op amp derivative taking circuits?
>
Only to a point -- then your performance is limited by the op-amp.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
From: Bret Cahill on 7 Aug 2010 11:38
> > To get to a higher frequency, is it possible to just use a smaller cap
> > and/or resistor on op amp derivative taking circuits?
>
> Only to a point -- then your performance is limited by the op-amp.


Thanks.


> Tim Wescott
> Wescott Design Serviceshttp://www.wescottdesign.com
>
> Do you need to implement control loops in software?
> "Applied Control Theory for Embedded Systems" was written for you.
> See details athttp://www.wescottdesign.com/actfes/actfes.html

From: John Larkin on 7 Aug 2010 12:12
On Thu, 5 Aug 2010 16:39:25 -0700 (PDT), Bret Cahill
<BretCahill(a)peoplepc.com> wrote:

>To get to a higher frequency, is it possible to just use a smaller cap
>and/or resistor on op amp derivative taking circuits?


What do you mean by "get to a higher frequency"? Do you mean "continue
to be accurate at a higher signal frequency"?

The size of the cap scales the constant K in

OUT = K * (dIN/dt)

but has nothing to do with how high a frequency the circuit will work
at. The opamp determines that.

The "pure" opamp differentiator, just a cap, a resistor, and an opamp,
seldom works. It tends to be unstable and oscillate.

Interestingly, its dual, the opamp integrator, has problems of its
own.

Do you have any specific performance goals in mind?

John

From: Bret Cahill on 7 Aug 2010 12:59
> >To get to a higher frequency, is it possible to just use a smaller cap
> >and/or resistor on op amp derivative taking circuits?
>
> What do you mean by "get to a higher frequency"? Do you mean "continue
> to be accurate at a higher signal frequency"?
>
> The size of the cap scales the constant K in
>
> OUT = K * (dIN/dt)
>
> but has nothing to do with how high a frequency the circuit will work
> at. The opamp determines that.
>
> The "pure" opamp differentiator, just a cap, a resistor, and an opamp,
> seldom works. It tends to be unstable and oscillate.
>
> Interestingly, its dual, the opamp integrator, has problems of its
> own.
>
> Do you have any specific performance goals in mind?

The derivative circuit needs to be linear to < +/- 1% over a range of
frequencies.

One solution is to move everything to lower frequencies which takes a
lot more time limiting use of the computer for hours/run. There's no
reason why SPICE calculations should take more time at low than high
frequencies. The time/step setting doesn't seem to help.

Is there any on line calculator that uses a faster computer?


Bret Cahill



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