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From: John Larkin on 7 Aug 2010 15:24
On Sat, 7 Aug 2010 09:59:59 -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?
>
>The derivative circuit needs to be linear to < +/- 1% over a range of
>frequencies.

What range?

John



>
>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.

Spice gets slow if there is a very wide range of time constants in a
circuit. It also slows down a lot if you use semiconductor models of
things like opamps. Behavioral models are faster. Ideal models are
fastest.

The fastest way to analyze most circuits is to not use Spice at all.

John

From: George Herold on 7 Aug 2010 16:14
On Aug 7, 12:12 pm, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Thu, 5 Aug 2010 16:39:25 -0700 (PDT), Bret Cahill
>
> <BretCah...(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

What problems do you see with an integrator? These always seem to
work just fine for me.
I find the State Variable filter a bit 'scary'. Whoever first
thought of putting to integrators in a row had a lot of 'guts'. But I
love the outcome.

George H.
From: Grant on 7 Aug 2010 16:41
On Sat, 07 Aug 2010 12:24:14 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Sat, 7 Aug 2010 09:59:59 -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?
>>
>>The derivative circuit needs to be linear to < +/- 1% over a range of
>>frequencies.
>
>What range?
>
>John
>
>
>
>>
>>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.
>
>Spice gets slow if there is a very wide range of time constants in a
>circuit. It also slows down a lot if you use semiconductor models of
>things like opamps. Behavioral models are faster. Ideal models are
>fastest.
>
>The fastest way to analyze most circuits is to not use Spice at all.

But that requires an imagination, no? Does growing up with Gameboys
and hi-tech toys stifle imagination or something? Or modern schooling
says you hafta simulate?

Grant.
>
>John
From: Bret Cahill on 7 Aug 2010 17:21

> >> >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.
>
> What range?

A couple of decades.

> >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.
>
> Spice gets slow if there is a very wide range of time constants in a
> circuit. It also slows down a lot if you use semiconductor models of
> things like opamps. Behavioral models are faster. Ideal models are
> fastest.

Thanks.

> The fastest way to analyze most circuits is to not use Spice at all.

It's valuable as a double check.


Bret Cahill





From: John Larkin on 8 Aug 2010 00:43
On Sat, 7 Aug 2010 14:21:17 -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?
>>
>> >The derivative circuit needs to be linear to < +/- 1% over a range of
>> >frequencies.
>>
>> What range?
>
>A couple of decades.


Ok, lets keep playing this game.

WHICH decades?

John


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