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From: Bret Cahill on 8 Aug 2010 14:06
> >> >> >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?

Any two that are next to each other.

The problem may have been coming from some other part of the circuit.
Everything was below 100 hz.


Bret Cahill


From: Jeff Johnson on 8 Aug 2010 14:42


"George Herold" <ggherold(a)gmail.com> wrote in message
news:52298b1c-8753-4a63-b795-e01e1a109268(a)d8g2000yqf.googlegroups.com...
> 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.

There are issues with dc offsets. If your signal has a dc offset then that
will get integrated over time successfully reducing your headroom.

e.g., In(t) = dc + f(t), Out(t) = dc*t + F(t). It may work fine for some
initial amount of time but eventually won't function at all. This is true
for all integrators and this is where choppers come into play.


From: John Larkin on 8 Aug 2010 16:41
On Sat, 7 Aug 2010 13:14:41 -0700 (PDT), George Herold
<ggherold(a)gmail.com> wrote:

>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.

They integrate their own voltage offset and bias current, of course.
For something like a magnetic field probe coil, that gets to be the
dominant error. Some cute periodic auto-zero becomes necessary.
Chopper amps are great, but noisy.

> 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.

We're just finishing up a product that jams 32 brutaly-pipelined
8-pole lowpass filters into one FPGA, sample rate 500 KHz per channel.
The cutoff range is 50 KHz down to 1 Hz, and original concept, classic
DSP butterfly stages, blew up mathematically. At 1 Hz we had allowable
coefficients errors like one part in 10^40, and 2-pole stage gains
like 10^17. This wasn't good. I suggested simulating a state-variable
lowpass digitally, and that worked, using the 48 bit MACs in the
Xilinx FPGA. The nice thing about state-variable filters is that you
can make the 2-pole stage gains exactly 1, and the coefficients scale
pretty much linearly on frequency.

I like SV analog filters, but sometimes a Sallen-Key is better,
because the DC gain is 1 and doesn't depend on resistor accuracy.

John

From: George Herold on 9 Aug 2010 15:57
On Aug 8, 2:42 pm, "Jeff Johnson" <Jeff_John...(a)Hotmail.com> wrote:
> "George Herold" <ggher...(a)gmail.com> wrote in message
>
> news:52298b1c-8753-4a63-b795-e01e1a109268(a)d8g2000yqf.googlegroups.com...
>
>
>
>
>
> > 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.
>
> There are issues with dc offsets. If your signal has a dc offset then that
> will get integrated over time successfully reducing your headroom.
>
> e.g., In(t) = dc + f(t), Out(t) = dc*t + F(t).  It may work fine for some
> initial amount of time but eventually won't function at all. This is true
> for all integrators and this is where choppers come into play.

Yeah, I forgot about that. Lately I've only been using integrators
that are inside a control loop. So the DC offset is not an issue.

George H.
From: George Herold on 9 Aug 2010 16:18
On Aug 8, 4:41 pm, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Sat, 7 Aug 2010 13:14:41 -0700 (PDT), George Herold
>
>
>
>
>
> <ggher...(a)gmail.com> wrote:
> >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.
>
> They integrate their own voltage offset and bias current, of course.
> For something like a magnetic field probe coil, that gets to be the
> dominant error. Some cute periodic auto-zero becomes necessary.
> Chopper amps are great, but noisy.
>
> > 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.
>
> We're just finishing up a product that jams 32 brutaly-pipelined
> 8-pole lowpass filters into one FPGA, sample rate 500 KHz per channel.
> The cutoff range is 50 KHz down to 1 Hz, and original concept, classic
> DSP butterfly stages, blew up mathematically. At 1 Hz we had allowable
> coefficients errors like one part in 10^40, and 2-pole stage gains
> like 10^17. This wasn't good. I suggested simulating a state-variable
> lowpass digitally, and that worked, using the 48 bit MACs in the
> Xilinx FPGA. The nice thing about state-variable filters is that you
> can make the 2-pole stage gains exactly 1, and the coefficients scale
> pretty much linearly on frequency.
>
" I like SV analog filters, but sometimes a Sallen-Key is better,
because the DC gain is 1 and doesn't depend on resistor accuracy."


I was measuring the DC gain of SV filters we are using a few months
ago. I was amazed at how accurate they were.
I can't recall the exact numbers, (My notebooks at work and I'm on
vacation.) but gain error was much less than the 0.1% resistor
tolerance.
They all used the same 10k 0.1% Sumuso (sp) resistors, I guess the
resistors matched much better than 0.1%. It's hard for me to measure
things to much better than 0.1%. I need another digit on my
voltmeter.

Say has anyone looked at the resistor values from 0.1% Sumuso (sp)
resistors? I wonder if they have the same bimodal
distribution that was claimed for the old 10% tolerance carbon
resistors. (where the 5% resistors were selected from the middle of
the
normal distribution.) For those who don't know the better Sumuso
resistors also come in 0.05% tolerance.

George H.



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