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Bret Cahill
Guest
Fri Aug 06, 2010 2:32 am
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
Tim Wescott
Guest
Fri Aug 06, 2010 5:59 am
On 08/05/2010 04:39 PM, Bret Cahill wrote:
Quote:
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
John Larkin
Guest
Sat Aug 07, 2010 5:12 pm
On Thu, 5 Aug 2010 16:39:25 -0700 (PDT), Bret Cahill
<BretCahill_at_peoplepc.com> wrote:
Quote:
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
Bret Cahill
Guest
Sat Aug 07, 2010 6:38 pm
Quote:
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.
Quote:
Bret Cahill
Guest
Sat Aug 07, 2010 7:59 pm
Quote:
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
John Larkin
Guest
Sat Aug 07, 2010 8:24 pm
On Sat, 7 Aug 2010 09:59:59 -0700 (PDT), Bret Cahill
<BretCahill_at_peoplepc.com> wrote:
Quote:
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
Quote:
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
Grant
Guest
Sat Aug 07, 2010 9:41 pm
On Sat, 07 Aug 2010 12:24:14 -0700, John Larkin <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
Quote:
On Sat, 7 Aug 2010 09:59:59 -0700 (PDT), Bret Cahill
BretCahill_at_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.
Quote:
John
George Herold
Guest
Sat Aug 07, 2010 11:14 pm
On Aug 7, 12:12 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
On Thu, 5 Aug 2010 16:39:25 -0700 (PDT), Bret Cahill
BretCah...@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.
Bret Cahill
Guest
Sun Aug 08, 2010 12:21 am
Quote:
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.
Quote:
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.
Quote:
The fastest way to analyze most circuits is to not use Spice at all.
It's valuable as a double check.
Bret Cahill
John Larkin
Guest
Sun Aug 08, 2010 5:43 am
On Sat, 7 Aug 2010 14:21:17 -0700 (PDT), Bret Cahill
<BretCahill_at_peoplepc.com> wrote:
Quote:
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
Jeff Johnson
Guest
Sun Aug 08, 2010 7:42 pm
"George Herold" <ggherold_at_gmail.com> wrote in message
news:52298b1c-8753-4a63-b795-e01e1a109268_at_d8g2000yqf.googlegroups.com...
Quote:
On Aug 7, 12:12 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 5 Aug 2010 16:39:25 -0700 (PDT), Bret Cahill
BretCah...@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.
Bret Cahill
Guest
Sun Aug 08, 2010 9:06 pm
Quote:
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
John Larkin
Guest
Sun Aug 08, 2010 9:41 pm
On Sat, 7 Aug 2010 13:14:41 -0700 (PDT), George Herold
<ggherold_at_gmail.com> wrote:
Quote:
On Aug 7, 12:12 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 5 Aug 2010 16:39:25 -0700 (PDT), Bret Cahill
BretCah...@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.
Quote:
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
George Herold
Guest
Mon Aug 09, 2010 10:57 pm
On Aug 8, 2:42 pm, "Jeff Johnson" <Jeff_John...@Hotmail.com> wrote:
Quote:
"George Herold" <ggher...@gmail.com> wrote in message
news:52298b1c-8753-4a63-b795-e01e1a109268_at_d8g2000yqf.googlegroups.com...
On Aug 7, 12:12 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 5 Aug 2010 16:39:25 -0700 (PDT), Bret Cahill
BretCah...@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.
George Herold
Guest
Mon Aug 09, 2010 11:18 pm
On Aug 8, 4:41 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Quote:
On Sat, 7 Aug 2010 13:14:41 -0700 (PDT), George Herold
ggher...@gmail.com> wrote:
On Aug 7, 12:12 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 5 Aug 2010 16:39:25 -0700 (PDT), Bret Cahill
BretCah...@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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