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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.
To get to a higher frequency, is it possible to just use a smaller cap
and/or resistor on op amp derivative taking circuits?
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 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
The derivative circuit needs to be linear to < +/- 1% over a range ofTo 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?
What range?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.
Spice gets slow if there is a very wide range of time constants in aOne 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.
But that requires an imagination, no? Does growing up with GameboysOn Sat, 7 Aug 2010 09:59:59 -0700 (PDT), Bret Cahill
BretCahill@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
What problems do you see with an integrator? These always seem toOn 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
A couple of decades.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?
Thanks.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.
It's valuable as a double check.The fastest way to analyze most circuits is to not use Spice at all.
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.
There are issues with dc offsets. If your signal has a dc offset then thatOn 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.
Any two that are next to each other.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?
They integrate their own voltage offset and bias current, of course.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.
We're just finishing up a product that jams 32 brutaly-pipelinedI 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.
Yeah, I forgot about that. Lately I've only been using integrators"George Herold" <ggher...@gmail.com> wrote in message
news:52298b1c-8753-4a63-b795-e01e1a109268@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.
because the DC gain is 1 and doesn't depend on resistor accuracy."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,
Susumu. They are fabulous, come from Digikey, and cost 1/10 of theOn Aug 8, 4:41 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
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.
The actual available values are bizarre. Maybe they made what specificSay 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.
Ahh Susumu, Thanks for the correction. I wonder if the 0.1% areOn Mon, 9 Aug 2010 13:18:18 -0700 (PDT), George Herold
ggher...@gmail.com> wrote:
On Aug 8, 4:41 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
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.
Susumu. They are fabulous, come from Digikey, and cost 1/10 of the
Vishay stuff.
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.
The actual available values are bizarre. Maybe they made what specific
customers wanted, then put them on the market. Or used a random number
generator.
We tested some of the 0.05% parts, for TC. We got numbers like 5 and 8
PPM/K.
John
I think the only fair thing to do is solder them to real surface-mountOn Aug 9, 11:00 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 9 Aug 2010 13:18:18 -0700 (PDT), George Herold
ggher...@gmail.com> wrote:
On Aug 8, 4:41 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
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.
Susumu. They are fabulous, come from Digikey, and cost 1/10 of the
Vishay stuff.
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.
The actual available values are bizarre. Maybe they made what specific
customers wanted, then put them on the market. Or used a random number
generator.
We tested some of the 0.05% parts, for TC. We got numbers like 5 and 8
PPM/K.
John
Ahh Susumu, Thanks for the correction. I wonder if the 0.1% are
rejects from 0.05% batches. (The 0.1% cost something like 1/5 as
much.) Maybe I'll try and measure some.... Say If I put them in a
bridge I can measure differences with a lot more resolution. Is there
any easy way to swap chip resistors into some test jig? I'll need to
keep the variations in the test jig resistance down below 0.1 ohm or
so.. (for 10k ohm samples).
Hmm If I want a quick measure of say 10 or 20 of them that sounds likeOn Tue, 10 Aug 2010 07:13:20 -0700 (PDT), George Herold
ggher...@gmail.com> wrote:
On Aug 9, 11:00 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 9 Aug 2010 13:18:18 -0700 (PDT), George Herold
ggher...@gmail.com> wrote:
On Aug 8, 4:41 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
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.
Susumu. They are fabulous, come from Digikey, and cost 1/10 of the
Vishay stuff.
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.
The actual available values are bizarre. Maybe they made what specific
customers wanted, then put them on the market. Or used a random number
generator.
We tested some of the 0.05% parts, for TC. We got numbers like 5 and 8
PPM/K.
John
Ahh Susumu, Thanks for the correction. I wonder if the 0.1% are
rejects from 0.05% batches. (The 0.1% cost something like 1/5 as
much.) Maybe I'll try and measure some.... Say If I put them in a
bridge I can measure differences with a lot more resolution. Is there
any easy way to swap chip resistors into some test jig? I'll need to
keep the variations in the test jig resistance down below 0.1 ohm or
so.. (for 10k ohm samples).
I think the only fair thing to do is solder them to real surface-mount
pads on a board, especially to measure TC. You never know how stresses
might effect things when you get to single digits of PPMs.
John
On Aug 10, 8:57 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 10 Aug 2010 07:13:20 -0700 (PDT), George Herold
ggher...@gmail.com> wrote:
On Aug 9, 11:00 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 9 Aug 2010 13:18:18 -0700 (PDT), George Herold
ggher...@gmail.com> wrote:
On Aug 8, 4:41 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
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.
Susumu. They are fabulous, come from Digikey, and cost 1/10 of the
Vishay stuff.
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.
The actual available values are bizarre. Maybe they made what specific
customers wanted, then put them on the market. Or used a random number
generator.
We tested some of the 0.05% parts, for TC. We got numbers like 5 and 8
PPM/K.
John
Ahh Susumu, Thanks for the correction. I wonder if the 0.1% are
rejects from 0.05% batches. (The 0.1% cost something like 1/5 as
much.) Maybe I'll try and measure some.... Say If I put them in a
bridge I can measure differences with a lot more resolution. Is there
any easy way to swap chip resistors into some test jig? I'll need to
keep the variations in the test jig resistance down below 0.1 ohm or
so.. (for 10k ohm samples).
I think the only fair thing to do is solder them to real surface-mount
pads on a board, especially to measure TC. You never know how stresses
might effect things when you get to single digits of PPMs.
John
Hmm If I want a quick measure of say 10 or 20 of them that sounds like
a lot of work. Unsoldering surface mount is always a bit of a PITA.
I use solder wick and then push with the iron...
Maybe I could stand the R's on end, solder one end to a PCB and touch
solder a bit of wire to the other end.
The Sumusu data sheet does list TC's at the few ppm level. Resistors
are pretty amazing. What's the TC of a piece of copper?.. one part in
10^4 or something like that.