If you make the S-K out of just unity-gain opamps, the overall DC gain is 1, as close as the opamps open-loop gain allow. Gain tempco will be close to zero with good opamps; it won't depend on the resistor values.
The version where the opamps have >1 gain is still considered to be a Sallen-Key. It allows the main RCs to be the same, which can simplify life, especially buying capacitors, but then the DC gain depends on resistor tempcos.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
of R1 and R2 sees a short at the other end of R3, making the edge step at the junction input step x (R3||R2)/(R1+(R3||R2))=input step x 0.1 . As the capacitors begin to charge the junction voltage exponentially charges to input x R2/(R1 + R2)= input x 0.2 . This is about what your scope image is showing. Dominant time constant is [sqrt(R1||R2) + R3/sqrt(R1||R2)]^2 x C2, which approximately agrees with image.
take in more charge, the circuit draws less current from the source.
Do you really mean 3 complex-pole pairs ?:-)
Could you have used some zeroes to steepen it up... they come for free with the quad-op-amp >:-} ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
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I love to cook with wine. Sometimes I even put it in the food.
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There's seldom a reason to go beyond duals.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
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The design I'm working on uses sixteen quads per board. Thirty-two duals wouldn't cut it. Getting the design tight is simply a matter of a little work (and two sides). In fact, thirty-two duals would be
*far* worse. You can pack a lot of 0402s in a small space.I don't know what you call volume. I'm talking at least a million units.
Power supply and signal range limitation are obvious. What you need to do is engineer the filter so that there is no gain at any op amp output at any frequency. I don't know how I can make this any clearer. That is how it is done if you do active filter design properly.
In addition, you insure no node is drooping either. That increases noise. Dynamic range adjusting is filters 101. But in analog, often close is good enough as long as nobody is out there doing a better job.
If you want a true 0dB loss, use the Fluke scheme. LTC and Maxim both made chips violating the patent. ;-)
Incidentally by design, this filter is not dynamic range adjusted at every node. The FB pin must peak.
Not only are zeros free, but you might be able to reduce the order of the filter.
So nobody has ever used a S-K in a million-unit application?
Makes no sense to me. All that matters is the final transfer function, and that it doesn't saturate in your range of expected signal levels. If some stage peaks some, and doesn't clip, what's wrong with that?
So it violates your rules.
That's a switched-cap implementation, noisy and nasty.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
I use them too, for the same reason. Of course in an instrument, the designer has more control over operating conditions than in chip design--if I really wanted to put a Q=5 stage in a filter, I could.
I also used the LT1062 way back when, because good op amps were expensive, and the zero offset idea was cool. Besides being horrifically noisy, the main problem was that it wasn't usefuly clock-tunable, because of the input RC. (Active filters are always horribly noisy, but not _that_noisy.)
My fanciest switched-cap filter was a Hilbert transform one made from an LMF100, which did SSB detection in a crossed-beam particle counter, circa 1991. It was magic--50 dB sideband rejection with zero adjustments. Noisy, but the dynamic range had been reduced in analogue beforehand, so it worked great.
Nowadays in instruments you just use filters to get the SNR and adjacent-channel selectivity you want and prevent aliasing, and then do any fancy stuff after digitizing.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Some of the noise peak in the LT1062 might be caused by clock jitter, modulating the attenuation in the transition region. That would be an interesting kind of noise, multiplicative onto the signal, not additive.
Switched-cap filter chips are awful. They kick clock spikes out of every pin, input and output, are noisy, and alias both signal noise and power supply noise back into the passband. Note that the noise curve of the 1062 is *microvolts* per root Hz, not nanovolts.
I did a double-conversion superhet FSK modem for Reuters, all PLLs and switched-cap filters. Sold a few before the internet blew away the old FSK leased-line news distribution thing.
Yup.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
all
with
at
...Jim Thompson | mens | | et | | | | |.- Hide quoted text -
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Wow! Dat's a lot of opamps!
George H.
Yup. That's what I was driving at. ...Jim Thompson
-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
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Two duals aren't much bigger than a quad, and routing can be easier with duals.
This thermocouple simulator board has 80 opamps, 16 channels at 5 opamps per channel. We used a mixture of singles and duals.
Oh, re another thread, here's the corresponding reference junction box:
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
...Jim Thompson | mens | | et | | |
Hi Jim, I'll make a small confession. I don?t have a good feel for how zero?s effect the filter response. (Off the top of my head, I?m not even sure how to add a zero to a filter. Does it have to be an active filter to add a zero... more than just an RL or RC? )
So sure if you write down some differential equation I can point to the zeros. (Well as long as it?s not some ugly monstrosity.) And yeah, the ?gain? goes to zero at the zero frequency. But more than that I don?t know. I understand ?all pole? filters in terms of simpl e harmonic motion, with resonant frequencies and damping factors. Are there the equivalent of damping factors in zeros? Maybe someone can suggest a good book or webpage?
George H.
...Jim Thompson | mens | | et | | | |
or
nimple
Oh, I found this,
In a passive LC lowpass filter, you add another series LC to ground, to short out the signal at some frequency just after the -3 dB point.
This becomes an "elliptical" filter. It greatly improves frequency-domain stopband performance and wrecks time-domain behavior.
There are mechanical equivalents.
Do you have Williams' filter book? It's a basic necessity.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
I have a long history of Laplace (actually Heaviside) notation, so I don't think differential equations (at least for filters).
My favorite book...
So ancient, it has no ISBN.
Library of Congress Number: 66-27547
"Synthesis of Filters" Jose Luis Herrero & Gideon Willoner © 1966 by Prentice-Hall, Inc., Englewood Cliffs, N.J.
(Peruse the old/used bookstores...
Math intensive, transforms required, not for those who can't already do a hand-math filter design with Laplace (and need to fall back on FilterPro :-)
My favorite section is the "General Parameter Filters"... put in your pass-band/stop-band requirements in Z-transform notation (not the same as today's sampled data Z) and get out the result in Laplace.
===
As for damping factors for zeroes... sure... any off-axis pole OR zero has a real part. But the only place I've specifically applied that is in all-pass structures. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
r
Thanks... I was thinking the twin T notch filter must have a zero.
I recall some ED or EDN article on poles and zeros that had some mechanical analogies... but I found the article mostly confusing.
This one?
I'll put it on my Xmas wish list, (along with socks.)
George H.
.highlandtechnology.com jlarkin at highlandtechnology dot com
r
Hmm, well I don't use filter pro, but I don't use Laplace transforms either.
Mostly I just write down the iw(omega) (= j*2*pi*f in EE speak) and go from there. But I rarely have to solve diff EQ's any more.
Ahh sure... (silly of me.)
Thanks Jim.
George H.
...Jim Thompson | mens | | et | | |ide quoted text -
quoted text -
Study this configuration...
Besides the advantages mentioned, it has no internal nodes with gross gains that may cause clipping as in other configurations.
Run the math, s=jw
And contemplate using the 4th OpAmp in the quad to generate a complex zero with just resistors. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Get the latest, third edition.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
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