Click "filter expert" then "dual supplies." Then click on a box on one of the surfaces.
I came up with a new-to-me filter architecture that *almost* works for me. It's a degenerated MFB lowpass, where the opamp itself replaces the usual opamp-r-c integrator.
formatting link
This is freaky because it's weird to see a cap from an opamp inverting output to ground. But the opamp is acting like an integrator, not an opamp. Its input is *not* a summing point.
Unfortunately, this circuit has too few parts, so not enough degrees of freedom to give me the gain I need (gotta have -5) and a decent filter response: I seem to be stuck with a +14 dB peak. Cute, though.
--
John Larkin, President
Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
Have you written the equation for the response ?:-) ...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.
No. I'm stuck with the opamp, TCA0372, and prefer to keep the resistor values, so all that's left is the cap value, so Spicing it was simple. I was already inside simulating a variant on a MFB lowpass when this ocurred to me, so I just tried it. Rob and I had a little fun with it on the whiteboard, in a semi-quantitative way, enough to understand what's happening.
Adding one more resistor, across the cap, gives me another knob to turn, namely control of Q, which gets rid of my 14 dB bump. But then I'm only one part away from a full MFB, so it's not worth it.
For a couple more parts I can do a nice MFB filter that uses available resistor and capacitor packs. There are 24 of these filters in the board, so using packs matters.
Still, the opamp integration constant is only a factor of 2 over what the MFB integrator needs to be, so a FilterPro design that assumes an ideal opamp has to be tweaked. And I want to use available resistor and capacitor packs, more reason to fiddle and see what happens.
This is better to simulate than to do analytically because the final criteria - phase shift and gain in the signal bandwidth, DAC arifact rejection higher up, waveform cosmetics - aren't clearly defined. So we model some choices that use available parts, print graphs, scratch heads, pick one that looks like a pretty good compromise.
I've thought about the virtues and vices of writing equations to describe circuits. In one sense, solving equations is a mechanical process that can be done without actually *understanding* what's going on, just as Spice lets one futz with a model without actually feeling, actually understanding, the dynamics.
John
--
John Larkin, President 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
Bwahahahaha ha! You can always be counted on for a laugh. If you can't do it, it's "mechanical" :-)
As long as you are well away from the GBW axis crossing, the OpAmp can be considered to have a (differential) gain of...
GBW/s
But you'd have to know how to use Laplace (actually Heaviside) notation :-) ...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.
Solving equations *is* mechanical, just like driving Spice. There's software, like the Wolfram stuff, now that will solve the equations for you. The point is, do you really understand what's going on, enough to design an efficient, reliable circuit. Analysis can only happen after you have something to analyze.
Of course. We all got that first year in college.
Have you ever used this filter form, where the open-loop opamp GBW replaced a conventional integrator in an MFB or state-variable filter?
--
John Larkin, President 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
Particularly true of resonant circuits and feedback loops, at least when done the textbook way. Damping factor isn't the most useful parameter for a feedback loop, where we hope that Q
Not yet. A quick eyeball of the analysis would suggest difficulties choosing rational components... like R2 VERY small to get your gain of
...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.
Then it's a new idea to you. Use it if you ever have a need, no charge.
A quick eyeball of the analysis would suggest difficulties
R2? In my lower circuit, with a few hundred pF for the cap, I get gain = -5 and about the same frequency shape, just scaled by the value of the cap, always with a 12 dB or so peak below the 3 dB point. To make this look Butterworth-y, it needs about 2K across the cap, with implications to offset and noise. At that point, it's just one part away from classic MFB.
I just thought it was weird enough to be interesting. In my current application, a classic 2nd order MFB is prudent. I will have to futz the integrator cap value down to make up for the fact that my opamp has fairly low GBW for this situation... FilterPro warns me to use about 10x the GBW that I have. That's a lot easier to simulate than doing analytically, especially since we're arbitrarily eyeballing several performance tradeoffs, like phase shift versus scope fuzzies versus standard valued R and C packs.
--
John Larkin, President
Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
What is the GBW of the OpAmp you are using? With 220pF, and a 10MHz GBW I get 14dB gain in the flat region, but a 28dB peak at 375kHz. ...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.
TCA0372, a dual 1 amp opamp with typ GBW of 1.4 MHz.
--
John Larkin, President
Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
That's a dumb idea, you're going to end up with positive feedback real fast= . The prototype MFB introduces a zero with its feedback capacitor, your cir= cuit has nothing. Then the dominant second order pole occurs at CRf/GBWo, s= o I don't see where that's very useful anyway. And what's this nonsense abo= ut the inverting input not being a summing junction? Are you saying the cur= rents don't sum to zero there? The analysis is trivial if you realize that = the voltage on the summing juncuion is Vout/A where A is amplifier gain ( c= omplex or whatever).
The prototype MFB introduces a zero with its feedback capacitor, your circuit has nothing.
The opamp has a capacitor inside. It's free.
that's very useful anyway. And what's this nonsense about the inverting input not being a summing junction? Are you saying the currents don't sum to zero there? The analysis is trivial if you realize that the voltage on the summing juncuion is Vout/A where A is amplifier gain ( complex or whatever).
I'm just replacing the integrator stage in the MFB with an opamp running open-loop, which is also an integrator. Whether it's dumb or not depends on whether it will work in a given situation.
--
John Larkin, President
Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
The prototype MFB introduces a zero with its feedback capacitor, your circuit has nothing.
that's very useful anyway. And what's this nonsense about the inverting input not being a summing junction? Are you saying the currents don't sum to zero there? The analysis is trivial if you realize that the voltage on the summing juncuion is Vout/A where A is amplifier gain ( complex or whatever).
--
| 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.
The prototype MFB introduces a zero with its feedback capacitor, your circuit has nothing.
that's very useful anyway. And what's this nonsense about the inverting input not being a summing junction? Are you saying the currents don't sum to zero there? The analysis is trivial if you realize that the voltage on the summing juncuion is Vout/A where A is amplifier gain ( complex or whatever).
--
| 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.
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