So this is a circuit that buffers a differential input and provides an isolated absolute value output:
Any thoughts or suggestions on how to improve it wrt speed, etc?
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Doesn't look quite right to me--for one thing, U6 isn't doing anything, since its output is trying to fight the output of U3 through a 10k resistor. So the topology is wrong.
Also you're trying to drive two LEDs in series off two (silicon) diode drops, so the LEDs will never turn on.
Even if they did, and the topology were fixed, your output linearity would be hostage to the CTRs of the two optos tracking each other, which they won't. Not only is the optical coupling in the package uncertain, but the CTR depends on the very poorly controlled variation of beta with collector current.
What's wrong with a garden-variety instrumentation amp, followed by a single-ended absolute-value circuit?
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
Belay that last. I've got ring modulators on the mind at the moment.
-- 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
The LT1360 offers a spectacularly high slew rate, and BAS70-04 Schottky dio des are quick and offer very low reverse capacitance, so that's what I used in my most recent absolute value circuit. The LT1360 doesn't offer anythin g marvellous in the way of offset voltage or low noise - as the LT1028 does - but getting an absolute value circuit to work at 17kHz callsed for a res pectable slew rate.
The AD797 beats the pants of the LT1028 when it comes to high speed perform ance - it's not a lot faster, but it is better behaved - if you really do n eed a low-voltage-noise op amp.
-- Bill Sloman, Sydney
-- If you'd upload the .asc file to dropbox then we could run it. John Fields
It seems to work OK in simulation. I didn't post the .asc file because I don't think anyone will have the bridge rectifier module. I can try to post the whole folder if necessary. I think to work OK the circuit would need to use a dual optocoupler with somewhat matched parameters.
I need complete galvanic isolation. So for the above solution I'd need a differential amp, a precision rectifier circuit using some op amps and assorted components, and then some kind of linear isolation amplifier to couple the signal. If I can find a cheap current sense amp to use instead of that weird LT part, I think my solution may be more cost effective.
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Galvanic isolation? Is this for hobby, or do you need to meet interntional AC mains isolation standards? Hospital patient contact leakage isolation? Magnitude? Noise floor? DC to ??Hz How accurate? Input impedance [loading to the monitored signal] Are you after the 'cheapest' way, in what volume of Production?
Many, many ways come to mind, hate to suggest a solution to have it shot down by one of those, "oh, by the way, I want this too" comments.
Isolate first, then perform ABS? ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
The signal is being coupled into a high voltage circuit...I don't need any op amps etc. on the HV side. I think it would be kind of a pain to build a separate isolated LV supply for the op amps post isolation to do the ABS.
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So where are you getting the Vcc shown on your schematic for the HV side? ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| 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.
This is just kind of a proof of concept. In real life the HV side will be different - the Vcc for the opto transistor will probably be derived from the HV via a cascode circuit. It may be possible to construct some kind of simple regulator to run opamps on the HV side, but I really would rather not have them there if I can avoid it.
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What kind of signal levels, frequency and accuracy? ...Jim Thompson
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| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: skypeanalog | |
| 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 use of two 4N28 is inferior to using a matched-pair of optoisolators;
at least has some limits on the current-transfer ratios between outputs. With a quad op amp and some diodes, there's more affordable ways to generate the absolute value.
You can get iso amps that provide isolated power, e.g. the AD215, but they're pretty expensive. They also tend to put the power supply on the output side, which appears not to be what you want.
The Avago ACPL-785 has an abs val output built in.
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
Maybe something like this would be better--
The positive and negative gains could be trimmed with two trimpots, one on each side of the isolation barrier. Better connected as a voltage divider than the rheostat connection shown, and could have a parallel precision resistor to eliminate the pot element tempco.
Best regards, Spehro Pefhany, Trexon Inc.
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Thanks!
Looks nice... then I find the TIL300 opto is listed as obselete. :( All the cool parts go away fast.
George H.
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