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| limitation of opamp in the single supply differential mode |
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| Zero999:
--- Quote from: Ian.M on April 12, 2019, 08:23:03 am ---Read the other thread (and :-- to O.P. for not mentioning this is a followup >:( ). You've been there before! If the O.P. has to use a ground-referenced low voltage OPAMP doing it with anything other than a gated precision differential amplifier configuration will be like listening for a mouse fart in a hurricane. He's interested in mV or uV level signals immediately after a 1.9V pk-pk pulse. That's going to make life interesting (in the Chinese curse sense). --- End quote --- Which thread? I've just looked through the original poster's recent threads and can't find anything similar on the first page of hits. I remember another recent thread about differential amplifiers but I think they might've been started by someone else. |
| Ian.M:
StillTrying linked to the middle of it above. Go back to page 1 and re-read. Unless the O.P's requirements have changed in the past 11 months, the small signal, and artificial constraints (+12V referenced sensing coil and 3.3V ground referenced single OPAMP supply) will make it very difficult to get usable results. @VanitarNordic, If you fail to provide pertinent information like other solutions that have been tried and failed and discussed here, and that you are interested in very low level signals preceded by a high amplitude pulse, you can expect a certain amount of opprobrium. I haven't questioned your intelligence, sanity or professional integrity, so if you feel insulted it must be because I linked a Wikipedia image to clarify the constraints on impedance ratios for a true differential OPAMP configuration. As you have decided to put personality before engineering, I'm just going to sit back and watch :popcorn: Good luck - you are going to need it! |
| VanitarNordic:
--- Quote from: Ian.M on April 12, 2019, 08:58:00 am ---StillTrying linked to the middle of it above. Go back to page 1 and re-read. Unless the O.P's requirements have changed in the past 11 months, the small signal, and artificial constraints (+12V referenced sensing coil and 3.3V ground referenced single OPAMP supply) will make it very difficult to get usable results. @VanitarNordic, If you fail to provide pertinent information like other solutions that have been tried and failed and discussed here, and that you are interested in very low level signals preceded by a high amplitude pulse, you can expect a certain amount of opprobrium. I haven't questioned your intelligence, sanity or professional integrity, so if you feel insulted it must be because I linked a Wikipedia image to clarify the constraints on impedance ratios for a true differential OPAMP configuration. As you have decided to put personality before engineering, I'm just going to sit back and watch :popcorn: Good luck - you are going to need it! --- End quote --- No, I never ever feel insulted by the technical talking or challenging the information. we learn from discussion. but please check you wasted how many hours to investigate someone else posts and make angry comments like that just because maybe the topic or content is repetitive or information is not enough. Generally, engineers never support each other and they have the habit to ruin each other work, that's why their hard work or high working stress gets paid much less than the people in medicine who make daily mistakes in their routine job and get high salaries just because they support each other, even if their colleagues kill somebody :( |
| Zero999:
--- Quote from: Ian.M on April 12, 2019, 08:58:00 am ---StillTrying linked to the middle of it above. Go back to page 1 and re-read. Unless the O.P's requirements have changed in the past 11 months, the small signal, and artificial constraints (+12V referenced sensing coil and 3.3V ground referenced single OPAMP supply) will make it very difficult to get usable results. --- End quote --- That was nearly a year ago. I didn't think it could be related to this thread. --- Quote from: VanitarNordic on April 12, 2019, 09:42:02 am --- --- Quote from: Ian.M on April 12, 2019, 08:58:00 am ---StillTrying linked to the middle of it above. Go back to page 1 and re-read. Unless the O.P's requirements have changed in the past 11 months, the small signal, and artificial constraints (+12V referenced sensing coil and 3.3V ground referenced single OPAMP supply) will make it very difficult to get usable results. @VanitarNordic, If you fail to provide pertinent information like other solutions that have been tried and failed and discussed here, and that you are interested in very low level signals preceded by a high amplitude pulse, you can expect a certain amount of opprobrium. I haven't questioned your intelligence, sanity or professional integrity, so if you feel insulted it must be because I linked a Wikipedia image to clarify the constraints on impedance ratios for a true differential OPAMP configuration. As you have decided to put personality before engineering, I'm just going to sit back and watch :popcorn: Good luck - you are going to need it! --- End quote --- No, I never ever feel insulted by the technical talking or challenging the information. we learn from discussion. but please check you wasted how many hours to investigate someone else posts and make angry comments like that just because maybe the topic or content is repetitive or information is not enough. Generally, engineers never support each other and they have the habit to ruin each other work, that's why their hard work or high working stress gets paid much less than the people in medicine who make daily mistakes in their routine job and get high salaries just because they support each other, even if their colleagues kill somebody :( --- End quote --- It's not like that where I work. Perhaps you should consider another job? Anyway, back on topic. The op-amp should be run off the 12V rail. An AC coupled differential amplifier could be used. Awhile go Audioguru did a schematic showing the inverting and non-inverting op-amp configurations, both DC and AC coupled. Here's my version, showing the differential amplifier configuration: bottom right. Since the inputs are DC biased at more than half the supply voltage, the polarity of the input capacitors will need to be reversed. |
| gf:
--- Quote from: Zero999 on April 12, 2019, 09:56:59 am ---Anyway, back on topic. The op-amp should be run off the 12V rail. An AC coupled differential amplifier could be used. --- End quote --- There also do exist 3V single-supply capable instrumentation amplifier ICs with CMRR of > 80dB, which is likely better than the CMRR achievable with a differential amplifier based on a single regular opamp. In order to fit into the amp's common mode input voltage range, only AC coupling is an option then, of course. The AC ripple on the 12V rail must not exceed the amp's peak-to-peak common mode input voltage range either. [ The first search result I found with Google is e.g. https://www.analog.com/media/en/technical-documentation/data-sheets/AD8223.pdf, but there are many others. ] |
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