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I have found this circuit of a millivoltmeter, and it seems like something which as a hobbyist I can do it, and the range of the measurement is very suiting one of my projects, but I would like to attach it to an Arduino interfaced precise 24 bit ADC, so my question is:

  • How to replace the ammeter with a voltmeter in order to interface an ADC?
  • Is there some better replacement of the IC LF351 in the same circuit?
  • What diodes I shall use as the best option?

enter image description here

JRE
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Nik Lozan
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    Um, that circuit is so old that the diodes used aren't even produced anymore; and the opamp used is also positively ancient. Modern self-compensating opamps can do this with far less error and less complex circuitry. This circuit is a non-start. It wasn't even a good way to start a measurement device when it was designed: diodes are among the least consistent and most temperature-dependent devices that exist, and you'd very much avoid using them without a closed loop in a measurement circuit. This, again, shows the age of the circuit: today, opamps are cheaper, so precision rectifiers exist. – Marcus Müller Jan 05 '20 at 13:25
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    So, maybe you want to **edit** your question and explain what you want to measure for what purpose – I think you'll get some helpful recommendations out of that; aside from "don't do what's depicted there". – Marcus Müller Jan 05 '20 at 13:28
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    The circuit is also not suitable because neither end of the meter movement is connected to ground. This would be a requirement for feeding into a microcontroller's analog input. – Transistor Jan 05 '20 at 13:35
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    Also, assuming your last "question" (you forgot to ask anything in there!) is related to this: Why did you suddenly go from measuring nanovolts (which is **hard**) to millivolts (which is quite literally a million times easier)? I think it's impossible to help you until you tell us what you want to do, in the bigger picture – this is the classical [XY Problem](http://xyproblem.info), so I'm voting to close your question as unclear until you define what you want to achieve, overall – Marcus Müller Jan 05 '20 at 13:36
  • Hey Marcus ! Thank You for the great points ! I am a bit disappointing to be honest that these things went not as I believed it will, but that's the life I presume. Well.. generally I have a few things in mind, but mostly experiments as a hobbyist and learner. I would really love to do something really working from the part of measuring at last in the millivolt range AC voltage with Arduino alikes. – Nik Lozan Jan 08 '20 at 22:26
  • so, you can measure millivolts directly with a modern microcontroller. You need to think about what you actually want to do and research what the tools available to you can already achieve – that's engineering. Again, editing the question and describing *explicitly* what you'd want to measure with this would make it possible to give an answer. Not doing that makes it impossible. – Marcus Müller Jan 08 '20 at 22:36

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