A DC micro-voltmeter

dcinstrumentation-amplifiermultimetervoltage measurement

Basically I will be calibrating circuits to a high degree of precision. Keeping it minimalistic, I figure all I needed for that are resistor networks, voltmeters and a few high precision voltage references. Like I said it has to be very high precision, so I figure the voltage differences would be in the microvolt range.

I've looked at different options from expensive high precision lab voltmeters to oscilloscope preamps (I suppose it could be adapted as a preamp for a voltmeter?). They are all so expensive.

Since my circuits have to work both with AC and DC signals, we could simplify everything by only calibrating against DC signals. So noise becomes a lot less of a problem, since it's DC.

So now I figure, just get a Low Noise Instrumentation Amplifier (or preamp) that will amplify by a fixed factor of 1000, to bring the microvolt to millivolt range, which my multimeter can then measure.

Is there any problem with this whole approach? If you have really good schematic designs for an Instrumentation LNA, can you guys share it with me?

Best Answer

Here are the pitfalls as I see them: -

  • The connection between two dissimilar metals is called a thermocouple and this can produce a voltage of several micro volts per degree centigrade. This is why thermocouple amplifiers use cold junction compensation and they are rigorous about this. Getting an accuracy below 0.1 degC is almost unfeasible. Note that I said accuracy and not resolution.
  • The best DC spec op-amp I use (and I do use some pretty good ones) has an offset voltage of typically 5uV. This represents an error in your measurement system and there's very little you can do about it except perform very exact calibrations quite often.
  • There is also a temperature related drift with this offset too so a stable environment is important
  • Amplifier bias currents can play havoc with DC voltage measurements if the source (the thing to be measured) has a significant output resistance. For instance a strain-gauge bridge as a resistance of typically 350 ohms in a lot of applications and, if you choose an amplifier (or op-amp) that has a bias or offset current of 10 nA you get an offset error voltage of 3.5 uV. Clearly for measuring sources that have a much higher resistance you have to choose amplifiers with much lower bias currents.

So, my advice is look at what is available and set your requirements to be achievable. I'm not going to give you diagrams because they have IP value and this is a free advice site.

Related Topic