Precision Voltage Adapter For Low Voltage Measurement

A typical 3 ½ or 4 ½ multimeter can measure voltage in the low mV range and current in the low mA range. Voltage measurement in the µV range and current measurement in the nA range are typically only available in the more expensive lab bench multimeters. In this post, I will show you a simple adapter circuit that can be used for precision voltage measurement down to the µV range. Using this circuit along with the current adapter circuit I discussed earlier you will be able to perform most of the low level measurements with a 3 ½ meter.

This circuit is built around TI’s programmable precision instrumentation amplifier LMP8358. The basic circuit is quite simple and is shown below:

Voltage Adapter

Voltage Adapter

The gain is determined by the DIP switches and the Op-Amp LM358 forms a virtual ground circuit so that voltages of both polarities can be handled. Using both supply rails also has some additional benefits. Because either positive or negative signals can be measured, this circuit is capable of measuring AC voltages with a frequency up to 80kHz at the gain of 1000 or 8MHz with a gain of 10.

The following picture shows the actual circuit built using a protoboard. I used three AA batteries to power it so that the output can swing between -2V and 2V, which ensures high resolution for using with 3 ½ meters (2,000 counts).

Voltage Adapter Board

Voltage Adapter Board

The differential input impedance is 50MΩ for LMP8358, which is sufficient for most of the measurement. Higher input impedance is possible with an additional differential input stage. LMP8358 has a typical input offset voltage of 1 micro-volt, which is good enough for our purpose. This offset voltage can always be cancelled out with the “Rel” function available on most meters. Since LMP8358 is a zero-drift instrumentation Op-Amp, the output will remain accurate once the offset voltage has been zeroed out.

If you don’t like setting the gains using DIP switches, you can use an MCU to control the gain settings.

Voltage Adapter

Voltage Adapter

The gain accuracy is guaranteed to be within 0.25% in the worst case scenario and the typical gain accuracy is at around 0.03%. Similar circuit can also be built with other low input offset and low drift Op-Amps. The major benefit of using LMP8358 is that precision gains are guaranteed by design, without the need of using external precision resistors.

LMP8358 is a much more capable instrumentation op-amp then what we are using it for here, and you can refer to the datasheet for more information.

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9 Comments

  1. Rick Rump says:

    Great project. I Googled the LMP8358 and the only place I could find it was Amazon at over $50 with shipping from China. Can you point me to a place to buy it (or do you have any you would sell?). I’m in thE Minneapolis area.

    Thanks,

    Rick

    • kwong says:

      Hi Rick,

      I think I got my LMP8358 from National before they were acquired by TI. I believe you can still get it directly from TI and it shouldn’t cost you more than $10 each. Alternatively, you can search other programmable gain instrumentation opamps and most of them would be able to do the same thing.

      • Patrick says:

        Awsome job. This looks great! I’m working on a low voltage measurement circuit myself and need it to be precise down to 0.5mV, looks like this may do the job when measuring up to 500mV VAC. How well do you think this does against line noise? And how fast do you see it settle down, less than 1 ms?

        BTW, I saw the LMP8358 selling on Digikey.com for less then $10.

        Thanks
        Patrick

        • kwong says:

          Thanks Patrick,

          The common mode rejection ratio (CMRR) for this chip is very high so line noise should not be an issue. The device itself is also low noise, the highest noise level (when using a gain of 10 according to the datasheet) is well under 1 uV. The gain bandwidth product is also quite high across all gain settings so settling time should be well below 1ms.

  2. Patrick says:

    Thanks for that. I do have one final question before building this. I use to use another instrumentation amp into a RMS to DC chip, I did this to virtually disconnect the ground on the device I was measuring so as not to introduce the measured ground into the rest of my MCU circuit. I noticed you had a 1Mohm resistor R7 to the virtual ground connected the VIN-, will this be a problem, or is this removable? Any reason you are pulling down VIN- to the negative rail from the LMP358?

  3. Paul says:

    Hi Kerry. I like your website. I have prototyped this circuit and it was fun. But I think VGND on the schematic should be replaced by GND on pin 8. Same thing for pins 12, 13 and 14. They should be allowed to go to GND through R4, R5 and R6. (VGND concerns only the signal to be amplified and not the digital inputs).

    Instead of using the “Rel” function of the multimeter, I am trying to add an offset adjustment potentiometer. I don’t see how to do this with the LMP8358 in this configuration at this moment. Any ideas ? Thanks anyway !

    • kwong says:

      Hi Paul,

      The VGND is used if you use dual supplies, if you are using a single supply then yes VGND would be the same as ground. Regarding offset adjustment, the easiest way is use an Opamp to form a simple adder circuit. You can then “substract” the offset voltage using a trimpot.

  4. Paul says:

    Thanks for your answer about the offset adjustment.

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