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I have a voltmeter and ammeter combo which sense the voltage and the current at the output. My problem is when nothing connected to the output it shows 0.06V else 0.00V. I tried to connect the output the GND but the value is still the same.

The other problem is it shows a little bit higher value than the real, for example 7.07V the original and 7.10V the displayed.

How can I modify this circuit? The OUTPUT(+) is the output where I want to sense the voltage. The ADC connect to an ADC but no marking on the IC.

The op-amp is LMV358 Rail-to-Rail.

No calibration potmeters.

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  • \$\begingroup\$ Open it up, twist the calibration screws until satisfied. The offset could perhaps be removed by a as-high-voltage-as-your-multimeter rated 1 Mohm resistor. \$\endgroup\$
    – winny
    Commented Aug 14, 2017 at 18:55
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    \$\begingroup\$ 0.03V out of 7 is 0.4% and I think that's pretty good already. Any chance you are using a 8 bit ADC? \$\endgroup\$
    – user3528438
    Commented Aug 14, 2017 at 19:04
  • \$\begingroup\$ What's the input impedance of the ADC? Even "rail to rail" op-amps can't source or sink very much current when close to the rails. You could try adding ~10 kohm to ground from the ADC node (but that will add a small error due to the divider with R6). \$\endgroup\$
    – The Photon
    Commented Aug 14, 2017 at 19:14
  • \$\begingroup\$ Across input terminals. \$\endgroup\$
    – winny
    Commented Aug 14, 2017 at 19:57
  • \$\begingroup\$ @Lobi If you delete the circuit diagram then the answer which you accepted doesn't make sense. \$\endgroup\$
    – Andrew Morton
    Commented Aug 15, 2017 at 19:45

1 Answer 1

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I think your values for R4 and R5 are backwards. That's the only way the numbers would make sense.

The only way to get it to read 0.0 Volts is to replace the op-amp with something that has a lot lower input offset. Take a look at the datasheet for LMV358 here: http://www.ti.com/product/LMV358

You can see that it has an input offset spec of 7 milliVolts. Your circuit has a gain of 14.66 (which is 100k / 7.32k + 1). So this input offset of the opamp of 7 mV * 14.66 = 0.103 Volts, so your 0.06 V output error is in line with what you should expect for this opamp.

But before you even go replacing the opamp, the bigger question is: why are you attenuating the input signal by 14.66 and then amplifying it by 14.66? Your overall gain = 1. Why don't you just configure your opamp as a voltage follower?

That alone will reduce your offset error from 100 mV to 7 mV. Reconfigure your circuit first, and then look for another opamp if you still need lower than 7 mV offset error.

(P.S. Why do you even need R3 50 mOhm there?)

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