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I am testing a buffer circuit. I came across a problem which i am not able to figure out why it is happening. Please help me out.

Problem:

In a buffer circuit the input voltage is measured across a 120k resistor. The opamp used is ADA4522-2.

Case 1:- When a single DMM is used to measure the input voltage, a particular value specific to the current flowing is obtained. When the same DMM is used to measure the output voltage a difference of 20mV from the input voltage is obtained.

Case 2 :- When i use 2 multimeters simultaneously for the measuring the input voltage and the output voltage with their respective negative probes connected to ground, the difference in the multimeter readings has been drastically reduced to less than 0.1mV. In fact when both of them are placed simultaneously, the output voltage(which had a 20mV difference is getting automatically reduced to the same value as the input)

What may be the reason behind this? Why was the buffer giving a difference in input and output at the first case? How can the difference between the buffer input and output voltage be nullified?

Thanks in advance.

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    \$\begingroup\$ put up a schematic \$\endgroup\$ – Neil_UK Mar 23 '17 at 7:47
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You measure a voltage across a 120k resistor, therefore your source impedance is 120k.

If the multimeter input impedance is 10 MOhms, which is now drawing current from your 120k source impedance, then it will cause about 1.2% error.

When you use two multimeters, the one at the input introduces an error, which is faithfully transmitter through your buffer, and measured by the one at the output.

No problem at all.

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  • \$\begingroup\$ Your first sentence is a bit of a stretch. It could easily be much less than 120K and (not so easily, with active circuitry) much more than 120K, depending on what else is connected to it. \$\endgroup\$ – Spehro Pefhany Mar 23 '17 at 11:38
  • \$\begingroup\$ He says he measures a current going through a 120k resistor, so I take that as high impedance. OP could give more details if he wanted! \$\endgroup\$ – peufeu Mar 23 '17 at 11:46
  • \$\begingroup\$ Hi Peufeu, Thanks for your reply. My circuit is functioning well.. I increased the input impedance of my multimeter and input and output values are matching with less than 0.2mV. \$\endgroup\$ – Ashok Mar 23 '17 at 14:19
  • \$\begingroup\$ OK! So, that was it then? \$\endgroup\$ – peufeu Mar 23 '17 at 14:36
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Your meters own input resistance has become part of the circuit. More of an issue is the circuit resistance is high or the meters input resistsnce is low.

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