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I want to use a shunt resistor to measure the current that flows from and in a battery. The current is set by power supply for charging(positive current) and by electronic load for discharging (negative current). I want to amplify the voltage drop over the shunt resistor with instrumental op amp that gives a gain equal to 50.4. The op amp that I'm using is AD620 with Rg resistor equal to 1k with tolerance 0.1%. I want to send the output of op amp in ADC converter to acquire the measurement.

The problem is that the results aren't what I expected, shown in figure, and I don't understand which is the problem. In figure there is a comparison with hall-effect sensor(black line), instead the red line is the current measured by shunt.

schematic

simulate this circuit – Schematic created using CircuitLab

Results obtained using the reported schematic

Results obtained connecting one of shunt terminal to ground

The second figure shows results obtained connecting one of shunt terminal to ground

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  • \$\begingroup\$ You need to tie the bottom end of the shunt to your ground on the AD620, or the inputs are not referenced. \$\endgroup\$ – Trevor_G Apr 4 '17 at 16:32
  • \$\begingroup\$ @Trevor Do I have to it using a resistor ? \$\endgroup\$ – Anto Apr 4 '17 at 16:33
  • \$\begingroup\$ You can but it will be more noise sensitive. \$\endgroup\$ – Trevor_G Apr 4 '17 at 16:34
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    \$\begingroup\$ yes, the battery circuit and the measurement circuit are totally decoupled \$\endgroup\$ – Anto Apr 4 '17 at 16:37
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    \$\begingroup\$ @peufeu You mean to connect ground of measurement circuit with ground of battery circuit? \$\endgroup\$ – Anto Apr 4 '17 at 16:44
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Would be better with Balanced sensing as shown here.

schematic

simulate this circuit – Schematic created using CircuitLab

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