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I have a problem with the circuit below:

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I'm using the op-amp as differential amplifier for low-side current sensing with gain of 100. All resistors are 0.1% and the load is just a variable resistor for testing the circuit.

The problem is no matter what load I have and how much current is going through the sense resistor the output of the op-amp is around 23 - 24 volts. I tried switching all of the components checked all the connections and even rebuilt the circuit on a bread board with no avail. I feel like there is something obvious that I'm missing.

I have the same circuit in simulator and it works just fine.

Update: As I mentioned I did miss something obvious and didn't check the datasheet fully.

enter image description here

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You can't drive the input common mode voltage to 0V.

Have you tried to connect the load to GND and the shunt to 24v?

TL084 Datasheet

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  • \$\begingroup\$ Thank you, I know I was missing something obvious; I built the circuit with what I had laying around and didn't check the datasheet fully. \$\endgroup\$
    – Oli
    Mar 4, 2021 at 20:20
  • \$\begingroup\$ I won't be able to connect the load to ground and shunt to the power since the final voltage of the load will be above what the op-amp can handle. \$\endgroup\$
    – Oli
    Mar 4, 2021 at 20:44
  • \$\begingroup\$ @Oli Analog Devices makes Over the Top opamps that can have their inputs go way over the supply voltage. \$\endgroup\$
    – Aaron
    Mar 4, 2021 at 21:07
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    \$\begingroup\$ Swapping the load & shunt will only help with the TL08xH version. The others can't operate with Vcm at Vcc (as shown in the datasheet snippet now posted by the OP). \$\endgroup\$
    – brhans
    Mar 4, 2021 at 21:49
  • \$\begingroup\$ What will the Isense voltage be max? Can likely run the chip from +/-12v just fine. \$\endgroup\$
    – rdtsc
    Mar 4, 2021 at 22:04

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