I have a problem with the circuit below:

enter image description here

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


enter image description here

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

  • \$\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 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 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 at 21:07
  • 2
    \$\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 at 21:49
  • \$\begingroup\$ What will the Isense voltage be max? Can likely run the chip from +/-12v just fine. \$\endgroup\$
    – rdtsc
    Mar 4 at 22:04

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