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OPA171: vout is not correct for current sensing application

Part Number: OPA171
Other Parts Discussed in Thread: TINA-TI

I am using this device to measure the current of a power supply

The device is configured as an inverting amplifier

Power supply +/-15V

Rfeedback = 100k, Rgain = 1k

also a 1k resistor from non-inverting input to ground for offset compensation

I am measuring current on the low side (sense resistor connected to ground)

Gain is 100

Rsense = 0.003 ohms

Current level about 1 amp

Vout = -100 *0.003*1 = 0.3Volts

Any suggestions will be appreciated

Thank you

Fausto bartra

  • Hi Fausto,

    Can you please give us a little detail on what is wrong with the output voltage? Is it railed to the positive supply? Is there more error than you are expecting? If so, how much? Also, scope shots and/or measurement readings can be very helpful when trouble shooting an issue. So if you can provide any measurements you have taken they may be very helpful.

    Is there anything connected to the output pin that is not shown in the schematic you provided?

    Thank you,

    Tim Claycomb

  • Tim:

    Vout is supouse to be -0.3V, but it is +0.45.

    Are there any limitations for the milivolt inputs?

    Thank you

    Fausto bartra
  • Hi Fausto,

    can you show a scope plot of the input voltage across the 3mR shunt and of the output voltage of OPAmp? Don't directly touch the output of OPAmp with the scope probe, though, but insert a 47...100R resistor.

  • Hi Fausto,

    There is no limit for millivolt inputs unless it violates the input common mode voltage of the device which is not the case for your circuit since you are using dual supplies. The typical offset voltage of the OPA171 is 250uV so for a 3mV input you may have ~8.3% error in your system just due to offset voltage.

    I have attached a TINA-TI file that simulates your circuit. I do not see the error you are seeing in simulation.

    How many devices have you tested? Do all devices have the same error?

    Are you measuring the current through the sense resistor? Just because a piece of equipment says the current output is at a certain value it is important to measure the current to get an accurate reading.

    Lastly, can you provide scope shots and measurement data of your system? If you sweep the input current do you have the same offset at the output?

    OPA171 low side current sensing.TSC

    Thank you,

    Tim Claycomb

  • Kai:

    I found the problem,

    the signal is very noisy so the reading from the voltmeter will always be the RMS vaklue

    thank you


  • Hi Fausto,

    I suspected this. That's why I asked for the scope plots... :-)

    Nice to hear that you could solve the issue!

    Good luck!