OPA171: vout is not correct for current sensing application

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.

Kai

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

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!

Kai