Hey,
I have an issue with a circuit designed to measure current on shut resistor, in my PCB i have 2 exactly the differential op amp circuit that uses rail to rail op amp with 50x gain . One of the circuit is fine and show the expected voltage 0.5v for 10mv input while the other for the same 10mv show 0.95v , i varified the resistors, ground and every thing seem fine what could be the causes?

 

Probably wired incorrectly.

 

both are on the single PCB?

check if connections are correct and soldering is really solid. reflow/resolder them all.
for example if shunt or feedback resistor have poor connection, it will appear as higher resistance and gain will be larger (and most likely unstable)

 

Is it a homemade board?

A schematic and well focused picture of the board would be helpful.

 

It is entirely possible that with the circuit exactly correct and all the parts values exactly as shown, the amplifier is simply runig out of headroom. What that means is that the value of the input differential is such that the output should be correct, that the actual value of one of the input lines is a bit beyond what the amplifier can work with.so even though the differential input voltage is within the limits, one of the actual input voltages is slightly beyond what the amplifier can work with. THAT can be a problem even with "perfect" circuits. A quick check will be to increase the supply voltage by half a volt. If that fixes the problem, there may be ways of a solution.

 

I found the issue, it was due to PCB layout the connection to the ground plane was on thr botttom to top was only thogth a narrow single trace.
Thank you for you help.

 

OK, the feedback voltage was developed across that impedance. THAT is a trap in PCB layout design work.

 

Hey,
I have an issue with a circuit designed to measure current on shut resistor, in my PCB i have 2 exactly the differential op amp circuit that uses rail to rail op amp with 50x gain . One of the circuit is fine and show the expected voltage 0.5v for 10mv input while the other for the same 10mv show 0.95v , i varified the resistors, ground and every thing seem fine what could be the causes?

The most likely cause is the resistor values. HOW did you verify the resistor values??? If the resistors are 5% tolerance devices then certainly the value can be different than the marked value by up to 5%.

 

I used 1% tolerance, but i also removed them from the circuit and measured it

 

OK, then it is a bit less clear. Usually 5% resistors are not within 1%. There might be a difference in some parameter of the op-amps. The data sheets usually give ranges, not exact numbers.