# Differential amplifier problem

#### kenlide

Joined Feb 23, 2017
24
Hello, I am a student working on a project that requires the use of a differential amplifier. I am trying to measure the active current of a micro controller.

Below is the schematic of a circuit I created to test out the differential amplifier. In the real circuit the 100 Ohm resistor would be exchanged by the micro controller.
I used Multisim to simulate it and it worked as intended. But when I tried it physically I got undesired values at the output. I would get 0.5V at the output even though the differential input was about the same as the simulation.

I have checked the connections multiple times and still don't see what could be causing the problem.

For the op-amps I am using the LM324-N from TI: http://www.ti.com/lit/ds/symlink/lm2902-n.pdf

I am supplying 9V to the LM324-N, I am using op amp 1 2 and 4, leaving 3 unused.

Could it be a problem with the Input Common-Mode Voltage?

Any help is appreciated!

#### GopherT

Joined Nov 23, 2012
8,012
@kenlide

Welcome to the site.

Your problem is that real op amps have a input offset voltage. Simulated op amps don't suffer that joy. Also real resistors have a tolerance of 1 to 10% (depending on how much you paid for them). Better resistors exist but they are not kept with all the stock drawer resistors because they get pricy.

You'll need to use op amps with with bias adjust pins. You'll need to sort through your resistors and find a set that match.

The input offset voltage is amost as big as your signal. (4mV). On top of that, you have three op amps that could all be off my that much.

The error in your resistors also multiplies.

AND - the input voltage is too high if your op amps are also running on 5 volts. There is a statement in the DATASHEET that input voltages must be 1.4 v less than supply voltage.

Feed the op amp with 6.5 volts or more.

Last edited:

#### Bordodynov

Joined May 20, 2015
2,430
Yes. A problem with the Input Common-Mode Voltage. Use Rail-to-Rail Input and Output OPAmp VCC=5V
Level input ~5mV (See Input Offset Voltage!)

#### Bordodynov

Joined May 20, 2015
2,430
Best decision of the problem