# How to amplify with op-amp?

1. Jul 22, 2011

### corona7w

So I have a load cell that outputs a + and - signal in the range of 0 to 30 mV depending on the force applied. I fed the +signal through a non-inverting amplifier that bumps up its value to about 4 V. However, when I applied force onto the load cell, the voltage range did not get amplified. The signal now varies from 4 V to ~4.03 V. Can anyone tell me what I am doing wrong here?

2. Jul 22, 2011

### uart

I've never used a load cell but I'm guessing that you probably need a differential amplifier rather than a simple non-inverting amplifier.

3. Jul 22, 2011

### artikk

What are the values for the DC power supplies you are using for the op amp? Because it could be the supplies are limiting your output voltage, depending on the output range of the supplies. If the supplies for your op amp are only capable of sourcing 4 V, you can't output higher than 4 V with your op amp. This is a saturation effect where the op amp stops behaving linearly and doesn't continue to amplify the signal beyond the DC supply levels.

4. Jul 22, 2011

### MisterX

What type of load cell is it? For example is it of the strain gauge Wheatstone bridge type, the piezoelectric type, or some other type? You should use a differential amplifier (such as an instrumentation amplifier) for the strain gauge wheatstone bridge type. Let me know if you want me to explain the the problem with using a non-inverting amplifier with a Wheatstone bridge.

I'm less experienced with the other types, but an instrumentation amplifier should be appropriate for them as well.

What are the forces that correspond to the range of 0 to 30 mV output? How much force did you apply? What is the gain of your amplifier?

The manufacturer/provider of the load cell likely provided information on how to convert voltage into force. For the strain gauge type this information may be at a specific voltage or current powering the bridge; if this is the case, make sure that this voltage or current is correctly applied to the load cell.

It makes sense to calculate the expected change in voltage from the applied force using the provided infomation and the amplifier gain. You may also want to make sure the offset (when no force is applied) is within reasonable bounds, based the manufacturer specified offset for the cell and the amplifier gain and offset. It may not be a bad idea to test the amplifier with a known signal.

And, as artikk mentioned, make sure you do not get output voltage close the the maximum output voltage for the amplifier, which typically depends on the power supply voltages.

Last edited: Jul 22, 2011