Hi all,
I'm trying to get a reading from a Digi-Star load cell.
I'm using the INA122 instrumentation amplifier, the schematic is attached.
Nothing complicated, I connect the cell output directly to the ina122 inputs, and I use a low gain (10K Rg).
Single power supply, 5V, to power the amplifier.

The problem is that at the output I have about the supply voltage.. about 4,8 V.

I used the same schematic as the one in the datasheet..
I don't understand why this behavior.. Any idea?

Thanks

 

My best guess is the load cell itself is incorrectly connected. I don't know which exact cell you have but you are sure the cell output and excitation are correct? What is the cell output to the IA?

Ron

 

The problem is that at the output I have about the supply voltage.. about 4,8 V.

What is the actual output load? If it's the micro as indicated, have you confirmed that the micro's pin is configured for analogue input and is not being driven high?

 

problem solved!
It was the different power supply between the amplifier and the cell. It has to be the same..

 

Something else you may want to consider is the excitation for the load cell needs to be a very stable supply. Any drift or change will impact the output and overall accuracy of the measurement plane. Glad you found the problem. I just assumed they were the same supply.

Ron

 

I use a 7805 voltage regulator, I hope it's enough for stability..

 

hi,
If you use a ratio metric ADC, which is common in most PIC's then a 7805 will be OK.

In your original circuit, there would have been a 6V Common mode voltage at the input of the IA, which was being powered by only 5v.!
E