# Amplified Difference op-amp configuration questions

Discussion in 'General Electronics Discussion' started by harris_p, Feb 18, 2013.

1. ### harris_p

2
0
Feb 18, 2013
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar6.html#c2

For the Amplified Difference Op-amp configuration on this page, there are two resistor values (R1 and R3) that must have exactly the same value.

Aside from using 4 extremely precise resistors or using two standard resistors and two pots, how can I ensure these values are the same?

Last edited: Feb 18, 2013
2. ### duke37

5,364
772
Jan 9, 2011
How accurate do you need to be?
Resistors are fairly cheap so you could buy a handful of 1% resistors and sort them.
If they all come from the same batch they will be similar and have a similar temperature coefficient.
You will need some high class measurement to do this.

3. ### harris_p

2
0
Feb 18, 2013
Well, let me expand on this...

I'm trying to control some galvos. These units require a balanced bipolar differential signal (max of +10V differential, min of -10v differential, center = 0v differential). The output on the ILDA_Y+ line will swing from -5V to +5V and the ILDA_Y- line will be the exact inverse. The Y line on the left side that runs to a 16 bit dac that has a min output of 0v and a max output of 4.096v.

This circuit has a few problems:
1. The op-amp voltage offset from zero issue. Due to this propperty of op-amps, I get a little bit of a "dead zone" in the central dac value range.
I'm using op-amps with a voltage offset of 500uV.. but ideally would like this to be zero. I'm not sure what else I can do here aside buying more expensive op amps?

2. The dual pot solution works, but is tedious to tune. I was hoping there was a way to do this with only one pot.

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Last edited: Feb 18, 2013