# difference amplifier input

• posted

I was reading up on op amp difference amplifiers

formatting link
one thing I haven't seen addressed explicitly is the impedance of your voltage signals. Suppose your input resistors are 100 and feedback resistors 1 Meg, for a gain of 10,000. Now let's say you are using a 100k pot to set one of the voltages going into the difference amplifier. Would this high impedance source affect the difference amplifier, like adding resistance to the input resistor, or does it matter?

Loading thread data ...
• posted

IF Op-amps were ideal, it would not effect it with the acception of the feed back resistor. if you look up the spec's of the op-amp, you'll find the gain of the op-amp and impedance. These figures can be used to get the exact gain over all. The gain is so high on most op-amps that these factors are almost non negotiable.

```--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.```
• posted

"kell"

** The voltage you work with is the one that really exists at a particular point in a circuit.

Where a voltage or signal has a significant series resistance that must always be taken into account.

** A difference amplifier responds to the voltage difference between the two inputs.

In the example you gave, the input imposes a 200 ohm load on that difference. If this seriously alters the signals you want to difference, then that particular circuit is no use.

When high impedance sources are involved, the solution is to add two more op-amps, wired as simple voltage followers, to function as input buffers for the difference amp's inputs.

Use FET op-amps for this and the effective input impedance can be G ohms.

....... Phil

• posted

Mr. Allison's advice is great. The setup he's talking about, with the voltage followers preceeding the difference amplifier, is generically called an instrumentation amplifier. It's good to use an IA when you have to worry about higher or uneven input impedances. Here's a reference:

formatting link

Also, I guess I should mention that your example is a little unrealistic. Gains of greater than 100 have to be done very carefully, and a gain factor of 1000 is pretty much the realistic limit for even the best instrumentation amp setups. You end up amplifying offsets and noise more than your signal. Also, your maximum input frequency goes way down.

Good luck with your studies Chris

• posted

An op-amp with feedback isn't a classic difference ( differential ) amplifier.

Graham

• posted

I have to set up the difference amp so that the output swings to the limit when the input differential reaches just a few millivolts. Can I do this, given the caveat you express about excessive gain? Frequency isn't a problem, it's very slow.

• posted

OK, Kell. You might want to just try it yourself, and see what happens. Nothing will blow up, and the Electronics Gods will still smile on the world.

I basically found out most of what I know about pulling millivolt-level signals out of the grass when I tried to do it for real. It's a good education.

Cheers Chris

• posted

Maybe I was thinking too hard. A comparator ought to do. I only need to keep the behavior of a physical system within certain bounds, not get an accurate measurement of it. Instrumentation amplifiers can wait for another day.

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.