# Testing instrumenation amplifier

1. Dec 2, 2017

### nothing909

1. The problem statement, all variables and given/known data
I just have some very simple questions to ask.

I'm testing a basic 3 OP instrumentation amplifier

I have to short V2 to ground and test V1 from -2V to 2V, in 0.25 increments.

I then have to short V1 to ground and test V2 from -2 to 2V in 0.25 increments.

the results i have for V1 are for example:

2 vout = ~12.2
1.75 vout = ~11.4
1.5 vout = ~ 10.8
1.25 vout = ~ 9.2
.
.
...

i then have this equation to find the gain:

∂vout/
∂vin1

when i'm doing this equation, i'm not undestanding the ∂. do i add up all the results and then divide it by all the vin1's added up like:

12.2+11.4+10.8.../2+1.75+1.25...

or will i just take one result like 11.4/1.75.

i know this is stupid question, i just wanna know if i'm doing it right. either way i do it, i get like ~6.

when i do both the tests, each gain is ~6. do both the gain of each of the buffer amplifier add up 6+6=12 and 12 is the overall gain for the INA?

2. Dec 2, 2017

### Baluncore

Make an accurate table of your outputs against inputs.
Then for each two rows of the table, compute the step change of input = dVi, and the step change of output = dVo.
The gain over that small range of input will be dVo / dVi.
You may find gain varies over the input voltage range and between inputs.
You may find limits to input voltages that produce sensible outputs.
INA is a difference amplifier so if gain of + side is 6 and - side is 6, differential gain is 6.

Last edited: Dec 2, 2017
3. Dec 2, 2017

### nothing909

I don't understand what u mean when you say "compute the step change of input..."

say my results are this:

how do i get the gain for these 2 result tables? they're very similar, but the negative and positives are opposite for each set of results.
can you explain a bit more simply how to get the gain using ∂vout/∂vin1?

thanks

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