INA128: gain not 1

Hi Robert,

What are the voltages that you are applying on the input pins? 

Also, what supply voltages are you applying? 

Best Regards,

Robert Clifton 

Supply voltages are + and - 12 volts at pins 7 and 4 respectively.  Decoupled with 0.1 uf caps.  The  analog input voltage applied to pin 3 with respect to pin 2 ranges from 0 to 2.8 volts.

Thanks

Hi Robert,

Thanks for replying quickly. I have a few follow up questions: 

- The differential voltage between the inputs ranges from 0-2.8V?

        - What is the common mode voltage? 

- Is this one part that is exhibiting it or multiple? 

        - If multiple is it all of them or a certain percentage? 

Best Regards,

Robert Clifton 

I wanted to post the schematic, but I cannot figure out how to do it.  I reconfigured the circuit and I will try to explain in words what it looks like.  +12 volts at pin 7, -12 volts at pin 4, pin 5 is grounded, +5 volts applied to pin 3, pin 2 grounded.  Nothing at pins 1 and 8.  This supplies a voltage of 5 volts at the + input with respect to the - input.  That would be a common mode voltage of 2.5 volts which is within spec.  There is a feedback resistor added from pin 6 to pin 3 of 105K, something the data sheet recommends.  (Taking it out had no bearing on the result).  For the input described, the output voltage at pin 6 with respect to ground is 6.15 volts, giving a gain of 1.22.  I bought seven of these ICs and all test out to give the same result.  Thanks.

One thing I should mention is that this is a wire wrap prototype (yes, I still wire wrap).  I suppose that if the resistance from pin 5 to ground is not exactly zero then the 40K resistor internally connected to pin 5 would change and possibly upset the gain calculation.  However, I ran the numbers out with an additional 8 ohms from pin 5 to ground and it made negligible difference in the gain of the internal differential amplifier.

Hi Robert,

Do you have a resistor going to ground for the non-inverting input pin (pin 3)? 

I'm thinking this has something to do with your common mode voltage being very low relatively to the reference pin. I'll see if I can test it in lab to try to recreate your results. 

Robert Cara said:

There is a feedback resistor added from pin 6 to pin 3 of 105K, something the data sheet recommends.  (Taking it out had no bearing on the result).

I've looked through the datasheet and didn't see this suggestion. Not that it's important since you have confirmed that it's not the problem.

Best Regards,

Robert Clifton