I am using AD8231 instrumentation amplifier in unity gain configuration. I used the inbuilt op-amp to provide reference voltage as the datasheet suggests. A 4.4kHz sine wave generated with my professional signal generator (PC sound card) is fed into the non-inverting input of the in-amp.

enter image description here

Channel one is the input signal and channel two is the output of the in-amp. For some reason the output clamps near the reference voltage (1.6V). enter image description here

The circuit is built on a breadboard and I haven't used decoupling and filtering capacitors since I'm prototyping. Here is a photo of the breadboard in case it helps:

enter image description here


1 Answer 1


Table 3 on page 5 of the linked datasheet gives a minimum input voltage of 0.05 V. It looks like you are driving the input voltage at +INA to below ground, so you are violating this requirement.

Probably there are esd protection diodes on the input pins that are short-circuiting the input to ground when you try to drive it negative.

  • \$\begingroup\$ Damn, it seems to be the case. I thought that setting the reference voltage to mid-supply will offset the input signal and allow the inputs to go negative. \$\endgroup\$
    – Ashton H.
    Commented Apr 22, 2015 at 22:31
  • 3
    \$\begingroup\$ If you offset the -INA input and also center the input signal around that voltage (for example, by ac-coupling), you should be able to get the results you want. \$\endgroup\$
    – The Photon
    Commented Apr 22, 2015 at 22:38
  • 1
    \$\begingroup\$ Alos, the REF input probably only affects the last stage of the amplifier. The input stages most likely are not affected at all by what you do wit the REF input. \$\endgroup\$
    – The Photon
    Commented Apr 22, 2015 at 22:39
  • \$\begingroup\$ I don't really "get" a single supply instrument amp. The sample circuit in the spec sheet are all for split supplies. \$\endgroup\$ Commented Apr 22, 2015 at 23:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.