Help with understanding why my instr. amplifier doesn't return anything

Question

I have a pressure sensor (output -> Wheatstone bridge) connected to an amplification stage, as shown here:

As you can see, four points in total are attached to the ADC of my MCU (STM32L476ZGT3.) The reason for that is to check the noise at different points.

The MCU is soldered and it works fine - I am able to program it and get responses.

I started soldering the sensor module step by step. The pressure sensor does its job: I see differences (mV) when I increase/decrease the pressure - it has a full scale = 52.5 mV.

Afterwards I soldered the first amplifier circuit (AD623) and here my problem came. At the output pin, I cannot see any difference when the pressure changes. I see constantly the ref value = 1.65V.

What can be the issue? The pinout is correct, there are no shortcircuits, supplies are fine with no noise.

Answer 1

You're exciting the sensor with a 5 V voltage, and this means that the common mode voltage at the sensor output will be around 2.5 V (if it's a Wheatstone bridge sensor; check this value on your circuit). If you look at the data sheet of the AD623 at p. 4, you can see that the (common-mode) input voltage range is up to \$V_\mathrm{S}-1.5\,\mathrm{V}\$: since you're powering the AD623 with \$3.3\,\mathrm{V}\$, this means that your maximum input voltage is \$1.8\,\mathrm{V}\$, well below the common-mode output voltage of the sensor.

Hence, the amplifier cannot work properly: you have to increase its power supply voltage, or choose an IA with rail-to-rail input capability.