Differential Air Pressure sensing (Guidance needed)

And power the sensor from same Vcc?

Yes. The very best solution it to create an analogue supple, AVcc. Supply this from Vcc through a resistor (33ohm will probably do), and put a 10uF capacitor from AVcc to Gnd, and a 100nF capacitor across the sensor supply. Power the sensor from AVcc. Power your external ADC from the Arduino Vcc (also with a 100nF capacitor across its supply), but connect its Vref pin to AVcc. This will give you a good, stable analogue supply with noise from the digital circuitry largely filtered out.

For anyone wanting to create a separate AVcc supply for their ATmega MCU, do not connect the Aref pin directly to any supply, including AVcc. Instead, leave it floating, or decouple it to 0V with a capacitor, and select internal AVcc as your ADC reference

When you fire it up and read the ADC value, you should get half-range with no pressure differential (a value of about 2048). Whatever this value is, store it as an offseet then subtract it from future readings, with the result stored as a signed integer. Now you should get zero with equal pressure, positive increasing values as the pressure at port P1 exceeds P2, and negative values as P1 pressure falls below P2. The scale should be 1.2Pa per increment, but you can calibrate it with a water column. Be careful to make sure water never enters the sensor, only air at the pressure you want to measure.

Let us know how you get on. :smiley: