The circuit board was reused from an earlier project to build my own flight recorder. When making that circuit board, I selected the wrong footprint device for the CPU, so I ended up with three scrap boards. One side has pads for the large surface mount pressure sensors, and the other was a footprint for an accelerometer (also the wrong size footprint) and an op-amp. So it was available for use on this project.
I chose this presure sensor for the noble reason that I had one. This was a pressure sensor from my Simple Altimeter project that didn't survive it's test flight (it seems that any of my rockets with electronic payloads in them are cursed), but it still appears to work. I don't have a spare (un-crashed) one on hand so it will have to do for now.
But ... this is a +5v device, so I needed to somehow make it work with a 3.3v A/D converter. The sensor output is near 5v at sea level, going to near 0v at about 36,000', it seems the easiest approach is to "invert" the signal. Referring to the National Semiconductor Application Note #31, Op Amp circuit collection, I selected a difference amplifier:
When all resistors are the same value, Vout = V2 - V1. By subtracting the sensor output from 5v, I get a voltage thats about 1.2v at my home (980ft MSL). My only concern is a shock (ejection or landing) causing the sensor to put out a spike over 3.3v. A 3.3v zener diode on the output of the op-amp should attenuate any spikes. I've found through trial and error that a 0.1uf capacitor on the "+" input is necessary to get rid of noise.
An alternative would have been to use a voltage divider. This would be made from a pair of same-value resistors: one from the sensor output and the other to ground to give me divide-by-two.
In the future, I plan to switch to an MP3H6115A6T1, which is a 3.3v device.
Here's what the back side, with all the sensors, looks like:
The power input is +5v, regulated by the regulator attached to the battery holder (with that chunk of aluminum for a heat sink).