Altitude Measurement

[octane-link]

Hey guys:

I am using a PIC18F452 to measure altitude with a Freescale pressure sensor.

I want to use a non-inverting op-amp so that when the plane is on the ground, I read an altitude of 0 ft/meters. Then as I go up and down, I get AGL altitude.

The op-amp will have a gain of 10, and I am going to use successive approximation to get the offset that goes in the (-) input. But I am not sure what the output should be on the ground. I am thinking close to 0 V, but then as altitude increases, voltage goes down. If the output on the ground is like 4 V, the voltage is solved, but I am not sure where to go on this.

Any help?

Thanks!

[thirsty]

Hey,

Why don't you just read the output of the sensor via your adc when powered up and store that value. Then you can use that stored value in calculations to work out the altitude above the ground. Obviously the sensor needs to be at ground level when powered up for this to work.
The MPX5010 and some other sensors output around .5 to 4.5 volts so should be able to work just fine without an opamp unless you want a buffer.

[octane-link]

I thought about that, but for the small pressure ranges that we fly in, I wouldn't get enough resolution on the ADC side.

By amplifying by a factor of 10, I can look then look at a 5 V window of the new 50 V range, giving me much greater accuracy in the measurement.

Its just that I am not sure how to rig the offset voltage.

[Zlatko]

Hi,

Have a look here
http://www.national.com/an/AN/AN-31.pdf
look at top of page 6 of that document.

Cheers