I need some advice comments on this circuit=20
http://www.vscp.org/images/barometer001.gif  You probably laugh your=20
guts out but but when you finished laughing please tell me what I have=20
done wrong.

My thought is to build an input stage for the Freescale MPX4115 pressure=20
sensor. This sensor outputs 0.2V - 4.8V for the pressure range 15 - 15kPa.

What I want to accomplish is:

- Something that can be calibrated with just software.
- That works over many pressure ranges not just barometer pressure ranges.
- Better than 0.1kPa resolution.

If I connected the sensor directly to the A/D input I would get 0.13kPa=20
resolution. (5/1023) /(4.8/115) =3D 0.117 kPa/bit.

By amplifying the signal by 10 I can get  0.012  kPa/bit but I then need=20
to shift it down to be able to read it with the A/D.

So I set up a difference amplifier with a gain of 10 and feed the sensor=20
output to the (-) and a 100 kHz PWM signal through a LPF to the(-)=20
getting (sensor - Vpwm) at the A/D input.

For the LPF I have set the corner frequency to about 16kHz. Don't=20
actually know how to do this for PWM but my thinking has been that the=20
choose values filter the PWM frequency away leaving the DC component.=20
(How is the correct thinking?)

Is this a solution that is worth trying?  I can take away any static=20
error during calibration but is there any sources for temperature or=20
other dynamic errors that I need to take care of.

Comments appreciated (when you stopped laughing.. ;-) )

/Ake

--=20
 ---
Ake Hedman (YAP - Yet Another Programmer)
eurosource, Brattbergav=E4gen 17, 820 50 LOS, Sweden
Phone: (46) 657 413430 Cellular: (46) 73 84 84 102
Company home: http://www.eurosource.se     =20
Kryddor/Te/Kaffe: http://www.brattberg.com
Personal homepage: http://www.eurosource.se/akhe
Automated home: http://www.vscp.org


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