At 08:43 PM 5/15/2005 +0200, you wrote:
>I need some advice comments on this circuit 
>http://www.vscp.org/images/barometer001.gif  You probably laugh your guts 
>out but but when you finished laughing please tell me what I have done wrong.

First off, you've got the + and - inputs swapped. When
that's fixed, the gain will not be 10, but approximately
100/11.8 = 8.47. The PIC output buffer will enter into it a bit.
If you don't like that, put a unity-gain op-amp buffer after the RC.
(with a dual op-amp, it's doesn't add any components).
Better yet make it a two or three pole filter with a time constant
in the tenths of a second. A single pole filter can only do so much.
Eg. if you use 100K and 1uF and PWM at 16kHz, then in one half-cycle
(31usec) the voltage will change by about t* (2.5/100K)*1/C = 0.7mV
at midscale, representing 7mV at the output or about 1.5 counts on a 10-bit 
ADc.
The ripple with your setup will be outside the range of my linear
approximation, many many counts.

There's nothing inherently wrong with your concept, however just using a
12-bit ADC would be much better for most purposes.


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

If the pressure transducer is ratiometric from the supply, then the PWM
is not a bad way of subtracting from that. You'll have to consider
the sensor accuracy itself. If you use a decent op-amp with low offset
voltage and decent resistors, they should not contribute substantially to
the drift.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




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