Spehro,

First of all thanks for noticing  the +/. swap. Had it on paper and the=20
component in eagle was the other way.  Probably should have taken me=20
several hours to find on a board... ;-(

When you say adding a 2nd pole filter do you mean to replace the PWM=20
LPF?  My thought was to also remove ripple in software. In this case it=20
has known properties so this should be possible.

Where does the 11.8 come from in 100/11.8?

Thanks your help/suggestions are very much appreciated.

/Ake

Spehro Pefhany wrote:

> At 08:43 PM 5/15/2005 +0200, you wrote:
>
>> 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=20
>> 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 =3D 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 =3D 0.7mV
> at midscale, representing 7mV at the output or about 1.5 counts on a=20
> 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=20
>> error during calibration but is there any sources for temperature or=20
>> 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=20
> reward"
> speff@interlog.com             Info for manufacturers:=20
> http://www.trexon.com
> Embedded software/hardware/analog  Info for designers: =20
> http://www.speff.com
>
>
>
>


--=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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