Kenneth, Thanks for many *very* good suggestions. In one of there appnotes=20 (http://www.freescale.com/files/sensors/doc/app_note/AN1646.pdf )=20 Freescale recommends a single pole filter with a cut off frequency of=20 650Hz and then use software. At the same time they say that the noise=20 is mostly white noise. Does a LPF realy help to get rid of white noice.=20 I mean you remove the high frequency parts but still have the lower=20 frequency noise left. /Ake Kenneth Lumia wrote: > Interesting idea. > > The MPX sensor output has a large amount of noise. You > will need to filter it extensively before applying it to your circuitry. > I seem to recall the app notes specifying a 750 Hz or so low > pass filter. You will probably want an even lower cutoff frequency > than suggested. A single pole RC is probably not sufficient for what > you are after - a multiple pole op-amp filter may be better. > Similarly, the PWM will probably need more filtering as well. > Extremely clean power to the circuit is a must. Also, choose your > sampling frequency carefully to avoid harmonics. Watch out for > non-linear regions of the op-amp near ground and 5V rails. > Finally, once you've treated it as best as is reasonable in > hardware, remember that software filtering is your friend. > >> 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. > > > I'd try it, with additional filtering of course. Temperature will > effect the results, you will need to determine the temperature > range and the accuracy required over that range to see if you > exceed the expected results (a rather complicated calculation, > considering all the "analog" parts and their tolerances). > > klumia@adelphia.net > > ----- Original Message ----- From: "Ake Hedman" > To: "Microcontroller discussion list - Public." > Sent: Sunday, May 15, 2005 2:43 PM > Subject: [EE] Comments on circuit > > >> 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. >> >> My thought is to build an input stage for the Freescale MPX4115=20 >> pressure sensor. This sensor outputs 0.2V - 4.8V for the pressure=20 >> 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=20 >> ranges. >> - Better than 0.1kPa resolution. >> >> If I connected the sensor directly to the A/D input I would get=20 >> 0.13kPa 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=20 >> need 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=20 >> sensor output to the (-) and a 100 kHz PWM signal through a LPF to=20 >> the(-) 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=20 >> the choose values filter the PWM frequency away leaving the DC=20 >> component. (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 Kryddor/Te/Kaffe:=20 >> http://www.brattberg.com >> Personal homepage: http://www.eurosource.se/akhe >> Automated home: http://www.vscp.org >> >> >> --=20 >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> > > --=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 --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist