Post by thread:[PIC]: PIC16LF627 and a wheatstone bridge.

Hello all, I was wondering how I'd go about using the ADC on the PIC16LF627 to measure the output of a wheatstone bridge (from a pressure transducer). Do I need any support components to bring the differental output back to ground? Can I just use the one voltage divider in the wheatstone or do I lose sensitivity or does the internal resistance of the pic (like on a DMM) come into play? Thanks, Aaron -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

At 01:59 PM 6/4/02 -0400, A.J. Tufgar wrote: >Hello all, > I was wondering how I'd go about using the ADC on the >PIC16LF627 to measure the output of a wheatstone bridge (from a pressure >transducer). Do I need any support components to bring the differental >output back to ground? Umm . . . The '627 does NOT have an ADC. You can fake one by making a dual slope convertor using one of the comparitor inputs but since you need a differential input, I'd seriously consider using an external dual op-amp configured as a diff in, diff out amplifier (needs only 3 resistors) followed by a LTC1298 12 bit a/d used in differential input mode. This is exactly what I use to measure strain guages and pressure sensors and it works well. You have one of the easiest types of sensor to measure - the output is ratiometric and is biased at half the excitation voltage. dwayne -- Dwayne Reid <.....dwaynerKILLspam@spam@planet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

It all depends on: What transducer make and model? What range of pressures? What accuracy you need? Depending on the answers to these question you may need anywhere from no additional circuitry to a bucket load of precision analog circuitry. The transducer manufacturer may have some helpful applications information. Paul > {Original Message removed}

To answer pauls questions: 1. This is the sensor I'm using from digi-key: MSP6800-ND 2. It's 0-5 psi guage. 3. All I need to detect is any negative pressure at all (so very low res.) Thanks for the advice dwayne, but I need really low res and sorry my error on the '627 mistook the comparator. My new ideas are: 1. Using the circuit from the following url and an 16C711. http://www.linear-tech.com/pdf/2053i.pdf 2. Or using the 627 and a ADC0831. Opinions? Thanks, Aaron -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

At 04:23 PM 6/4/02 -0400, A.J. Tufgar wrote: >To answer pauls questions: >1. This is the sensor I'm using from digi-key: MSP6800-ND >2. It's 0-5 psi guage. >3. All I need to detect is any negative pressure at all (so very low >res.) If all you need to determine is a negative pressure, I think the '627 may be all you need. It just depends upon how large the input offset voltage is compared to the minimum negative voltage you need to see. Connect the sensor to the '627's comparitor in differential mode. You may also want to add an offset pot to one input to trim out any offset or to ensure that the output is in the state it is supposed to be with zero pressure applied. dwayne -- Dwayne Reid <EraseMEdwaynerspam_OUTTakeThisOuTplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

A.J., That pressure sensor measures gauge pressure (0 psi = 1 atm), not absolute pressure. Does the voltage actually swing negative when the pressure drops below atmospheric? Usually the pressure gauges I work with will just stop at 0 V. Mark >At 04:23 PM 6/4/02 -0400, A.J. Tufgar wrote: >>To answer pauls questions: >>1. This is the sensor I'm using from digi-key: MSP6800-ND >>2. It's 0-5 psi guage. >>3. All I need to detect is any negative pressure at all (so very low >>res.) -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

I had a tough time actually getting this information out of the company I'm dealing with. I believe the voltage actually does swing negative and positive in relation to the pressure placed on it. The engineer I taked to said it worked both ways so I'll take his word on it. They sent me a couple of samples though. Aaron {Original Message removed}

On Tue, 4 Jun 2002, A.J. Tufgar wrote: >I had a tough time actually getting this information out of the company >I'm dealing with. I believe the voltage actually does swing negative and >positive in relation to the pressure placed on it. The engineer I taked >to said it worked both ways so I'll take his word on it. The voltage on all bridge pressure sensors changes sign when they are driven with 'opposite' pressure. Usually there is a limit on the reverse pressure (it is lower than the forward pressure). Yuo need to account for the offset of the bridge to determine zero. The offset will change with the bias voltage. Peter -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu