At 09:57 PM 5/18/00 +0100, you wrote: >I am wanting to read a thermocouple via a/d into a 'f873 but i am told that >this is very difficult. > >I have the circuit to give 0-3.6 volts at .004v/oC from the therm, this >gives a range of 0 to 900oC. > >How would i be able to read in and represent .28 volts in the pic ? A thermocouple is a low impedance, low voltage source. It puts out only a few millivolt signal, but is able to support very high currents. The problems encountered with thermocouples generally are in the area of drift and AC noise pickup. The standard solution is to read the thermocouple with a chopper-stabilized op-amp to eliminate drift. However, you can get very low offset, very low drift op-amps now inexpensively. Typically, you'd amplify the thermocouple difference voltage with a 100x-200x amplifier, and introduce a low pass filter (e.g., 3-5 Hz) to take care of noise. If you use a LMC660 or other CMOS opamp, you'll get low offsets with low drift. You can use input (precision) resistors of ~ 1k and feedback resistors of 100k. With the LMC660, you can use a single 5 V supply, with the common point at 2.5 V. You send the output of the amplifier/filter to the PIC ADC. You will need to measure the temperature at the input connections as well, using a thermistor or solid-state sensor, unless you know you will be measuring only very high temperatures. You can deal with offset with a single-point calibration. It's simpler to amplify the signal than use a 24-bit converter! ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causas scire" ================================================================