Well, let's see. The first stage has a gain of about 201. Input offset voltage is multiplied by this to appear at the output (the second stage has a gain of 1). Input offset voltage is 3.8mV at temperature extremes, giving 763mV output offset. Since this would be the output offset if only one of the input op amps had input offset, it COULD be double that if both have input offset in opposite directions, or 1.5276V. Looking at input offset voltage drift, there's no maximum listed, but typical is 1uV/degree C. If we double this for the two input op amps, then multiply by the gain of 201, there's 402uV / degree C of output offset drift. A 10 degree C temperature change would give you 4mV of output change. Then, there's input bias current. If the op amps are driven by a low Z source, the output due to input bias is Ib*If. The input bias current here is 10pA max, giving 1uV for each input op amp. Bias current appears to be a minor contributor! Since input offset current is half this, it's even less of a contributor. So, it looks like input offset voltage is the major contributor, but you could subtract that out, since it's fixed. Input offset voltage drift would account for about 1/10 of the drift you're seeing with a 10C change. Next would be resistor temperature coefficient. Because the bias current is so low, changes in the feedback resistors do not make significant error contributions with bias current. However, assuming there's a common mode voltage (and I assume there is, since you're using an instrumentation amp), minor changes in resistance, especially in the "third" op amp input circuit, make significant deterioration in CMRR. It would be interesting to see if the output drifts with both inputs to the instrumentation amp grounded. As you bring the inputs up to the transducer output voltage, I expect you'll see output variation due to common mode gain (which is ideally zero). So, I expect the problem is resistor temperature coefficient which is causing variation in the common mode gain, which is giving you an output due to the common mode input. My solution: buy an instrumetation amp chip! These have precisely matched resistors on the chip. Good luck! Harold On Fri, 26 Jul 2002 10:44:34 -0400 "A.J. Tufgar" writes: > Hello all, > I'm using the classic instrumentational three opamp setup > with > all R = 100K except the one going between the two neg terminals > which is > 1K. > > I was using a LM324 but as recommended by the list I tried a better > CMOS > opamp and went with the LMC6482. > > I'm having the same problem I was having with the LM324. With the > LMC6482 my output voltage starts at 0.04V and goes to 0.08V in a > matter > of minutes, without any changeon the transducer. > > I don't think it's temprature drift as the LMC6482 has a relatively > low > drift. So anyone have any idea how I can get this annoying drift to > go > away?? There must be some way to correct for this. > > Thanks, > Aaron > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com Reach broadcasters, engineers, manufacturers, compliance labs, and attorneys. Advertise at http://www.hallikainen.com/FccRules/ . ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/web/. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads