At 11:11 AM 10/19/99 -0400, Wagner wrote: >Your doubt about negative input, or inverting buffer with a positive >input, is the most common and old problem in instrumentation. This is so >often that they invented the "instrumentation amplifier". You just need >a reference lower than the signal itself. Those amplifiers accept >several types of transducers and sensors, mostly bridge configurations, >and they amplify the difference between the + and - inputs. The same >for any other amplifier? yes, but the negative feedback is NOT done to >the - input, so it doesn't interfere with the input itself. Those >amplifiers (as any other) need the inputs inside the common mode voltage >(Power + and -), and they are great to reject noise at both inputs >(common mode reject), as 60Hz for example. Instrumentation amplifiers >can be build with 3 regular op-amp (LMC660 for example), but I usually >do it with only 2 with the same effect, so one LMC660 can produce 2 >instrumentation amplifiers. The good point is that in your case, the >inverting output just need a fixed reference voltage at the + input to >work as adjustment to the output dc component setup (what voltage will >be at the output when the input is zero). A good inst-amplify cost more >than $10 while the LMC660 is around $2. Actually, using a difference amplifier solves the requirements for the pH application with the LMC660, since it's got such a high input impedance. You'll have to deal with the offset, however, since it's a measurable error compared to the 60 mV per pH unit. A good chopper-stabilized instrumentation amplifier actually costs only $3, not a whole lot more than the LMC660. The advantage is that the offset voltage problem is removed. But, again, both chips work fine with only a single 5 V supply. ================================================================ 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 causae scire" ================================================================