At 10:24 PM 5/8/99 -0400, you wrote: >Hi all, > >I'd appreciate it if anyone could give me some hints on a strange op-amp >problem I'm having: > >I have an LMC662 dual CMOS rail-to-rail op-amp hooked up so that one of the >two amps is in a non-inverting buffer config and then other has both inputs >tied to Vdd (5v in this case). Vss is 0 volts. > >When I apply a voltage to the non-inverting input,the output follows it >perfectly until I get to 3.7v,at which the output suddently swings up to >5v. If I decrease the non-inv input,the output stays at 5v until I get >below 3.7v at the input at which time it begins to follow the input again. >I also have a scope hooked up to the output and I detect no oscillation or >other strange behavior throughout the entire input range from 0 to 5v. Hi Ran into a similar problem in my ignorance a few years back. Check the data sheet, and find the "schematic" for the guts of the part. Take a good look at the input lines. Many op amps have protective diodes internally on the input lines, and cannot accept a differential input voltage more than a volt or so. Differential input voltages exceeding the diode drops will cause the outputs to typically pull to one rail or the other. A 3.7 volt input for this effect sounds a bit hi, but it is easy to check out in the data sheets--and experimentally as you have inadvertently done. There are a number of parts that will not give this problem, you just have to check the data sheets thoroughly. OP27x will definitely do this to you. Paid for this knowledge with many hours of frustration. Pass it on when you have a chance. **************************************************************************** ******** All legitimate attachments to this email will be clearly identified in the text. Please note our new address and phone numbers. William K. Borsum, P.E. DASCOR P.O.Box 462885 Escondido, CA 92046-2885 V&M&F) 760-796-7785 Direct) 760-796-7788