Sean Breheny 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.
snip

Check the specs, does the common mode input voltage extend to Vdd ?

As you say that it is a rail to rail op-amp, the common mode input
should start from zero, but it will probably only go to vdd less a volt
or so (1.3 V in your case)

--
Peter Cousens
p.cousens@cwcom.net