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Op Amp Gain error


Mike Blankenship

Jan 1, 1970

I have a 2 stage Op Amp Curcuit. The first stage works good as a voltage
and out puts 100mv. The second stage is what I am having problems with. The
stage uses a MAX400 with a gain of 10. However the out put always rails to 5
The input does receive 100mv. Here is my setup :

Stage 1 >>>>>>> 100mv

Stage 2 (MAX400)

Pin 1 ... Vos TRIM
Pin 2 ... -IN
Pin 3 ... +IN
Pin 4 ... V-
Pin 5 ... No Connection
Pin 6 ... Vout
Pin 7 ... V+
Pin 8 ... Vos TRIM

The output of My Stage 1 Op Amp goes directly into Pin3 Max 400
Pin 6 is connected to Pin 2 with a 100K resistor. And Pin 2 is connected
to ground with a 10K resistor. This should give me a Gain of 10. I have
2 different OP Amps with no sucess. The 100mv signal is good with very
little noise ~5mv

Any help would be appreciated.




Jan 1, 1970
you should check the voltage on Pin 2,to do what you are claming it
has to be lower than Pin 3.
also there is another possibility
i don't know your full circuit but if you are running a
+/- supply then the + input has to be near 0 volts.
or at least less than the - input..
if you do have a well balanced supply then i think the
- input is getting 0 volts center and you are injecting 100 mv's in the
+ side hence the full rail to what i assume 5 volts to be your Vcc.
in other words its acting as a voltage comparator.
using an OP with very low offset makes it a very sensitive device.

Mike Blankenship

Jan 1, 1970
opps Newby error .... The Common mode voltage for this part has to be at
a 1 volt input. It not rail to rail inputs. The Max400 can go +5 -5 then I
could measure
the 100mv input.

Now I need to generate a -5V power supply. Thanks for the info.