Maker Pro
Maker Pro

Op Amp Gain error

M

Mike Blankenship

Jan 1, 1970
0
Hi,

I have a 2 stage Op Amp Curcuit. The first stage works good as a voltage
follower
and out puts 100mv. The second stage is what I am having problems with. The
second
stage uses a MAX400 with a gain of 10. However the out put always rails to 5
volts.
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
tried
2 different OP Amps with no sucess. The 100mv signal is good with very
little noise ~5mv

Any help would be appreciated.

Thanks

mike
 
J

Jamie

Jan 1, 1970
0
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.
 
M

Mike Blankenship

Jan 1, 1970
0
opps Newby error .... The Common mode voltage for this part has to be at
least
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.

mike
 
Top