Gain of Op-Amp is varying by itself !!!

[nikhilsigma]

Hello friends....
I am using CA3140 an CMOS op-amp from intersil for amplifying small signals (of the order of few mili volts...)
I am using it in Non Inverting configuration, where Gain=(1+R2/R1).... the circuit is given below...



here the gain is perfectly coming out to be 10, as set by the resistor values...(1+ 90k/10k)=10. But the problem creeps in when I try to give inputs of lower value....as in the following figure, configuration is same, only input voltage is changed to 10mV and the Gain becomes 15 !!!



again if I decrease the input voltage, the gain increases further, for 1mV input the gain becomes 60 !!!



I have tried to use dual supply i.e. +-12V, but the problem remains the same.....
I am NOT able to find out the problem which is causing it....!!!! Please HELP!!!

 

[keith1200rs]

Two possibilities - firstly the model probably includes an input offset voltage and secondly the opamp output range does not include the negative rail.

Keith

 

[crutschow]

Check the output for 0V input. You will likely find an output of 50mV giving an input offset of 5mV, not uncommon for a standard op amp. For a much smaller error go to a low offset op amp such as an op-27.

 

[FvM]

Yes, real OPs expose offset voltages. Instead of suspecting outrageous behaviour you should question your OP knowledge (!!!).

OP simulation models are often including typical offset voltage numbers. I personally doubt about it's purposefulness. But as today's engineers are used to absolutely believe in simulation results, it's probably necessary.

 

[keith1200rs]

If you want to check for input offset you need to do it with the output in its working range i.e. away from the negative rail. The easiest way to check it in simulation is as a unity gain buffer and a fixed input voltage in the middle of the opamps input/output range.

Keith

 

[nikhilsigma]

Ahh !! The culprit was the offset voltage itself... silly me...
Thanks alot for answering..... :-D