yamanoor sairam wrote:
> However, I tried to understand the concept of offset voltage in an
> opamp from the link below:

First you have to understand how a ideal opamp works.  It takes the voltage
on the + input minus the voltage on the - input, then multiplies that by a
large number (the gain).  Often you consider the gain of a ideal opamp to be
infinite.  You get useful linear results with this by using feedback.

So with a perfect opamp, if the - input is a 1.000 volts and the + input is
at 0.999 volts, then the output will slam to the low supply rail.  If the +
input then moves to 1.001 volts, then the output slams to the high supply
rail.  For a ideal opamp, this is still true for the + input being 1.0001,
1.00001 and 1.000001 volts since those are all greater than the 1.0 volts
the - input is being held at.

However, real opamps have a error tolerance when comparing the + and -
inputs.  This is called the "input offset voltage", and is usually a few
millivolts.  Let's say your opamp has a 2mV input offset voltage.  If the -
input is still held at 1.000 volts, then you don't know what the amp will do
if the + input is within 2mV of that, meaning from 0.998 to 1.002 volts.
The + input has to be outside that range for the opamp to reliably determine
that it is higher or lower than the - input.

The equation for a ideal opamp is

  Out = Gain(PosIn - NegIn)

When the input offset voltage of a real opamp is taken into account it
becomes:

  Out = Gain(PosIn - NegIn + Err)

Where Err can vary from amp to amp, over temperature, and possibly other
parameters.  You don't know ahead of time what Err is except that it is
bounded to be within the input offset voltage.  In the example above, Err
can therefore be anywhere from -2mV to +2mV.

How the input offset voltage effects various circuits and how to deal with
it are long topics beyond the scope of this post.


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