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Design >> Analog Design >> offset voltage in opamp
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Message started by shri on Jan 18^th, 2005, 3:55am

Title: offset voltage in opamp
Post by shri on Jan 18^th, 2005, 3:55am

Hello,
Please can you help me how to measure offset voltage in an opamp in cadence env, it would be great if you can elaborate on the simulation setup. Thanks in advance.

Shri

Title: Re: offset voltage in opamp
Post by sheldon on Jan 20^th, 2005, 5:13pm

Shri,

Just setup your circuit as you intend to use it and
measure difference, run a dc operating point simulation,
and then in the voltage of the input terminals. There are
several cavaets:
1) If the op-amp has a biplolar input pair you will need to match
the resistances at the input. There will be a slight amount
of residual offset due to offset current.

2) Or if you are worried about the effect of offset current
then you can take the difference in the base-emitter
voltage.

3) The bigger issue to the above discussion is that the
measured offset voltage is the "systematic" offset
voltage. In order to measure the offset voltage due
to mismatch in the devices, you will need to run
Monte Carlo analysis.

Accurate Monte Carlo analysis is non-trivial since you
need to setup the model files correctly, see the
following link:
http://www.designers-guide.com/Modeling/montecarlo.pdf

4) For a quick estimate, you can also use dc mismatch
analysis. It assumes a 3 sigma mismatch in devices
- dc mismatch needs special model setup, different
than the setup mentioned above.
- dc mismatch only works for selected models: bsim3v3,
VBIC, and 2 terminal resistors.

Regards,

Sheldon