about input offset voltage

[shmilyzhyao]

input offset voltage

Hi,everyone. I am confused about the concept of input offset voltage.
I have designed a fully-differential OTA. I know that the input offset voltage is always introduced in layout due to the unsymmetry or other non-idealities. However, what i am doing is just designing this circuit and simulating it by Hspice, which is not related to the layout procedure. Need I find the input offset voltage of the circuit before measuring any AC performance such as open loop gain?
In a word, does the input offset voltage exist in the procedure of SPICE simulation? If so, what is the causation?
Can anybody help me?
THX in advance!

 

[ravirajdv]

offset voltage

Hi,
In any amp, the offset can be systematic or dynamic. The one which you are referring to is the systematic offset and proper layout methodologies should help you minimize it. However the dynamic offset can be because of random effects, like noise. You need to simulate and check for these kind of offsets.

One thing what you can do, is to hold one input at some reference level and vary the other input slowly (a ramp of 10u), then check the output. The output should be zero when both the inputs become equal, if there is any mV of difference with o/p at zero, then this should be your offset.

Regards,
RDV

 

[shmilyzhyao]

input offset voltage from dc sweep

Hi,RDV . Thanks a lot for your kind reply.
I followed your method and got that there was no offset at all. The output voltage is zero when the tow inputs are all VDD/2. Is it right?
Regards

 

[ravirajdv]

finding offset voltage with spice

Hi,
I guess you have checked this using a transient sim setup.
Also check with a dc sweep, wherein you sweep 1 input ( and other as constant). This will also help in analyzing the offset. You can also check if there is any hysteresis coming into picture.
Unless your circuit is perfect you should see some offset (atleast offsets of the order of 1mV -2mV).

Regards,
RDV

 

[timo67]

site:www.edaboard.com offset

Hy
If you want to have an idea about the input offset voltage during Spice simulation you should introduce parameter dispertion of you technonogy and proceed to a monte carlo simulation

 

[shmilyzhyao]

ota input offset simulation

Thank you all. I agree with timo67. I think input offset voltage is just the deviation in process. And if we want to simulate it, we should use Monte carlo simulation.