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Design >> Analog Design >> Transisent simulation of fully differential OTA
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Message started by seahs on Jan 5^th, 2009, 6:40pm

Title: Transisent simulation of fully differential OTA
Post by seahs on Jan 5^th, 2009, 6:40pm

I am trying to use resistors to feedback the ouputs to inputs to form inverting configuation, since OTA can not drive resistive load, I put the reisistor 100Mohm. But in this way, the the rising time increases a lot due to large time constant associated with the resistor and capacitance of OTA.

Is there any good condifuration for the transisent simulation of fully differential OTA?
Thanks

Title: Re: Transisent simulation of fully differential OTA
Post by sheldon on Jan 5^th, 2009, 8:13pm

Seahs,

Assuming that you have an OTA for use in switched capacitor type
application and you want to check the behavior stand alone. Also
assuming that you are using Spectre. Try the the following:

1) Use capacitive feedback to close the loop. Cinput=Cfeedback
2) Use the the analysis dependent switches to set the transient
analysis initial conditions, that is, short the input to the output.
You should also short the input and output for dc analysis and
transient initial conditions. The switch should be open for transient
analysis.
--> Setting the transient initial conditions emulates the operation
of the switched capacitor circuit when the feedback switch
is closed and sets the operating point for the OTA.
3) Once the initial conditions are set then apply the sine wave during
transient analysis.

This approach generally works at 0.18um and above, not sure
whether this will work with 65nm, ... due to drift of the operating
point due to gate leakage.

Best Regards,

Sheldon