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Measurements >> Other Measurements >> Measuring Gm in presence of offset
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Message started by rajdeep on Aug 14^th, 2011, 12:33pm

Title: Measuring Gm in presence of offset
Post by rajdeep on Aug 14^th, 2011, 12:33pm

Hi All,

Can anybody tell me how one can measure in ATE gm of an OTA in presence of input referred offset Vos?

The easiest method I can think is that I apply a difference voltage V1 across the OTA and measure the output current Iout1 of it, then Iout1 = Gm(V1 - Vos) [assumed a - sign]. Then we apply V2 as the i/p diff voltage such that V2 has the same polarity as V1. Then Iout2 = Gm(V2 - Vos). Then Gm = (Iout1 - Iout2)/(V1 - V2).

Do you think it is correct? Can the input referred offset Vos change its
value during the 2 measurements as the operating point is changing?

Thanks,
Rajdeep

Title: Re: Measuring Gm in presence of offset
Post by boe on Aug 15^th, 2011, 10:06am

Rajdeep,

why not use small-signal analysis so you don't have to worry about changes in operating point?

B O E

Title: Re: Measuring Gm in presence of offset
Post by rajdeep on Aug 15^th, 2011, 11:33am

BOE,

Can you pls elaborate? I have never used any test methods that uses small-signal analysis in ATE.

Well in my post I should have mentioned that V1 and V2 should be small enough. The OTA has constant Gm for +-50mV. So I can safely use V1 = 10mV, and V2 = 15mV without really going out of the nice region where Gm is designed to be linear. This is assuming the offset is not that large to drive it out of the nice region. So in this case do you think this method will have any problem? I just dont like the fact that I am not trimming the offset first and measuring Gm in presence of it, although I cannot see any technical difficulty in this method. I am kind of happy and not! :-/

Thnx!
Rajdeep

Title: Re: Measuring Gm in presence of offset
Post by boe on Aug 17^th, 2011, 2:08am

Rajdeep,

Oh sorry, I didn't read your post accurately enough and overlooked the "ATE".
But in this case your method should be OK. I don't expect the offset to change much here.

B O E

Title: Re: Measuring Gm in presence of offset
Post by rajdeep on Aug 17^th, 2011, 3:41am

Ah! That's too bad! I was desperate to find out something bad about it, as before this I thought of a much more subtle (read complicated) way of doing this! But this is so darn easy :-/

Anyway thanks for confirming this!

Rajdeep

Title: Re: Measuring Gm in presence of offset
Post by nehaali on Jun 16^th, 2013, 9:43pm

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It is really very nice and informative forum thanks for sharing ....