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Simple distortion model for modern MOSFET

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exp

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Hi,

Using the square law model, one can derive the distortion of a differential pair using NMOS as:

HD3 = 1/32 (vi/Vov)^2

I successfully used this approximation for an 180nm process and it was still reasonably accurate.

Now for a 28nm process, this formula seems to be unuseable. As example, I use vin=1mV. Vov is not very well defined but dc analysis in spectre shows me Vdsat=130mV. This yields -114dB.

Using the values vgd-vth from dcOpAnalysis I get 0.52489361-0.43463317=90mV, resulting in -108 dB. Close enough.

However, measuring the HD3 or THD with tran and dft gives me -76dB ... orders of magnitude different!


How do you deal with distortion in a modern process?
 

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