Hi,

I was trying to model in Verilog-A a differential OTA by the equations

  ΔId = K.ΔVin.√( 4*Iss/(2.K) - sq(ΔVin) )   for   |ΔVin| <= √2.(Veff)  -- (1)

  ΔId = Iss     for   |ΔVin| >  √2.(Veff)  -- (2)

  ΔId = -Iss    for   |ΔVin| <  √2.(Veff)  -- (3)

Simulating in transient mode with the OTA in a negative feedback configuration, I got convergence problems. I had to change (1) such that      ΔId = K.ΔVin.√abs( 4*Iss/(2.K) - sq(ΔVin) )    to get around the convergence problems.  

But this method failed when I used the OTA in a switch-cap circuit at the top-level. There were no error messages, but the output was wrong and it started to occur when   |ΔVin| <= √2.(Veff)  .

Anyone knows the way to get around such problems of "piece-wise" functions? Apologize if I got the term "piece-wise" wrong, but I hope u get the idea. Thanks!!

Smoothing is rarely, if ever, needed. I suspect the key here is not to make your transfer curve smooth, but to make it continuous. What is ΔId when ΔVin == √2×Veff? It should be Iss, is it?

-Ken

It generally means that O1P has suddenly become floating.

-Ken