Hi Ken,

When I was simulating my continuous-time Delta-Sigma with transient noise in Spectre, I found that the switch transistors in the current-steering DAC1 were having a major impact on the SNDR. I expected the tail current source to dominate noise performance but the switch pair has about 4 times more noise power than the tail! To make a long story (~two weeks) short, I've simplified the circuit by looking at only the DAC unit current cell: NMOS Diff pair, ideal noise-free tail current source and two noisefree resistors to bias to Vdd.

I've found that when comparing the PSD (A^2/Hz) of the switching diff pair for PSS+Pnoise and transientnoise+FFT, the transient noise shows a lot more noise (especially low frequency noise). At 10MHz, the PSD is about 1000e-27 A^2/Hz and at 300MHz about 100e-27. Pnoise shows a very small noise from the switch transistors of ~20e-27 A^2/Hz at both 10MHz and 300MHz, which I believe is correct.

(When having a larger risetime, the noise at 300MHz matches better but the 10MHz noise (which is in the band of interest) is approx the same.)

I've attached the two testbenches: PSS and tran noise. The tran noise does FFT in Cadence and a script to get the PSD is attached in the Ocean view. (Normally I do this in Matlab but to simplify debugging, I did it in Cadence). I've tested UMC130 and ST65 PDK and the results are similar.

For stationary noise (e.g. resistors), PSS and tran noise match. I would be very happy if somebody could try this and confirm my problems.

Best regards,
Mattias

Hi Sheldon,
Sorry that I haven't replied earlier but I've been busy.

The devices are specified in um so that's probably why it says 2m. The sizes are W=2um, L=200nm and ID=10uA in ST 65nm process.

I've downloaded a 32nm model (http://ptm.asu.edu/) and included it in the testbench, see attachment, so we can use the same model. The transient noise has decreased with this model but there is still 4 times more noise in transient noise compared with PSS, so it seems like the problem remain.

When you look in the PSS noise summary, the resistors doesn't produce noise, right?

Thanks,
Mattias