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Design >> Mixed-Signal Design >> How to find out the BW of a dynamic latch, such as strong arm latch?
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Message started by Frank_Heart on Nov 17^th, 2017, 10:22am

Title: How to find out the BW of a dynamic latch, such as strong arm latch?
Post by Frank_Heart on Nov 17^th, 2017, 10:22am

Hi, Everyone,

If there is no pre-amp, just a dynamic latch, such as strong arm lath, in the CMP design, how we calculate its BW?

Thanks
Frank

Title: Re: How to find out the BW of a dynamic latch, such as strong arm latch?
Post by DanielLam on Nov 27^th, 2017, 7:33pm

I usually just sim it with the worst case corner.

Otherwise, this is what I would try.

1) Pick smallest input amplitude
2) Find output time constant (need to spec and output load). The time constant will be gm/Cl or gm/(2*Cl) depending on how you want to calculate the output gms.
3) Find number of time constants needed to amplify input to a "valid" output". For example maybe when the differential output voltage is > 0.6 Vdd.
4) BW = 1/number of tau constants

This would be a rough approximation as the strongarm input transistors go through a saturation to linear mode. For very small inputs, I think you might need to break the amplification time into 2 parts, a linear and a positive feedback portion.

Note: You should just sim it with parasitics. And I've never tried the method I described.

Title: Re: How to find out the BW of a dynamic latch, such as strong arm latch?
Post by Berti-2 on Nov 28^th, 2017, 4:30am

Why do you want to calculate a bandwidth?
I suggest to just consider and specify the delay of your latch (for the smallest input -> LSB/2).