about match of layout

[lhlbluesky]

in sc-circuit, the ratio of capacitor and switch must be in proportion, to reach better layout performance, but i don't know how to match the switch size.
the following is a 2X circuit, vout = vout+ - vout- = 2 * (Vin+ - vin-) - (vref+ - vref-) , to reach better match permance, Cs = 2Cf, but how to set the size of the switches? if the size of S1 is W=3u/0.35u, L=1u/0.35u, what is the size for other switches (S2~S6)? and pls explain to me why the size is used to make better layout match performace. thanks all.

 

[erikl]

... the size of S1 is W=3u/0.35u, L=1u/0.35u, what is the size for other switches (S2~S6)? and pls explain to me why the size is used to make better layout match performace.

I guess you thought of PMOS=3u/0.35u, NMOS=1u/0.35u ? ;-)

Assuming the Cf (Cr) switches (S3,S6) have to transfer only ½ of the charge of the Cs switches during the same time interval (surely a coarse approximation), it seems to be justified to assign them only ½ of the W/L ratio of the S1,S2 switches. But what about S4 and S5 ? Could be quite different. So usually all switches have the same size, and this also facilitates the switches' layout symmetry, which is an important factor for the ADC's accuracy/resolution, too.

For more detailed info about switched capacitor application and the selection of switches and switch sizes see the thesis PDF below.