layout problem about scaling cap in dac

[stocol]

hi all,
a charge-redistribution dac is needed in my design.
the scaling capacitor is 32/31 times the size of the unity capcitor,
to achieve good matching,how to design the layout?
ur suggestion appreciated.

 

[protonixs]

hi all,
a charge-redistribution dac is needed in my design.
the scaling capacitor is 32/31 times the size of the unity capcitor,
to achieve good matching,how to design the layout?
ur suggestion appreciated.

what does "the scaling capacitor is 32/31 times the size of the unity capcitor" mean?

 

[stocol]

shown as shown in the pic

 

[protonixs]

shown as shown in the pic

your first statement was 32/31 but what is in the snapshot is 16/15.
for 16/15, you may layout a 4x4 structure of the unit capacitor. connect the 15 caps in series(the result is 1/15) and connect them in parallel with the remaining one ( 1 + 1/15 = 16/15).
for the 32/31, you may layout a 8x4 structure of the unit capacitor. connect the 31 caps in series and connect them in parallel with the remaining one.
i hope you got it correctly.
regards

 

[stocol]

yeah
i just want to point out that what 'scaling capacitor' means
in my design, that capacitor is 32/31 times the size of 1LSB capacitor

hi protonixs:
1LSB capacitor is 100fF, and it cannot be divided into 32 parts for the process precision. if i choose a large 1LSB capcitance, the cap array will consume much larger area and sample rate falls

 

[Hsuan66]

32C/31 = 1/(31/32C) = 1/[(16/32c)+(8/32C)+(7/32C)] , 32C/7=4C+(4C/7)

and 4C/7 = 1/[(1/4C)+(2/4C)+(4/4C)],

16/32C = 1/2C : two caps(1LSB) connect in parallel
8/32C=1/4C : four caps(1LSB) connect in parallel
4C/7 : connect the 4C and the 2C and the 1C in series

Therefore, you total only need 17 unit caps.