layout problem about scaling cap in dac

Status
Not open for further replies.
S

stocol

Newbie level 6
Joined
Oct 24, 2007
Messages
13
Helped
1
Reputation
2
Reaction score
0
Trophy points
1,281
Visit site
Activity points
1,353
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.
 

stocol said:
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.
Click to expand...

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

stocol said:
shown as shown in the pic
Click to expand...



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
 

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
 

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.
 

    S

    stocol

    Points: 2
    Helpful Answer Positive Rating
Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…