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c-2c ladder network rather than r-2r (dac question)

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treehugger

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c-2c dac

hi there,
for building a digital to analog converter (dac) how do we use capacitive rather than resistive voltage dividers?

i have found many r-2r structures but failed to find any c-2c structure. any source, idea?
 

r2r dac

Although a c-2c ladder n/w can work as a voltage divider; I don't think it can be used as a DAC. b/c once the capacitors charge up with a particular voltage, you will have to provide a discharge path (using resistors), before they can be used for a new input value.

Even if you do provide the resistors for the discharge path, the RC time constant will limit the response of the circuit.

Finally, its usually difficult and costly to fabricate capacitors in ICs, whereas resistors can be easily implemented using o/p impedance of current sources.
 

c-2c ladder

zeeshanzia84 said:
Although a c-2c ladder n/w can work as a voltage divider; I don't think it can be used as a DAC. b/c once the capacitors charge up with a particular voltage, you will have to provide a discharge path (using resistors), before they can be used for a new input value.

Even if you do provide the resistors for the discharge path, the RC time constant will limit the response of the circuit.

Finally, its usually difficult and costly to fabricate capacitors in ICs, whereas resistors can be easily implemented using o/p impedance of current sources.

thanks for answering but i have 2 objections:

1) i am quoting from "electronic analog-to-digital converters" by Seitzer and Pretzl:

"..in view of the progress of MOS technology with its high-quality capacitor capability, proposals have been made to use capacitive rather than resistive voltage dividers. Further improvements to reduce the number of capacitors lead to a C-2C structure."

2) i understand and appreciate your suggestion to use output impedances of MOS current sources as resistive elements. but then, how are you going to get rid of those current sources. since i need to have just the resistors not current sources?
 

r-2r dac

Maybe the C-2C structure is a switched-C strucure which emulates resistors using caps...
 
c2c dac

If you come to the IC implementation point C2C ladders have floating nodes. So if you think off a DAC it would work only for AC.

The capacitive dividers used in DACs are typical charge redistribution networks. There each node have sometimes a connected to a defined potential.
 

r2r network

i understand and appreciate your suggestion to use output impedances of MOS current sources as resistive elements. but then, how are you going to get rid of those current sources. since i need to have just the resistors not current sources?

well, thats how resistive characteristics are usually implemented in ICs.

ΔI = ΔV/R

where R is the o/p resistance of the current source and ΔV is the voltage change on the o/p node of the current source. We use this change in o/p current of the current source to sense the voltage change.

IC design has a completely different methodology as compared to discrete circuit design. We try to use MOSFETS for everything we need; b/c of their small size and ease of fabrication. Otherwise, a megaohm sized resistor would cause a huge penalty on the IC area and still have a high tolerance value.
 

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