1. DAC architectures

2. Tao Chen
Office: Coover 3108

3. Last week: Unbuffered String DACs

4. You can also choose to make the fine segment using a ladder structure.
For the 3+3 bit structure, what resistance values should be used?

5. Current-Mode R-2R LadderRF
IOut
VOut
Vout=RF*Iout

6. Current-Mode R-2R LadderRF
IOut
VOut

7. Current-Mode R-2R Ladder
Vref
Vref/2
Total current:
Vref/R I1 I2 RF
IOut
Ron
VOut
Series R can be used to adjust gain of DAC
R_out is code dependent, leading to DAC nonlinearity if un-buffered
But can use all NMOS switch, -tive gain buffer

8. Thermometer current DACs
Output node at virtual ground
Each resistor contributes either 0 or Vref/R
If input digital code = k, k of the 2N – 1 switches will be turned on
I_out = kVref/R
Inherently monotonic

9. Comparison Pros Cons R2R Simple Easy to extend resolution
Matching is poor
MSB/LSB same variation
Thermometer
Inherently monotonic
Balanced variation
Large area
Complex digital decoder (area)

10. Can combine thermometer code and binary ladder:

11. 12-bit Hybrid R2R DAC Design
Segmentation
NM-bit MSB + NL-bit LSB
R2R:
Each bit has 3R (2R+1R)
An additional 2R
3*NL+2
Thermometer:
(2NM -1)*2
MSB-ISB
4-8
5-7
6-6
MSB Area 30 62
126
LSB Area 26 23 20

12. 12-bit Hybrid R2R DAC Design
Switch
Ron needs to be small
(simulate)
Ron Mismatch
Can use only NMOS

13. 12-bit Hybrid R2R DAC Design
Clock
Use DFF for Din
Use clocked buffer
CLK