1. More DAC architectures

3. Unbuffered String DACs
Fine string // R  7R/8
Total resistance = 8R, current = Vref/8R
Voltage across fine string = Vref/8R * 7R/8 = 7Vref/64
Each resistor in fine string has voltage = Vref/64

4. Switch design All switches should have small Ron
Coarse string switches can be NMOS, PMOS, or transmission gates
Fine string switches all need to be transmission gates
Ron of coarse string switch can be lumped into part of the top and bottom resistors of the fine string

5. Ron When settled, current through switches = 0 Vds = 0
Ron = {uCox(W/L)*Veff}-1
Veff = |Vgate – Vtap|
As code changes, Vtap changes
For small Vtap, use NMOS with Vgate = VDD
For large Vtap, use PMOS with Vgate = gnd
For T-gates: one of each

6. Vout buffer
Need buffer between the fine string tap voltage (labeled as OUTPUT) and the DAC output
Purpose: remove loading to the string
Challenge: Buffer amplifier needs large input common mode range
Warning: cannot use negative gain configuration of op amp

7. Current-Mode R-2R Ladder
Series R can be used to adjust gain of DAC
R_out is code dependent, leading to DAC nonlinearity

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

11. Use Matched Current Sources
Current mirrors to create identical current sources
CMOS current sources are compact
Cascode current mirrors to increase output inpedence

12. Use Complementary outputs
Each current source and the two switches for a differential pair
Differential pair input connects Vinh and vinl that are enough to steer all tail current either to left or right side
All tail currents in current mirror connection
The on side diff pair transistor and tail current transistor in cascode
Can cascode tail current source, to achieve double cascode R_out

13. Binary-Weighted Current Sources Switched into a Load

14. Segmented Current-Output DACs
Instead of turning on and off current, use current steering;
Current steering is much faster, suited for high speed
Use differential pair as steering switch
Use just enough diff voltage to steer the current completely

15. Thermometer + thermometer segmentation
Example use

16. pMOS version of steering switches

17. Chapter 16 Figure 22