1. DAC architectures

2. 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

3. 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

4. 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

5. 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

6. Unbuffered String DACs

7. HW
Suppose we want an 8-bit DAC, with 4-bit coarse string and 4-bit fine string, with no buffer in between. Modify the calculations for the tap voltages.

8. HW:
Is it possible to make the fine segment using a ladder structure.
For the 3+3 bit structure, what resistance values should be used?

9. Current-Mode R-2R Ladder
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

10. 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. Can combine thermometer code and binary ladder:

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

13. 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