DAC architectures

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

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

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

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

Unbuffered String DACs

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.

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?

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

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

Can combine thermometer code and binary ladder:

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

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