More 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

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

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

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

Binary-Weighted Current Sources Switched into a Load

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

Thermometer + thermometer segmentation Example use

pMOS version of steering switches

Chapter 16 Figure 22