More DAC architectures 1 2 Unbuffered String DACs

More DAC architectures 1

2

Unbuffered String DACs • • Fine string // R 7 R/8 Total resistance = 8 R, current = Vref/8 R Voltage across fine string = Vref/8 R * 7 R/8 = 7 Vref/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 = {u. Cox(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 5 • 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 6

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

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

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

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 12

Binary-Weighted Current Sources Switched into a Load 13

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 14

Thermometer + thermometer segmentation Example use 15

p. MOS version of steering switches 16

Chapter 16 Figure 22

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