Simple DAC architectures 1 Basic DAC with External

Simple DAC architectures 1

Basic DAC with External Reference • • Most DACs require that their reference voltage be within quite a narrow range whose maximum value is less than or equal to the DAC's VDD, Multiplying DACs (or "MDACs"), will work over a wide range of reference voltages that may go well outside their power supply 2

1 -Bit DAC d 0 • If d 0 = 1 • Output = Vref • If d 0 = 0 • Output = gnd 3

The Kelvin Divider (String DAC) V 7 V 6 V 5 HW: synthesize a 6 bit decode V 4 V 3 V 2 V 1 V 0 • N-bit version consists of 2 N equal resistors in series and 2 N switches • Need N-bit binary to “ 1 out of 2 N” decoder 4 • If input code = k, analog output = Vk

Chapter 16 Figure 01 5

Chapter 16 Figure 03 6

Chapter 16 Figure 04 7

Trimming the INL • Trim the four main resistors after fabrication so that 3 major nodes are at correct potential • Four point trimming can reduce the INL by a factor of four. 8

Digital Potentiometer VTAP=VB+D*(VA-VB) • Widely used in appliances, entertainment electronics, … 9

Chapter 16 Figure 05 10

Segmented Voltage-Output DACs • Coarse string has 2 M equal resistors, must be accurate to M+K bit level • Fine stage K-bit DAC must have K-bit accuracy • Fine stage could be string DAC or R-2 R DAC • Use buffers to isolate the two stages, buffers need to be accurate to 11 the M+K bit level

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 12

4 -Bit R-2 R Ladder Network • • One of the most common DAC building-block structures uses resistors of only two different values with ratio 2: 1 An N-bit DAC requires 2 N resistors, easily trimmable two ways in which the R-2 R ladder network may be used 13 as a DAC

Voltage-Mode R-2 R Ladder • "rungs" or arms of the ladder are switched between VREF and ground • Output node has output impedance = R, may need to fuffer 14

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 15

Equal Current Sources Switched into an R-2 R Ladder Network • DAC output impedance is constant and equal to R • Used in high speed video DAC • Low output impedance may be problem 16

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 17

Use Matched Current Sources • Current mirrors to create identical current sources • CMOS current sources are compact • Cascode current mirrors to increase output inpedence 18

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 19

Voltage-Mode Binary-Weighted Resistor DAC 20

Capacitive Binary-Weighted DAC in Successive Approximation ADC 21

Binary-Weighted Current Sources Switched into a Load 22

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 23

Thermometer + thermometer segmentation Example use 24

p. MOS version of steering switches 25