DAC0854CIWM datasheet(9/14 Pages) NSC

9 / 14 page

Applications Information

FUNCTIONAL DESCRIPTION

The DAC0854 is a monolithic quad 8-bit digital-to-analog

converter that is designed to operate on a single 5V supply

Each of the four units is comprised of an input register a

DAC register a shift register a current output DAC and an

output amplifier In addition the DAC0854 has an onboard

bandgap reference and a logic unit which controls the inter-

nal operation of the DAC0854 and interfaces it to micro-

processors

Each of the four internal 8-bit DACs uses a modified R-2R

ladder to effect the digital-to-analog conversion

(Figure 5)

The resistances corresponding to the 2 most significant bits

are segmented to reduce glitch energy and to improve

matching The bottom of the ladder has been modified so

that the voltage across the LSB resistor is much larger than

the input offset voltage of the buffer amplifier The input

digital code determines the state of the switches in the lad-

der network The sum of currents IOUT1 and IOUT2 is fixed

and is given by

IOUT1 a IOUT2 e

VREFbVBIAS

J 255

256

The current output IOUT2 is applied to the internal output

amplifier and converted to a voltage The output voltage of

each DAC is a function of VBIAS VREF and the digital input

word and is given by

VOUT e 2(VREFbVBIAS)

DATA

256

511

128

VBIASb

255

128

VREF

The output voltage range for each DAC is 03V – 28V This

range can be achieved by using the internal 265V reference

and a voltage divider network which provides a VBIAS of

140V

(Figure 6) In this case the DAC transfer function is

VOUT e 25

(DATA)

256

0310

The output impedance of any external reference that is

used will affect the accuracy of the conversion In order that

this error be less than

LSB the output impedance of the

external reference must be less than 78X

TLH11261 – 16

FIGURE 5 Equivalent Circuit of R-2R Ladder and Output Amplifier

TLH11261 – 17

FIGURE 6 Generating a VBIAS e 140V from the Internal Reference

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