DAC8420

REV. 0

–13–

must always be at least 2.5 V less than the positive supply to

ensure linearity of the device. For these applications, the REF43

is an excellent low drift 2.5 V reference that consumes only

450

µA (max). It works well with the DAC8420 in a single 5 V

system as shown in Figure 29.

DAC-8420

DIGITAL

CONTROL

4

9

15 16

14

12

11

10

8

2

3

6

7

1

5

DAC A

DAC B

DAC C

DAC D

VOUTA

VOUTB

VOUTC

VOUTD

DIGITAL INPUTS

GND

VREFLO

0.1µF

+5V SUPPLY

VREFHI

REF-43

VOUT

GND

VIN

+5V SUPPLY

2.5V

0.1µF

6

2

4

Figure 29. +5 V Single Supply Operation Using REF43

Isolated Digital Interface

Because the DAC8420 is ideal for generating accurate voltages

in process control and industrial applications, due to noise,

safety requirements, or distance, it may be necessary to isolate it

from the central controller. This can be easily achieved by using

opto-isolators, which are commonly used to provide electrical

isolation in excess of 3 kV. Figure 30 shows a simple 3-wire

interface scheme to control the clock, data, and load pulse. For

normal operation, CS is tied permanently LOW so that the

DAC8420 is always selected. The resistor and capacitor on the

CLR

pin provide a power-on reset with 10 ms time constant. The

three opto-isolators are used for the SDI, CLK, and LD lines.

One opto-isolated line (LD) can be eliminated from this circuit

by adding an inexpensive 4-bit TTL Counter to generate the

Load pulse for the DAC8420 after 16 clock cycles. The counter

is used to count of the number of clock cycles loading serial data

to the DAC8420. After all 16 bits have been clocked into the

converter, the counter resets, and a load pulse is generated on

clock 17. In either circuit, the DAC8420’s serial interface pro-

vides a simple, low cost method of isolating the digital control.

10k

Ω

10k

Ω

+5V

10k

Ω

LD

SCLK

SDI

+5V

REG

+5V

POWER

HIGH VOLTAGE

ISOLATION

+5V

+5V

0.1µF

+5V

CLR

CLSEL

CLK

SDI

LD

CS

4

5

VREFHI

VREFLO VSS GND

1µF

10k

Ω

DAC-8420

VOUTA

VOUTB

VOUTC

VOUTD

REF-43

VOUT

GND

VIN

+5V

6

2

4

2.5V

9

8

1

6

7

3

2

11

10

14

16

15

12

VDD

Figure 30. Opto-lsolated 3-Wire Interface

Dual Window Comparator

Often a comparator is needed to signal an out-of-range warning.

Combining the DAC8420 with a quad comparator such as the

CMP04 provides a simple dual window comparator with adjust-

able trip points as shown in Figure 31. This circuit can be

operated with either a dual or a single supply. For the A input

channel, DAC B sets the low trip point and DAC A sets the up-

per trip point. The CMP04 has open-collector outputs that are

connected together in “Wired-OR” configuration to generate an

out-of-range signal. For example, when VINA goes below the

trip point set by DAC B, comparator C2 pulls the output down,

turning the red LED on. The output can also be used as a logic

signal for further processing.