DAC8420FSZ2 datasheet(19/24 Pages) AD

19 / 24 page

DAC8420

Rev. B | Page 19 of 24

For a single 5 V supply, VVREFHI is limited to at most 2.5 V, and

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 34.

DAC8420

DAC A

DAC B

DAC C

DAC D

DIGITAL

CONTROLS

GND

DIGITAL INPUTS

VREFHI

VREFLO

+5V SUPPLY

0.1µF

VOUTA

VOUTB

VOUTC

VOUTD

REF43

VIN

GND

VOUT 6

0.1µF

+5V SUPPLY

2.5V

Figure 34. 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,

from the central controller; 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 35 shows a simple 3-wire

interface scheme for controlling 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 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 serial interface of the DAC8420

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

10kΩ

REG

HIGH VOLTAGE

ISOLATION

POWER

SCLK

SDI

REF43

VIN

GND

VOUT 6

+5V

10kΩ

0.1µF

VOUTA

VOUTB

VOUTC

VOUTD

GND

VREFLO

VSS

VDD

VREFHI

CLR

CLSEL

DAC8420

CLK

SDI

2.5V

Figure 35. Opto-lsolated 3-Wire Interface

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