DAC8420FSZ datasheet(18/23 Pages) AD

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DAC8420

Data Sheet

Rev. C | Page 18 of 23

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 can 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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