DAC8420_07 Datasheet, PDF (19/24 Pages) Analog Devices – Quad 12-Bit Serial Voltage Output DAC

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.
+5V SUPPLY
0.1µF
REF43
2 V_IN
4 GND V_OUT 6
2.5V
+5V SUPPLY
VREFHI
5
1
0.1µF
DAC8420
DAC A
7
VOUTA
DAC B
6
VOUTB
DIGITAL
CONTROLS
DAC C
DAC D
3
VOUTC
2
VOUTD
10 11 12 14 15 16
DIGITAL INPUTS
9
4
8
GND VREFLO
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.
DAC8420
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.
POWER
HIGH VOLTAGE
ISOLATION
5V
REG
5V
LD
SCLK
SDI
5V
10kΩ
5V
10kΩ
5V
10kΩ
+5V
REF43
2 V_IN
V_OUT 6
5V
4 GND
2.5V
5V
5
1
10kΩ VREFHI VDD
15 CLR
0.1µF
7
16 CLSEL
14 LD
6
DAC8420
12 CS
3
11 _CLK
2
10 SDI
VREFLO VSS GND
4
8
9
0.1µF
VOUTA
VOUTB
VOUTC
VOUTD
Figure 35. Opto-lsolated 3-Wire Interface
Rev. B | Page 19 of 24