DAC8420 Analog Devices, DAC8420 Datasheet - Page 19

• Manufacturer Part Number: DAC8420
• Description: Quad 12-Bit Serial Voltage Output DAC
• Manufacturer: Analog Devices

Specifications of DAC8420

• Resolution (bits): 12bit
• Dac Update Rate: 125kSPS
• Dac Settling Time: 8µs
• Max Pos Supply (v): +15.75V
• Single-supply: Yes
• Dac Type: Voltage Out
• Dac Input Format: Ser

For a single 5 V supply, V
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.
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.
0.1µF
+5V SUPPLY
Figure 34. 5 V Single-Supply Operation Using REF43
10
DIGITAL INPUTS
11
DAC8420
2
4
CONTROLS
V
GND
12
DIGITAL
IN
REF43
14
V
15
OUT 6
VREFHI
16
9
2.5V
GND
is limited to at most 2.5 V, and
DAC A
DAC B
DAC C
DAC D
5
VREFHI
4
VREFLO
+5V SUPPLY
8
1
7
6
3
2
0.1µF
VOUTA
VOUTB
VOUTC
VOUTD
Rev. B | Page 19 of 24
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.
SCLK
SDI
POWER
LD
Figure 35. Opto-lsolated 3-Wire Interface
HIGH VOLTAGE
ISOLATION
5V
5V
5V
10kΩ
10kΩ
10kΩ
REG
5V
+5V
5V
2
4
10kΩ
0.1µF
REF43
V
GND
IN
5V
V
OUT 6
15
16
14
12
11
10
VREFLO
VREFHI
CLR
CLSEL
LD
CS
CLK
SDI
DAC8420
4
5
2.5V
VSS
8
VDD
5V
1
GND
DAC8420
9
7
6
3
2
0.1µF
VOUTA
VOUTB
VOUTC
VOUTD