AD420
THREE-WIRE INTERFACE
ASYNCHRONOUS INTERFACE USING OPTOCOUPLERS
Figure 8 shows the AD420 connected in the three-wire interface
mode. The AD420 data input block contains a serial input shift
register and a parallel latch. The contents of the shift register are
controlled by the DATA IN signal and the rising edges of the
CLOCK. Upon request of the LATCH pin the DAC and inter-
nal latch are updated from the shift register parallel outputs.
The CLOCK should remain inactive while the DAC is updated.
Refer to the timing requirements for three-wire interface.
FAULT DETECT
V
CC
V
LL
FAULT
DETECT
LATCH
CLOCK
DATA IN
LATCH
V
CC
10k
FAULT
DETECT
V
CC
V
CC
AD420
DATA
OUT
I
OUT
R
LOAD
LATCH
CLOCK
DATA
IN
GND
AD420
DAC2
DATA
OUT
I
OUT
R
LOAD
CLOCK
DAC1
DATA
IN
GND
The AD420 connected in ASYNCHRONOUS INTERFACE
mode with optocouplers is shown in Figure 9. Asynchronous
operation minimizes the number of control signals required for
isolation of the digital system from the control loop. The resistor
connected between the LATCH pin and V
CC
is required to
activate this mode. For operation with V
CC
below 18 V use a
50 kΩ pull-up resistor, from 18 V–32 V use 100 kΩ. Asynchro-
nous mode requires that the clock run at 16 times the data bit
rate, therefore to operate at the maximum input data rate of
150 kBPS an input clock of 2.4 MHz is required. The actual
data rate achieved may be limited by the type of optocouplers
chosen. The number of control signals can further be reduced
by creating the appropriate clock signal on the current loop side
of the isolation barrier. If optocouplers with relatively slow rise
and fall times are used, Schmitt triggers may be required on the
digital inputs to prevent erroneous data being presented to the
DAC.
+24V
23 V
CC
100k
7 LATCH
Figure 8. Three-Wire Interface Using Multiple DACs with
Joint Fault Detect
USING MULTIPLE DACS WITH FAULT DETECT
CLOCK
+5V
2
8
V
LL
CLOCK
The three-wire interface mode can utilize the serial DATA
OUT for easy interface to multiple DACs. To program the two
AD420s in Figure 8, 32 data bits are required. The first 16 bits
are clocked into the input shift register of DAC1. The next 16
bits transmitted pass the first 16 bits from the DATA OUT pin
of DAC1 to the input register of DAC2. The input shift regis-
ters of the two DACs operate as a single 32-bit shift register,
with the leading 16 bits representing information for DAC2 and
the trailing 16 bits serving for DAC1. Each DAC is then up-
dated upon request of the LATCH pin. The daisy-chain can be
extended to as many DACs as required.
AD420
9 DATA IN
DATA
11 GND
GALVANIC ISOLATION
BARRIER
Figure 9. Asynchronous Interface Using Optocouplers
–8–
REV. F
AD [ ANALOG DEVICES ]
