ad420-32-reel Analog Devices, Inc., ad420-32-reel Datasheet - Page 8

• Manufacturer Part Number: ad420-32-reel
• Description: Serial Input 16-bit 4 Ma-20 Ma, 0 Ma-20 Ma Dac
• Manufacturer: Analog Devices, Inc.

AD420
THREE-WIRE INTERFACE
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.
Figure 8. Three-Wire Interface Using Multiple DACs with
Joint Fault Detect
USING MULTIPLE DACS WITH FAULT DETECT
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.
LATCH
CLOCK
DATA IN
LATCH
FAULT
DETECT
CLOCK
DATA
IN
GND
AD420
DAC1
DATA
OUT
I
V
OUT
CC
V
CC
R
LOAD
10k
FAULT DETECT
V
FAULT
DETECT
LATCH
CLOCK
DATA
IN
GND
LL
AD420
DAC2
DATA
OUT
I
V
OUT
CC
V
CC
R
LOAD
–8–
ASYNCHRONOUS INTERFACE USING OPTOCOUPLERS
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
activate this mode. For operation with V
50 k pull-up resistor, from 18 V–32 V use 100 k . 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.
Figure 9. Asynchronous Interface Using Optocouplers
CLOCK
DATA
GALVANIC
BARRIER
+5V
ISOLATION
100k
+24V
CC
11
23
8
7
2
9
below 18 V use a
V
LATCH
AD420
DATA IN
GND
CLOCK
V
CC
CC
LL
is required to
REV. F