AD7849
APPLICATIONS
Opto-Isolated Interface
31. This leaves 800 ns for the LDAC pulse. Of course, as the
update rate requirement decreases, the number of possible
channels increases.
In many process control type applications it is necessary to
provide an isolation barrier between the controller and the unit
being controlled. Opto isolators can provide voltage isolation in
excess of 3 kV. The serial loading structure of the AD7849
makes it ideal for opto-isolated interfaces as the number of
interface lines is kept to a minimum.
The sequence of events to program the output channels in Fig-
ure 25 is as follows.
1. Take the SYNC line low.
2. Transmit the data as four 16-bit words. A total of 64 clock
pulses is required to clock the data through the chain.
Figure 25 shows a 4-channel isolated interface using the
AD7849. The DCEN pin must be connected high to enable the
daisy-chain facility. Four channels with 14-bit or 16-bit resolu-
tion are provided in the circuit shown, but this may be expanded
to accommodate any number of DAC channels without any
extra isolation circuitry. The only limitation is the output up-
date rate. For example, if an output update rate of 10 kHz is
required, then all the DACs must be loaded and updated in a
time period of 100 µs. Operating at the maximum clock rate of
5 MHz means that it takes 3.2 µs to load a DAC. This means
that the total number of channels for this update rate would be
3. Take the SYNC line high.
4. Pulse the LDAC line low. This updates all output channels
simultaneously on the falling edge of LDAC.
To reduce the number of opto-couplers, the LDAC line could
be driven from a one-shot which is triggered by the rising edge
on the SYNC line. A low level pulse of 100 ns duration or
greater is all that is required to update the outputs.
V
V
V
V
DD
DD
DD
DD
DATA OUT
CONTROLLER
SDIN
CLOCK OUT
SCLK
V
OUT
V
OUT
A
SYNC
AD7849*
LDAC
SYNC OUT
+5V
+5V
+5V
+5V
DCEN
SDOUT
CONTROL OUT
SDIN
SCLK
SYNC
AD7849*
LDAC
V
V
OUT
B
C
D
OUT
QUAD OPTO-COUPLER
DCEN
SDOUT
SDIN
SCLK
SYNC
AD7849*
LDAC
V
V
OUT
OUT
DCEN
SDOUT
SDIN
SCLK
SYNC
AD7849*
LDAC
SDOUT
V
V
OUT
OUT
*ADDITIONAL PINS OMITTED FOR CLARITY
DCEN
Figure 25. Four-Channel Opto-Isolated Interface
–14–
REV. B
ADI [ ADI ]
