Datasheet

AD7849
Rev. C | Page 17 of 20
APPLICATIONS INFORMATION
OPTO-ISOLATED INTERFACE
In many process control 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 because the number
of interface lines is kept to a minimum.
Figure 28 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 resolution are provided
in the circuit shown, but this can be expanded to accommodate
any number of DAC channels without any extra isolation circuitry.
The only limitation is the output update rate. For example, if an
output update rate of 10 kHz is required, then all DACs must be
loaded and updated in 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 is 31, which
leaves 800 ns for the
LDAC
pulse. Of course, as the update rate
requirement decreases, the number of possible channels increases.
The sequence of events to program the output channels in
Figure 28 is as follows:
1. Ta ke t he
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.
3. Ta ke t he
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 optocouplers, the
LDAC
line can be
driven from one shot that 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
DD
V
DD
V
DD
V
DD
DATA OUT
CLOCK OUT
SYNC OUT
CONTROL OUT
CONTROLLER
V
OUT
A
V
OUT
B
V
OUT
C
V
OUT
D
5V
QUAD OPTO-COUPLER
*ADDITIONAL PINS OMITTED FOR CLARITY.
LDAC
SDIN
V
OUT
DCEN
SDOUT
AD7849*
5V
LDAC
SDIN
V
OUT
DCEN
SDOUT
AD7849*
5V
LDAC
SDIN
V
OUT
DCEN
SDOUT
AD7849*
5V
SCLK
SYNC
LDAC
SDIN
V
OUT
DCEN
SDOUT
AD7849*
01008-031
SCLK
SYNC
SCLK
SYNC
SCLK
SYNC
Figure 28. 4-Channel Opto-Isolated Interface