AD7849 Analog Devices, AD7849 Datasheet - Page 17

• Manufacturer Part Number: AD7849
• Description: Serial Input, 14-Bit/16-Bit DAC
• Manufacturer: Analog Devices

Specifications of AD7849

• Resolution (bits): 16bit
• Dac Update Rate: 143kSPS
• Dac Settling Time: 7µs
• Max Pos Supply (v): +15.75V
• Single-supply: No
• Dac Type: Voltage Out
• Dac Input Format: Ser

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
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.
*ADDITIONAL PINS OMITTED FOR CLARITY.
CONTROLLER
CONTROL OUT
CLOCK OUT
DATA OUT
SYNC OUT
QUAD OPTO-COUPLER
AD7849
Figure 28. 4-Channel Opto-Isolated Interface
V
V
V
V
DD
DD
DD
DD
Rev. C | Page 17 of 20
The sequence of events to program the output channels in
Figure 28 is as follows:
1.
2.
3.
4.
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.
Take the SYNC line low.
Transmit the data as four 16-bit words. A total of 64 clock
pulses is required to clock the data through the chain.
Take the SYNC line high.
Pulse the LDAC line low. This updates all output channels
simultaneously on the falling edge of LDAC .
SCLK
SYNC
SCLK
SYNC
LDAC
SCLK
SYNC
LDAC
SCLK
SYNC
LDAC
LDAC
AD7849*
AD7849*
AD7849*
AD7849*
SDOUT
SDOUT
SDOUT
SDOUT
SDIN
SDIN
SDIN
SDIN
DCEN
DCEN
DCEN
DCEN
V
V
V
V
OUT
OUT
OUT
OUT
5V
5V
5V
5V
V
V
V
V
OUT
OUT
OUT
OUT
B
C
D
A
AD7849