REV. B

–14–

AD7849

APPLICATIONS

Opto-Isolated Interface

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.

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

31. This 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 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.

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.

DATA OUT

CLOCK OUT

SYNC OUT

CONTROL OUT

CONTROLLER

+5V

QUAD OPTO-COUPLER

*ADDITIONAL PINS OMITTED FOR CLARITY

SCLK

SYNC

LDAC

SDIN

DCEN

SDOUT

AD7849*

+5V

SCLK

SYNC

LDAC

SDIN

DCEN

SDOUT

AD7849*

+5V

SCLK

SYNC

LDAC

SDIN

DCEN

SDOUT

AD7849*

+5V

SCLK

SYNC

LDAC

SDIN

DCEN

SDOUT

AD7849*

Figure 25. Four-Channel Opto-Isolated Interface