REV. A

–11–

AD7243

Common clock, data, and synchronization signals are applied to

all DACs in the chain. The loading sequence starts by taking

SYNC low. The data is then clocked into the input registers on

the falling edge of SCLK. Sixteen clock pulses are required for

each DAC in the chain. The data ripples through the input reg-

isters with the first 16-bit word filling the last register in the

chain after 16N clock pulses where N = the total number of

DACs in the chain.

When valid data has been loaded into all the registers, the

SYNC input should be taken high and a common LDAC pulse

used to update all the DACs simultaneously.

APPLICATIONS

OPTO-ISOLATED INTERFACE

In many process control type applications it is necessary to pro-

vide an isolation barrier between the controller and the unit be-

ing controlled. Opto-isolators can provide voltage isolation in

excess of 3 kV. The serial loading structure of the AD7243

makes it ideal for opto-isolated interfaces as the number of in-

terface lines is kept to a minimum.

Figure 17 shows a 4-channel isolated interface using the

AD7243. The DCEN pin must be connected high to enable the

daisy-chain facility. Four channels with 12-bit resolution are

provided in the circuit shown, but this may be expanded to ac-

commodate any number of DAC channels without any extra

isolation circuitry.

The sequence of events to program the output channels 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 50 ns duration or greater

is all that is required to update the outputs.

SCLK

AD7243*

SYNC

LDAC

SDIN

DCEN

SDO

SCLK

AD7243*

SYNC

LDAC

SDIN

DCEN

SDO

SCLK

AD7243*

SYNC

LDAC

SDIN

DCEN

SDO

SCLK

AD7243*

SYNC

LDAC

SDIN

DCEN

SDO

*ADDITIONAL PINS OMITTED FOR CLARITY

DATA OUT

CLOCK OUT

SYNC OUT

CONTROL OUT

CONTROLLER

QUAD OPTO-COUPLER

Figure 17. Four-Channel Opto-lsolated Interface