AD7398: Usage of LDAC pins

Q I am a little confused over the use of the LDAC control pin on the AD7398. How do I go about updating all four DACs simultaneously? A The idea behind the LDAC- control pin has to do with the double buffered input structure. Double buffered means that there are two layers of registers: Input registes and DAC registes. You write to the input register. The DAC register directly controls the analogue output. So once you have written to the input register you use the LDAC- signal to transfer the new data from the input register to the DAC register, thus updating the Analogue output. While LDAC- is held high, you can write to the input registers "in your own time" without changing the values held in the DAC registers, therefore the Analog outputs do not change when the part is written to. Then, at a later time you can update the DAC register, and the Analogue Output to the new value, by pulling the LDAC- input low. While the LDAC- input is low, the DAC register is equal to the input register. In effect, the DAC register is transparent, as described by Table 1 on page 6 of the datasheet. When LDAC- returns high, changes to the input register will again have no effect on the DAC register, therefore the rising edge is the critical edge, because the data present at the time this edge occurs is the data which will remain in the DAC register until the next LDAC- update. The LDAC- pin is most useful in a multiple DAC, like the AD7398, where several DACs can be written to over the same interface. Clearly, you can only write to one DAC at a time, yet you may wish all the DAC outputs to be updated at the same instant in time. By keeping the LDAC- pin high during the writes, you can write to each DAC input register in turn without changing the Analogue output. Then, once all the input registers have been written to, you strobe LDAC- low and the DACs are all updated simultaneously. On the AD7398, the LDAC- input is level sensitive. If you were to tie LDAC low permanently, then the values in the DACs would be updated on completion of the serial write operation, since the DAC register is transparent in this case.

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AD7398 Production
The AD7398 / AD7399Â family of quad, 12-bit/10-bit, voltage output digital-to-analog converters (DACs) is designed to operate from a single 3 V to 5 V...
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