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AD5422 daisy-chain output jumps

Question asked by leonwang on Apr 15, 2015
Latest reply on Apr 20, 2015 by tjohnson

I have two AD5422 in daisy-chain mode for 0~20mA. The MCU operates the first AD5422 through 3 optocouplers, with noncontinuous clock. By inserting proper software delays, it just works well. But suddenly I found the second AD5422 outputs erroneous current, randomly, from time to time. The software actually is a endless loop to send data to AD5422 repeatedly, so it just looks the output current jumps. In the mean while, the first AD5422 output is quite stable.

I tried sending a fixed data number. 0 and 0xffff seems OK, but any other values just jump, yes, it jumps, not precision problem, for instance from 10mA (data 0x8000) to about 3mA etc. But the first AD5422 are always good with whatever number.

Here the second AD5422 I mean the chip with SDI connected to the first AD5422 SDO, i.e. it gets data from the first AD5422, not from the MCU directly.

I'm using internal Vref and DVcc, and checked the digital level OK.

Anything else I could do next? Thanks very much in advance!