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AD5422 with a boost transistor output offset problem

Question asked by SteveR1 on Dec 4, 2013
Latest reply on Dec 11, 2013 by SteveR1

Hello,

I have designed a board which contains four AD5422B DACs.Each DAC has been programmed to generate 4mA output.

Here are the four output currents measured. Output 1 = 3.999mA, Output 2 = 4.001mA, output 3 = 4.053mA, Output 4 = 3.999mA.

The load resistor for this test is 250Ohms.

I am trying to determine why output 3 is so high. The measured reference voltage is  4.9976 mV.

I programmed the part  initially to use Rext and changed to Rint without any noticeable difference.

Because AVDD comes up after DVCC in this design, a reset command is issued to all DACS first. Sheet 29 recommends a settling time of 40usec but I increased it to 100 useconds but again didn't  notice any difference in output. 

The boost transistor is recommended in the data sheet.

Attached is a page from the schematic showing two of the AD5422B circuits.

I would appreciate any ideas.

Steve Rose

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