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AD420 Drift in output

Hi there

We manufacture a range of instruments that we have designed the AD420AR-32 chip onto the pcb to supply a 4-20mA signal.

We are able to use the chip successfully and have used about 60 of these chips. We have noticed that the output mA drifts upwards constantly to varying degrees on many of these boards. For example the output is set to 4.000mA and without sending the chip any new data the output goes up by 0.001mA every second until it reaches as much as 13mA. We are using a Fluke multimeter to measure the mA directly. We also tried a load of 1000 ohms on the current loop circuit  but the output still drifts upwards. On some boards there is no variation at all. The software used is exactly the same on all boards. Below is our schematic for the application of the chip.

This is a major problem as it appears to our customer as though the output from our instrument is changing. Can you assist to identify what the problem is?


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