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?
is the behaviour that you are seeing only a recent issue for this design - is it an older design that is now only experiencing the issue in a recent build?
Are all the devices that you are using from the same lot and can you send lot numbers for any devices you have seen this trend with?
We have isolated the problem. On the output of the AD420 (the 4-20mA Iout line pin 18) we had a transient voltage suppressor which when remove led to the drift being eliminated. For some reason the transorb was causing the analog drift on those pcbs.
Senior Electronic Engineer
DebTech - De Beers
Tel: +27 (0)11 309 3824 (Please note this is my new telephone number)
Fax: +27 (0)11 309 3260
From: MClifford email@example.com
Sent: 16 September 2011 21:17
To: Nabbie, Arshad
Subject: New message: "AD420 Drift in output"
Analog Devices EngineerZone <http://ez.analog.com/index.jspa>
AD420 Drift in output
reply from MClifford <http://ez.analog.com/people/MClifford> in Precision DACs - View the full discussion <http://ez.analog.com/message/32373#32373