MSP430 --> AD421 DAC current loop

I hope someone with some experience with the AD421 can point us forward with a problem we have.

We've got an MSP430F149 driving an AD421 16 bit DAC as a 4/20mA current loop driver. The AD421 is isolated using H11L1M optocouplers, the F149 is battery powered and completely isolated from the AD421, we have used the circuit recommended in the data sheet.

The problem is that the output current is offset and non linear at the low end:

Data Current

---- -------

00000 10mA Should be 4mA 26000 10mA+ O/P current ramps up ok from here, proportional to data. 65535 20mA

Apologies for the length of this post, I figure the more info the better.

Some things we have tried / observed:

1) The data/clk/latch signals look fine on the CRO. This is backed up to some extent by the circuit operating ok above about 26000 counts.

2) LV pin - set the AD421 voltage to 3/3.3/5V without any positive effect - we tried the data sheet 16k resistor non-linearity fix for 3V operation - didn't fix.

3) I've checked the clk/data/latch line rise times and tweaked the opto pullup resistors etc to get the times

Hi Jeff, tried with loop supply between 9 --> 20 Vdc with no effect, loop termination/sense resistor is either R=100 ohms or just an ammeter.

It's not a stupid question, I reckon we've all been caught by things like that one or twice.... regards rob