Question
I work on a AD420 based application. I wish to have a current and a voltage
output with the same device. I made a prototype with your AD420, the current
output works very well. But with the voltage output, I don't have a good
linearity. I use optocoupler with the SPI bus, the range select pins are
connect to ground, a regulator 5V is connected to Vll, Vcc = 24V, I use trimmer
on REF IN and OFFSET TRIM and I disconnect Iout and Boost. CAP1 and CAP2 are
10nF. I try to adjust trimmer but I have 40mV instead of 0V and 4.996V instead
of 5V. The middle scale is well. Is there any issue? T
Answer
The output resistor is not precision so only Iout is guaranteed, Vout can have
quite a large tolerance.
There are some new parts due to release in early to mid 2008 which will be a
great improvement on the AD420 (better accuracy, higher loop voltage etc).
The resistance value has never been guaranteed for the AD420 and there has
always been the possibility that the value could change with random process
variations. Since the resistance isn't tested, these parts are shipped as good
parts. The voltage output has always been an auxilliary function to the AD420,
the only guanrateed accurate output is the current output.