We are using AD5422 DAC IC in an industrial control application and we are currently experience a problem with this particular IC.
It is about the FAULT pin/function of the IC - pin 3 for the TSSOP-24 package, which we are using.
What we are observing is when at current mode and nothing connected to the output terminal:
CASE 1 - CORRECT!
FAULT pin = LOW (we have a 10k pull up resistor and an LED for indication, connected to DVdd) therefore the LED is glowing, which is correct!
11V <= AVdd <= 15V
AVss = 0V
DVcc = 3.3V
If data is send to Iout (pin 19 for TSSOP), it shows correct current output!
CASE 2 - ERROR?
FAULTpin = HIGH and the LED is not glowing as per expected?
AVdd > 15V
AVss = 0V
DVdd = 3V
If data is send to Iout (pin 19 for TSSOP), it shows correct current output! Only error is that if the output is not connected (simulating open-circuit) the FAULT pin will not go LOW, it will remain HIGH, therefore the LED is not glowing.
The problem is that the FAULT pin will not go LOW when the Iout terminal is unconnected when the AVdd is above 15V. We have tested several different boards and different firmware and all show same problem.
I have eliminated possible problem with the protection circuitry that I have on the current outputs (which are per Analog Devices datasheets!) by removing these components and retested it again - shows the same symptoms!
I am using clean power supply (no noise) for AVdd and DVdd.
The IC has separate Power and Ground Planes (4 layer PCB) and all signals run over its corresponding planes and no overlapping of different planes or signals.
Any help will be greatly appreciated