Hello. I am consulting for a client based in US. We have designed a device for them that is based on a D/A converter (AD5420), which is supposed to output a 4~20 mA signal to a variable valve to regulate pressure and flow.
Electronically, I am using a FT4232H USB to SPI converter to send data via the PC's USB port to the AD5420. With this setup, I have been able to successfully communicate with the AD5420, issue it a RESET command and write data to its internal Data registers, and see a respective current output from the DAC.
The present problem I am facing is that the /FAULT line activates, and upon reading the Status register, I see a code of 7 (decimal) - indicating :
a) Over temperature,
b) Slew-rate in progress,
c) IOut fault.
The circuit is built AS indicated in the Application notes and AD5420 datasheet - no change, except that I am presently not using an External Rset resistor. The Control register has thus been programmed to use the internal Rset resistor.
The AVdd line of the D/A is powered by 12V linear supply (the datasheet specifies a range from 10.8V to 20+ volts). So that is in spec too.
On the output, i am using a PBSS8110Z NPN transistor with a Breakdown voltage of 100V, so that is in spec too. After that, I am using a 100 Ohm current shunt resistor from the Output to Ground, and measuring signal in Volts on a DMM.
Can you please help me ascertain what might the problem be? The product is almost in the finalization stages and BOM is to be ordered.
Thank you for your help.