AD5422 'FAULT pin' problem

Hello,

 

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

Thanks

  • 0
    •  Analog Employees 
    on Mar 6, 2012 1:31 PM

    Hi,

    I am currently looking into this issue and will reply when I've some data. 

    Does the Fault Data register show the same response as the Fault pin?

    Regards,

    Michael Collins

    PDA Applications

  • Hi,

     

    I have performed the tests that you have mentioned and these are the results:

     

    When I read back the information from the DACs I get exactly the same information as the external FAULT LED is showing (except in case of toggling)

     

    I found that till 13.80V power supply DAC operates correctly, but after exceeding 13.80V  and depending on DAC output current values fault status will start toggling first and then  after increasing power supply voltage  will stop fault pin functioning.

    I have attached Excel file with test results. In testing I have tested two cases 1) without connecting DAC channel to outputs. 2) Connecting with DMM for measuring current on channels for both DACs side by side. I found that both DAC have small voltage difference in fail cases.

    Let me know if you want me to perform any more test.

    Note: In design, I am using two DAC in daisy chain connection and fault pin is connected to active low LED

    Regards,

    Pathik Shah

    AD5422Test_Conlab1.xls
  • 0
    •  Analog Employees 
    on Mar 8, 2012 4:20 PM

    Hi,

    Thank you for the data.  I've tried the same sequence on my set-up here an not observing any issues.

    Can you ensure the following.  DVcc = 3V.  This is DVcc supply for the AD5422.  Also ensure that the

    pull-up resistor on the Fault output pin goes to this same potential.  Also can you ensure that DCC Select

    (Pin 16 of the TSSOP) is connected to GND.  If you can send on a snapshot of the /Fault signal as well,

    showing the levels and also showing one of the toggle plots, along with DVcc it may point to the reason

    for the issue you are seeing. 

    Also in your daisy chain operation where you are connecting both /FAULT pins together I assume you have

    a single 10k pull-up resistor?

    Feel free to contact me directly at michael.collins@analog.com

    Regards,

    Michael (PDA Applications)