• TI Thinks Resolved

DAC161S997: DAC connection issues

Prodigy 120 points

Replies: 11

Views: 101

Part Number: DAC161S997

Hello, 

I have an issue with DAC161S997 circuit , when i tried to power up the circuit  transistor Q1 is fully on and huge amount of current flowing the it

I have powered DAC with internal 3.3V regulator and 24V for loop supply from the board itself. 

So anything i have missed ?

  • Hi Chetan,

    It is important to ensure that the ground net (COMA/COMD) is not connected to the LOOP–.  You must ensure that the LOOP supply can be a "floating" supply, in that its LOOP– can be a lower potential than GND.

    If they are shorted together, then the DAC161 cannot regulate the current.

    Thanks,

    Paul

  • In reply to Paul_Frost:

    Thanks Paul for the reply,

    So if i m using 24V internal supply for current loop , i must keep IC ground and Loop- Should be in different level ?

    Regards,

    Chetan

  • In reply to chetan karkera:

    Hi Chetan,

    You are correct, LOOP- will always be a lower potential than COMA/COMD.  

    Thanks,

    Paul

  • In reply to Paul_Frost:

    Hello Paul,

    Tested current loop(-) with different ground still the same issue. I am not able to understand why output voltage at the base is high at around 980mV , Because of the is external transistor is alws ON

    Regards,

    Chetan

  • In reply to chetan karkera:

    Hi Chetan,

    Let's check some basics.  First, are AGND and DGND connected somewhere in the design?

    What is the voltage difference between COMA and OUT? OUT must be negative with reference to COMA.

    Can you draw a figure showing how your power supplies are connected? Where you measuring the loop current?

    Thanks,

    Paul

  • In reply to Paul_Frost:

    Any updates?

  • In reply to Paul_Frost:

    Hello Paul,

    Are AGND and DGND connected somewhere in the design? - Yes, connected to other analog and digital circuit grounds

    What is the voltage difference between COMA and OUT? OUT must be negative with reference to COMA - Yes, it is below COMA

    We are measuring the out at the 250ohm external resistor as shown in the below diagram

  • In reply to chetan karkera:

    Hi Chetan,

    So from this figure, I can assume your COMA and COMD are not connected to the "common" ground signals? The grounds of the system (COMA/D) are completely floating?

  • In reply to Paul_Frost:

    Hello Paul,

    No, they are all tied together with common , in fact we have tried different loop supply(seperate to system supply) to BJT to get 4-20mA output. But still there is no improvement 

  • In reply to chetan karkera:

    Hi Chetan,

    I suspect that is your problem.  The 24V cannot be connected to the DAC161S997's COMA/D pins.  The grounds must be separate.