DAC161S997: DAC connection issues

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

Hi Chetan,

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

Thanks,

Paul

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

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

Any updates?

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

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?

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 

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.