DAC8760EVM: I-out current range out of spec.

Hi Muhammad,

Welcome to the e2e forums!

Please verify the state of all jumpers on the board.  Do you have anything connected to JP2 or connector J4? If the RSET resistor has another value in parallel with it, then the output will be skewed.  Can you confirm the voltage you are using for the AVDD supply?  What is the voltage of the reference (REFIN, test point VREF)?  Also, confirm that your meter does not have any parallel resistance when you do the measurement, as that would skew your current value.

Thanks,

Paul

Hi Paul,

Thanks for looking in to this problem. So my test conditions are as following:

AVDD = 15VDC (tried 12V and 24V didnt make any difference)

VREF = 4.995V 

JP2 shunt completely removed (HART not being used)

JP6 inserted so the 300ohm resister is in parallel to JP5. Meter used is a Fluke 87 and no external resistors attached to the probes.

I also noticed that if I set the meter to current measurement, connect the probes across J5 (basically creating a dead short) I am able to produce 4-20mA accurately so I am not sure if there is some special settings that I need on the board or in the GUI?

Regards,

Muhammad. 

Hi Muhammad,

Just to confirm, when you monitor the voltage across R4, is JP5 populated or not?

Can you try both combinations (JP5 open and shorted)?  Please verify that all the solder joints around the boost components look good (Q1, R1, R2).

Can you also remove JP6 and confirm the resistance of R4 with your meter?

Thanks,

Paul

Any updates?

Hi Paul,

My comment in red.

Just to confirm, when you monitor the voltage across R4, is JP5 populated or not? Yes JP5 is inserted

Can you try both combinations (JP5 open and shorted)?  Please verify that all the solder joints around the boost components look good (Q1, R1, R2). The measurements I gave you were with JP5 inserted, if I remove it i.e. using the external FET, the current goes down and that causes the voltage to go down as well.

Can you also remove JP6 and confirm the resistance of R4 with your meter? It measures 299.5 ohms.

Regards,

Muhammad

Hi Muhammad,

Has this EVM been modified in any way? Is it new?

Given that using the meter as the shunt to ground (across J5) seems to work with JP5 shorted, I suspect that there is something wrong with the boost circuitry.  Can you check the solder joints around R1 and Q1? With JP5 open, can you measure the voltage on the left side of R2 (the IOUT pin)? If there is some kind of open path, then this voltage should probably be hitting the compliance limit (~AVDD-2V).

Thanks,

Paul

Hi Paul,

So the board is in original condition hasn't been modified. I removed JP5, measured the voltage on the left side of R2 i.e. base of Q1/Iout it measures 3.1v. I also noticed that if I remove JP6 (remove the 300ohm load), the voltage on the base of Q1 goes to 4.484v, why would this voltage change... why should this voltage change? Also if I touch the DAC8760 IC, it feels warm I am now thinking that the controller is internally damaged. 

Regards,

Muhammad.

Hi Muhammad,

If you remove JP6, then there is no more current path to ground, so the output should rise to the compliance voltage.  In fact, I would expect it to approach the AVDD supply, which you said was 12-24V - is that still correct? Are you using a 5V supply for AVDD?

Paul,

I am using a 15V as my AVDD and thats what I was also expecting to see but it stays at 4.484V. if you have a known working board can you please verify these values on your end as well?

Muhammad.

Hi Paul, any updates?

Muhammad,

Were you able to make any of these measurements or otherwise make progress on debugging this? May we request an update?

Muhammad,

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