Hardware Failure on DAC 1220 - Pin C1always equal to 0V

Hi Sylvain,

We're on break for the Thanksgiving Holiday here in the US through Monday next week.  We'll get back to you on this question as soon as possible.

Hi Sylvain,

Have you checked the crystal oscillator/clock to make sure that it is active?  Do you have a low impedance connection between the analog and digital grounds?  Is it possible for you to show us your schematic?

Sometimes solder connections can degrade from thermal cycling.  Have you looked at all your solder connections and voltages besides C1 and C2?

Best regards,

Bob B

Hi Bob,

Thanks for your help.

Unfortunately, I wasn't able to investigate this defect further. I had to repair the board very quickly in order to send it back to our customer.                                                   (I replaced the defective DAC by a new one).  I was wondering if the DAC broke down because the Digital Power supply might have been, at some point,  higher than the Analog Power supply.

Right now, I am also encountering another defect with 2 other electronic boards, that have the same design.  There is, on each boards, two DACs 1220. On each board, there is one defective DAC and a good one. The output Vout of the defective chips seems to be "locked".

As soon as the defective DAC chips are powered, they generate a 2.782V voltage on Vout.

I send a command in order to send the defective DAC in Sleep mode, Vout goes to 0V.

I send a command in order to send the DAC in Normal mode again, Vout goes back to  2.782V.

I send a command in order to set a new voltage on Vout, No effect.

I recalibrate the DAC, Vout goes to 2.78301V .

I send another command in order to re-calibrate the DAC. Vout goes back to 2.782V .

I tried to change the capacitors: no effect.

If I change the Defective DAC: the Board works fine again.

During all the tests:

- The clock Xin is active, I also have a clock on Xout.

- The communication between our microcontroller and the defective DACwas active (signals SCLK, SDIO, CS)

-  Voltage on pins Vref, C1 and C2 were0830.DAC design.pdf correct.

N.B.: in our design, the clock Xin is supplied by the microcontroller.

Sylvain,

Thanks for the further info and schematic.  I find your power scheme rather interesting.  Is there a particular reason you have separated the digital supply for each DAC1220?  Is there a common component that is showing a problem in each case? 

I see that you have an inductance between the grounds, with schottky diodes to keep the grounds at the correct voltages, but I don't see that same type of thing between AVDD and DVDD.  When you separate the supplies, you really should have a schottky diode here as well.  See Figure 6 on 8 of the datasheet.  Perhaps you have already seen that figure, and that is why you ask the question about the digital supply.

Transients can be a real problem if the voltage is drop occurs across the inductance.  Trace inductance can be as big a problem as the inductors themselves. I'm guessing you have a problem where a transient is loading the inductor so that DVDD is higher than the AVDD by more than 300mV and this is damaging the DAC1220.

Best regards,

Bob B

Thanks,

I always suspected that the missing schottky diodes  (between AVdd and DVdd) could be the cause of the failure.

I need to tell you, however, that the schottky diodes (D1 and D2) used to separate the grounds, have been shorted with 0Ohm resistors. In our design, the Analog and Digital Ground are at the same voltage.

Since it is a very old design, I have no idea why the DAC have different digital power supplies.

Best Regards,

Sylvain