• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 46 subscribers
  • Views 695 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DAC8563: EMC/EMI/RFI Issues

Alan Wilson1
Prodigy 20 points
Part Number: DAC8563

I have a new sensor design using the DAC8563 chip.  The DAC output signals (VoutA, VoutB) are fine until I begin the RFI/EMC immunity tests. 

When the DUT is subjected to an RF power of 10V/meter, the DAC output voltages shift.  During an RF sweep of 80 - 250 Mhz, this voltage shift shows up at various frequencies & amplitudes.  The worst-case shift error is as much as 35 millivolts at 123 Mhz..  Other analog  and digital circuits on the PCB do not seem to be affected by the RF field.  The power supplies and Vref output pin are steady & clean.  

I have tried various PCB and cabinet grounding configurations, ferrite beads, shielding etc., but the voltage error is still present.  This problem looks like the RF rectification that sometimes shows up in op-amps and instrumentation amps when subjected to an RF field. 

Has anybody seen this problem with a DAC?  

Thanks, Alan                  

  • 0
    TI__Genius 14220 points

    Hi Alan,

    Welcome to E2E and thank you for your query. The DAC outputs are more susceptible to RF noise probably because they might have cables that might be acting as antennas. Ferrite beads work at the frequency range you have indicated but they circuitry, component selection, and the component positioning are very important for proper common-mode filtering. Is it possible for you to share the DAC part of the schematics, especially the output circuitry, power-supply connections etc? 

    Regards,

    Uttam Sahu

    Applications Engineer, Precision DAC

  • 0 in reply to Uttam Sahu
    Prodigy 20 points

    Hello Uttam,

    I have tried Ferrite beads on the I/O cables and single wires, metal shields, bypass capacitors, etc.  None of these actions had any effect on the voltage-shift error at the DAC output pin. 

    I tried adding a 3.3K ohm resistor to the VoutB DAC output pin to GND;  this dropped the shift error in half ! 

    As per our schematic, the output resistance to GND is 11K ohm (RN23).  Adding the 3.3K resistor dropped the effective resistance to 2.54K ohms.  Apparently, the DAC8563 is susceptible to RFI with a light load.  I removed the 3.3K ohm resistor, and added a 120 pF capacitor (refer to the schematics).  This completely removed the voltage-shift error while in an RF field. 

    The DAC8563 data sheet specifies good output stability with up to a .003 uF / 2K ohm load.  I believe adding this 120pF cap with the 11K ohm resistance of RN23 will not cause problems.  I plan to test for stability issues, and will let you know what I find.

    Thanks for you help,  Alan          

  • 0 in reply to Alan Wilson1
    TI__Genius 14220 points

    Thanks for the update. Closing the thread now.

    Regards,

    Uttam