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AD75019 ICs work consistently as expected at 4Mhz but inconsistently at lower frequencies.

AD75019 IC , The VCC is +12V, VSS = -12V , VDD = +5V, DGND = 0V,

Pclk , Sclk and Sin are driven from an FPGA . There is a voltage translator which converts the FPGA signals at 3.3V to 5V signals.

We have been using this IC in our project for several years now, with the same configuration and have been running the control signals at 2Mhz. 

In the latest set of PCBs that we got from the manufacturer, We saw that the ICs didn't work consistently at 2Mhz. By this I mean, 1 out 100 times all the Switches would turn on on one IC and none of the switches would turn on on another IC. I verified that the protocol and bit selection is correct. The signal looks unchanged when the switches configure correctly vs when they dont.

Suspecting timing issues I reduced the clock frequency to 1Mhz. The inconsistency was more pronounced now. It failed to configure correct on every other run.

I increased the frequency to 4Mhz and the inconsistency went away. 

The rise time remains unchanged at apprx 2ns with varying frequency.. I have attached the scope traces at 1Mhz.  Please advice on what might be cause of this issue? 

In the scope trace, blue is SIN, Green is SCLK, yellow is Pclk, purple is DGND, Pink is VCC

 

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  • Hi newEngineer, 

    This is a strange one.  I am not sure what the cause maybe.  But here are some ideas.  Did you try slowing the clock way down to see does the failing switch occur in a very simplified case? Another check to do is go around all the pins on the switch with a DMM and check the voltage levels and make sure they are what they should be.  We have seen issues with pcbs and floating pins in the past with some customer builds, e.g. ground pins not being ground.  A thought that comes to mind is its possible a higher frequency signal might get over some parasitic series capacitance problems better than a lower speed/DC signal which would be blocked.  Maybe there is something in that to give us a clue what's going on.  

    Eric.

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  • Hi newEngineer, 

    This is a strange one.  I am not sure what the cause maybe.  But here are some ideas.  Did you try slowing the clock way down to see does the failing switch occur in a very simplified case? Another check to do is go around all the pins on the switch with a DMM and check the voltage levels and make sure they are what they should be.  We have seen issues with pcbs and floating pins in the past with some customer builds, e.g. ground pins not being ground.  A thought that comes to mind is its possible a higher frequency signal might get over some parasitic series capacitance problems better than a lower speed/DC signal which would be blocked.  Maybe there is something in that to give us a clue what's going on.  

    Eric.

Children
  • Hi Eric, 

    The voltages seem fine on all the pins. they are as expected. Another recent observation we had is that at lower frequency when we put a probe on The Y8 pin which is under use, the inconsistent switching goes away. Is there any impedance requirement for the output pins for the switch to work consistently? 

    Thanks