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SN74CB3Q3257: Value of Internal Isolation Resistance

Expert 7735 points

Replies: 7

Views: 50

Part Number: SN74CB3Q3257

Hello Guys,

Good day.

Our customer is using four SN74CB3Q3257PWG4 in their design. Pin 10 of the parts have been lifted and not connected to the PCB pad.  He then measure the PCB pad to VCC and it is open.  When he measure the lifted pin 10 to VCC, he got 324k. Is this normal? He is expecting resistance in Megaohms.

They are connecting measurement points through these switches to DMM inputs.  A pull up at the least alters the measurement.  At worst, is connecting VCC to o their measurement point through an effective resistance of 324k.

He noticed this problem because he is tasked with proving no pin has an unexpected voltage. The switch pins are at about 2.5V, which is because he have a resistor to ground across the DMM probes.

Thanks in advance!

Art

7 Replies

  • By the way attached is their schematic.

    Thanks!

    Art

  • In reply to Art Mecina:

    Hey Art,

    What/how are they measuring the 320k with? Just a multimeter probe? Has he measured the device not on the board? 
    There is a path from Vcc to the I/O pins via an internal ESD diode to Vcc connected to the signal path. Do they have means of testing this diode? Typically there should be a diode setting on a multimeter that should show a drop or confirm an existence of a diode or not. He is right in expecting megaohms on this though. 

    Additionally, just to clarify here, you're saying that the voltage on the supply lines are at 2.5V? More just looking for a confirmation that all the signals that are applied here fall within the recommended range.

    Thanks,
    Rami 

  • In reply to Rami Mooti1:

    Thanks for looking into this Rami!

    I will forward your inquiries to the customer and get back to you once I have the answers.

    Thanks!

    Art

  • In reply to Art Mecina:

    Hello Rami,

    I might direct the customer to this thread to explain his use case better.

    Art

  • In reply to Art Mecina:

    Hey Art,

    Any update on this? 

    Rami

  • In reply to Rami Mooti1:

    Hello,

    Apologies for the delay.  While the behavior of the part (324k isolation resistance measured from any switch pin to VCC) is curious, in the end this was not our problem.  Thank you.  Please close this case.

    Regards,

    Jeff

  • In reply to Rami Mooti1:

    Hello Rami,

    Thanks for looking into this. The customer advise to close this ticket now.

    Thanks a lot!

    Art

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