• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 16 subscribers
  • Views 8018 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.

CD74HC4067: channel off leakage

Yourij Bosco1
Intellectual 410 points
Part Number: CD74HC4067
Other Parts Discussed in Thread: SN74LV8151

The circuit fragment shown above comprises a multiplexer and a Schmitt trigger. The idea was to make a sort of sample & hold circuit. There can be a logical level "1" or "0" at the DD14:1.The address bus "SET5" switches quickly from 0 to 15 cyclically. The duration of connection condition between DD14:1 and any of I0...I15 is about 1 ms. Any capacitance connected to I0...I15 is quickly charged if OUTSET5 logical level is high.
After the address changes the capacitance discharged within several milliseconds to about 2 V. So the trigger DD10 can show a logical "1" or logical "0".

I don't see why and how the capacitances discharge so quickly. The CD74HC4067 datasheet doesn't give information about the IC inside circuitry.

Could somebody explain that fact described above?

  • 0
    TI__Intellectual 2900 points
    Hi Yourij,

    I am not aware that CD74HC4067 can help discharge the output voltage on your loading cap. I would propose two experiments that might help to isolate the problem.


    1. Remove C45. capture the waveform on I1.
    If it stops discharging on I1, it has something to do with your 1000pF capacitor. Otherwise, it's other issues.

    2. Remove SN74LV8151. capture the waveform on I1.
    If it stops discharging on I1, it has something to do with SN74LV8151 IO structure. Otherwise, it's other issues.


    Please let me know the experiment results and we can go from there.
  • 0 in reply to Fan.Wang
    Intellectual 410 points

    Hi!

    I can do it some time later. Not now, sorry.

    But meanwhile I researched the problem from another side and I found from the datasheet that the leakage current could be as much as 8 uA. This fact explains the behavior of the circuit.

    The datasheet doesn't answer why CD4067 has only 100 nA or less leakage current and CD74HC4067 has up to 8uA! There is something wrong inside the chip circuitry.

  • 0 in reply to Yourij Bosco1
    TI__Intellectual 2900 points
    Hi Yourij,

    It is a good observation. The CD74HC4067 family and CD4067 family are targeting two different use cases. You can easily tell by the VDD range, power consumption, bandwidth and 1ku price.

    CD74HC4067 is mostly used in more digial signal driven use cases where speed, power consumption and price are some key considerations.

    CD4067 is more industrial or data acquisition applications where leakage current is a key factor while price or power consumption is not quite sensitive.