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SN74HC4851 Analog MUX reading problems

Hai,

      Iam using  8 channel analog MUX SN74HC4851, in a temperature reading application.

Here there are 24 NTC thermister to read with one MCU analog pin , using  3 MUX.

In my circuit all channel selection pins and common  output pin s are connected parallel, and MUX selection is done by ENABLE pin separately, and read all MUX sequently.

     My problem is , suppose channel 1 of MUX 1 is HIGH, and all other channels LOW practically..But corresponding pin of MUX 2 also shows high.I use proper delay and all other precautions, but no use. Individual MUX gives good values in all  channels

Please give me the error possibilities.

  • Here is my schimatic.

    Here , output signal conditioning , MCU and thermister circutary are omitted.

    All MUX pins are connected together

  • Hi Binu ,

    you see correct values on individual Mux when connected separately but once connected in parallel you see the issue ?
    did you measure the Enable signals on each of the Mux individually and when connected in parallel ,any differences observed ? seems that either connecting in parallel enabling the other Mux too or signal is coupling from output to input being in close proximity .
  • Hai,
    The Enable pin function properly.
    My application is NTC thermister reading. Yesterday I found one more thing that, when the temperature reading is high in a channel, the succeeding channel in the same MUX shows the near value with respect to the previous.
    This time, I gave a 100 us delay. After that I increased the delay to 1 ms and it shows even less value compared with the previous. I tested with different values, and when I tested with 200 ms delay it shows perfect individual values, in my application.
    But according to the datasheet , all maximum switching delays are in nanoseconds!.
  • Hi Binu ,

    Does that mean the root cause was the delay you provide in the software to read the thermistor values ?
    The nano seconds in the datasheet is for each device , but does not apply for multiple devices connected together and doesn't account for the software overhead involved . I suspect there could be a setup issue with individual Mux working however when connected together is causing issues. Disconnecting one Mux at a time can be tried if it improves the situation as a debug step .

  • Hai ShreyasRao,
    The same crosstalk issue exist even in a single MUX .
    I cant point out the issue, if anybody else the same issue faced?
  • Hi Binu ,

    I haven't seen any questions and or issues come in for this device in the recent past . You can also search in the e2e database if you find anything any clue regarding your issue seen by others .
    I believed that the single device worked without issues as mentioned by you in the initial post. The crosstalk could relate back to layout where traces are close to each other which results in coupling of the signals . running them perpendicular to each other or having sufficient distance for isolation can reduce it to a large extent .