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PCA9306: The value for stray capacitor

Part Number: PCA9306

Hello all,

I want to use the I2C voltage level translator,
For pull up resistor, I can understand the value should be determined by rise and stray capacitor.
I want to ask how to calculate value for capacitor or how to measure?
Because this capacitor is from PCB, layout, etc. I can't know the exactly value.

  • Hi James,

    If you are talking about your bus capacitance this value is made up from multiple sources:

    1. First you have the capacitance of your actual bus pins. These are the SDA/SCL pins of each device that connect to the bus. Each device should have a value in it's datasheet that tells you expected capacitance that it will add to the bus.
    2. Next you have the capacitance of your trace/cable. Depending on how you connect all your I2C devices to each other this could be a mix of both.
      1. For trace capacitance you need multiple details of your trace in order to get a very accurate measurement,
      2. I assume this is what you mean when you talk about stray capacitance. However, these values are hard to get especially if you are not designing your PCB so a good estimate is 0.8pF per cm for a narrow trace. Copper has a capacitance of roughly 20 pF per cm^2 and each via is around 0.5 pF.
      3. Now if your trace goes into a cable you will have to find out the capacitance of your cable per meter. This can vary greatly from cable to cable so I would check with the manufacturer of your cable. Most likely any connectors on your cable will also have their own added capacitance.
    3. Lastly if you add any capacitance in the form of filtering you will have to add this in as well.

    This should be all the sources of any capacitance that can be found on an I2C bus. These all have to be taken into account to make sure you are not exceed I2C spec.

    Best,

    Chris

  • Thanks your apply. Those information is helpful for me.