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PCA9306: Use as analog limiter for high speed ADCs possible?

Robin Gruber1
Prodigy 30 points
Part Number: PCA9306
Other Parts Discussed in Thread: THS4561, , LSF0102, LSF0101

I am currently looking for a solution to limit the output voltage of a differential opamp (like the THS4561) supplied with 3,3 or 5V when feeding its output to a high speed ADC that accepts a maximum voltage of 1.8V only. After evaluating some topologies with transistors, I noticed, that the PCA9306 features exactly the same topology that I inteded to use. Since I don't assume that any switching circuitry is inside that may cause interference and usually an integrated solution is more convenient that the discrete solution, my quesion is: Although not intended for this use case, is it possible to use the PCA9306 for analog signal limiting when interfacing an ADC driver to an ADC?

Best regards and thanks in advance,

Robin

  • 0
    TI__Mastermind 48308 points

    Hi Robin,

    Do you have a block diagram to help describe what you are trying to accomplish here? 

    My first thoughts when you mention "analog signal limiting when interfacing an ADC driver to an ADC" is some sort of voltage translation while also having some sort of driver capability. 

    The PCA9306 is a passive device that simply level translates I2C signals (i.e. 3.3V SDA/SCL to 1.8V SDA/SCL via open drain driver). Signals pass through the device only when a logic LOW (open drain driver pulls to GND) is present. When the I2C bus is released, the respective voltages on each side of the device passively rise to their voltage rails via pull-up resistors, and the internal pass-FETs of the device become HIGH-Z separating the I2C bus internally. PCA9306 does not drive signals.

    Please help me to better understand this statement:

    Robin Gruber1 said:
    Although not intended for this use case, is it possible to use the PCA9306 for analog signal limiting when interfacing an ADC driver to an ADC?

    Regards,

    Tyler

  • +1
    Guru 317370 points

    The PCA9306/LSF0102 contains nothing but three MOSFETs (and ESD protection), as shown in the LSF appnote. It works as an analog switch for voltages below VREF1, and is an open switch for larger voltages. So it can indeed be used as a voltage limiter.

    (The LSF0101 has one less channel.)

  • 0 in reply to Tyler Townsend
    Prodigy 30 points

    Hi Tyler, sorry IT security forbids uploading of anything with a browser. But imagine this: A THS4561 supplied with 5V is fed through a PCA9306 (with additional kickback filter) to the input of an ADC that allows 1,8V on its inputs at max.