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ISO1228: Unused inputs

Maria Vittoria Pelosi
Prodigy 30 points

Part Number: ISO1228

Hello,

I am using ISO1228 receiver to read inputs from active-low proximity switches. Since I only need four input channels, I would like to know if the component will still work properly if I connect unused input pins to AVCC pin. Alternatively, I wonder if a single pull-up resistor for the four channels may be enough. I would also ask how the LED pins of unused channels must be connected. 

Thank you in advance for your attention,

Maria Vittoria

  • 0
    TI__Mastermind 34580 points

    Hello Maria, 

    Thanks for reaching out. ISO1228 will operate with only 4 inputs connected. It is preferred to leave unused INx and LEDx pins floating. There is no need to connect the inputs to AVCC or through a pull-up resistor. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 30 points

    Hello Andrew,

    Thanks for your answer.

    However, while testing our circuit with ISO1228, we observe that with only four input channels connected, output signals are wrong. We then connected the unused inputs to AVCC: in this configuration outputs are correct but we measure a residual voltage of 3V at input pins instead of 0V. Has this behaviour already been observed? How can we set up our configuration?

    Thank you,

    Maria Vittoria

  • 0 in reply to Maria Vittoria Pelosi
    TI__Mastermind 34580 points

    Hello Maria,

    As I said previously it should be okay to leave unused INx and LEDx pins floating. In this state the corresponding OUTx should be in the default state (low).  

    If you are seeing something else please attach a schematic of your design and waveforms showing the incorrect OUTx with the inactive INx.

    Best,

    Andrew