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LM293: coupling between positive inputs

Cornelius Kress
Prodigy 10 points
Part Number: LM293

Tool/software:

Hi Team,

Description: I am using the LM293 comparator as part of a two-stage digital input circuit. Each input stage consists of a separate comparator and is intended to function independently. The outputs of the comparators are connected to a GPIO pin of a microcontroller.

I have the following issue: When I apply a 5V input signal to Channel 1, I observe a capacitive voltage spike at Channel 2 (positive input of the comparator), even though Channel 2 is not connected. What is the source of this coupling? Is internal coupling of the positive inputs possible?

conditions: 

- IN_CH1: 5V

- IN_Ch2: NC

- VCC: 12V

- Vref: ca. 2V

The oscilloscope picture shows the behaviour.

oscilloscope:

- IN_CH1: 5V, IN_CH2: NC

- RED (positiv input comparator 1)

- BLUE (positiv input comparator 2)


In my expectation, there should be no coupling between the positive inputs.

I'm looking forward to hearing from you.

Thank you, best regards,

  • 0
    Guru 290400 points

    What exactly do you mean with "not connected"? The input should never be left floating. Is R11 present?

    The schematic does not show any decoupling capacitors.

    Please show the board layout.

  • 0
    TI__Guru 54446 points

    Hello Cornelius,

    From the schematic - there should not be any coupling. You could remove the comparators to verify that they are not the source.

    As Clemens mentioned, is there a bypass capacitor on the comparator supply AND the pull-up voltage?

    I am assuming that the IN2 node is floating, but R11 and D2 are present.

    What it really looks like is capacitive coupling between the input lines IN1 & IN2.

    IN1 has a fast 5V edge and can easily couple into the ~33k input impedance of IN2 through a few pF of stray capacitance.

    Are the input traces running in parallel together? Or in a cable? This looks like a layout or routing issue.

  • 0 in reply to Paul Grohe
    Prodigy 10 points

    Thanks for your reply. Your right it's a layout issue. The LM293 working as expected.