Why won't my comparator output high?

The most likely reason your comparator won’t output high properly is that it has an open collector or open drain output scheme. As an example, take a look at the functional block diagram in the datasheet of AS393AMTR-G1DITR-ND.

This device has an open collector output, which requires a pull-up resistor at its output to be able to output a high state (the same is true if it were open drain). Other integrated circuit outputs for devices other than comparators can have the same open collector/drain style, always check a part’s datasheet for details.

For more detailed explanations of open collector and open drain see:

What does “open collector” mean?
What does “open drain” mean?

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