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LM293: LM293A

Part Number: LM293

I saw someone use one series RC cross positive input and output of LM293. Usually, it composes an integral circuit if RC is cross the negative input and output of one OPA. However, I do not know why the similar circuit is used in comparator. Could you give me some comments?


  • Hello Liming,

    This is AC Hysteresis (positive feedback) and is quite common in high frequency/fast comparator applications.

    It works similarly to the resistor type feedback, but only works for a short time. Normally AC hysteresis is used where a shift in the DC level is not acceptable and hysteresis is only needed for a short time. It is commonly seen in zero crossing, clock buffers or other continuous AC signals.

    The capacitor passes the fast edge of the comparator digital output, which combined with the input series resistance, creates a brief "kick" to the positive input signal, briefly moving it away from the threshold and stopping any further chatter/oscillation.

    The amount and duration of the "kick" is dependent in the series resistance and the size of the capacitor (RC time constant).

    Our friends at TINA have a blog about it:

    Typically the capacitor is small, just a few pF. In order for it to work, there MUST be series resistance on the positive input for the cap to "work against". Theoretically, the "kick" amplitude will be the P-P output of the comparator, so a resistance in series with the cap is used to attenuate or set the actual amplitude. Normally the kick is just a few mV.

  • Hi Paul,

      Thanks a lot for the nice answers.