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LM393: Properly terminating an unused op amp.

SWAROOP R SADARJOSHI
Intellectual 725 points
Part Number: LM393


Hi all,

I am employing TI's LM393 operational amplifier in my custom PCB. I have few questions regarding the design.

I am employing the LM393 as a speed sensor as shown in below circuit: 

as shown in the above circuit I am employing only one op amp from LM393.

My queries as follows:

Q1. What is the best practice to terminate an unused op amp to reduce noise, excess power consumption?

Q2. As shown in the circuit above if the source for inverting (- pin number 6) and non inverting terminals (- pin number 5) of the LM393 are far away (2 meters away from LM393) what impact will it have on the performance of the LM393? What precautions should I take?

thanks and regards

  • 0
    TI__Guru 61746 points
    Hello Swaroop,

    For clarity - the LM393 is a comparator - not an op-amp. Very different when it comes to dealing with unused inputs.

    As to your questions:

    Q1 - the inputs should be tied to different voltages. Contrary to what some datasheets state, do NOT tie the inputs together or to ground. This can cause oscillations when if/when the internal offset crosses through zero. The voltages should be stable and within the operating range of the inputs. For your circuit, you could ground one input and tie the other input to either the divider reference voltage or the cathode of the LED. Leave the output floating - do not ground or tie it to a supply.

    Q2 - The inputs are high impedance. Long input lines would pick up noise from the surrounding environment. Depending on what is around, you may get false triggers when large loads turn on.

    Of course, for best results, the comparator and the sensor should be together. It is always more difficult to send small signals long distances than to send large signals (the output) long distances.

    If you must place the 2 meters away, I would use a coaxial cable (RG-174 or similar) to provide shielding. You do not need to place the 10k divider resistors at the sensor. At minimum, twist the sensors emitter and collector wires together to minimize noise pickup.

    You have a large 1uF cap on the input which will slow things down. There is not much you can do with 2 meters of cable that would affect the response time more than adding that cap (the 1uF cap swamps out any cable capacitance).

    What is your supply voltage? The inputs must remain at least 1.5V below VCC.