# Inverting, single-supply comparator not working as expected

I am trying to convert an input sine wave to a square wave. The sine wave is centered at 2.5V and has an amplitude of 2V, with a frequency of 100kHz.

I am looking for a hysteresis of 100mV centered around 2.5V. (ie. 50mV below and 50mV above 2.5V)

I am trying to achieve something like the following (except that the output is inverted), where both waves are centered about 2.5V:

To do so I looked through Design with Operational Amplifiers and Analog ICs, and I thought that I could work with a VTC offsetting single-supply inverting Schmitt trigger.

I then calculated the resistance values according to my parameters.

VTH = 2.55V
VTL = 2.45V
VCC = 5V


Assuming R3 = 100kΩ and R4 = 2.2kΩ, I calculated R1 and R2 both being 4.1kΩ`.

I proceeded to construct the circuitry on Proteus software using the LM311 comparator.

And here is my output.

Yellow is Channel A, Blue is Channel B. I zoomed in on the square wave can be seen from the dials.

The output is not only attenuated, but also unevenly centered. The book mentions none of these effects, so I suspect that I am missing something more fundamental here. Any suggestions on what I am doing wrong would be appreciated.

• You have both scope channels set for AC coupling - that will remove any DC component from the signal, so you won't know where the signal is relative to Ground. The two vertical position sliders have different non-zero settings which will also change the vertical position of the traces. – Peter Bennett Nov 14 '18 at 17:28
• Also you have not said what the input signal level is not have you said whether it is adequately biased at 2.5 volts. – Andy aka Nov 14 '18 at 17:43
• Thanks, updated images of oscilloscope. @Andyaka, input signal level is adequately biased at 2.5V, with an amplitude of 2V. – Rrz0 Nov 14 '18 at 17:45
• LM311 has enormous gain, and large output-stage current surges during transitions. You may have oscillations. Use a Ground plane. Use a very small package, so inductance in the VDD and GND pins are minimal. – analogsystemsrf Nov 15 '18 at 3:56