When a sine wave is converted into a square wave by a voltage comparator (operational amplifier), why is the upper and lower amplitude of the output square wave asymmetrical?
This is caused by the improper setting of the reference voltage of the comparator. Adjusting the reference voltage can change the duty cycle.
In fact, the simplest method is to use an inverter to shape, such as CD4069. Sine wave signal will be converted into positive square wave with good quality as long as it is inverted twice at most.