Transform a sinusoidal wave into a square wave with op-amp
simulate this circuit &ndash; Schematic created using CircuitLab

There is one basic DM modulator circuit from textbooks that you might be looking for.

Low pass filter band limits the input signal.

Clock determines the sampling interval.

Comparator is used to compare with previous sample and to produce a '1' or '0'. 

Staircase generator is basically an up/down counter that feeds a DAC.

DAC converts the digital output from staircase generator to analog for comparison with current value. DAC resolution determines the step size.

Sampling interval and step size are two vital design aspects that determines the precision of the system; possible slope overload distortion/ granular noise.