PWM output and ADC sampling on input have two things ... which means a higher frequency AC signal, but in the ADC case we’re interested in the DC component of a low-frequency signal.

It precisely maintains a constant output current (Iout) by forcing the OUTPUT pin to ... Figure 3 shows the idea fleshed out into a complete PWM-controlled 18 V, 1 A grounded-load negative current ...

To get these currents into a workpiece, [bwang] used a TL494 PWM controller as an oscillator. The output of the TL494 is filtered and amplified a few times to generate a huge amount of AC current.