To ensure that the circuit can be used universally, the power and data transmission were isolated. There are many reasons to isolate circuits, most notably to avoid ground loops, eliminate conducted noise in electronic circuits and protect from high volt voltage areas. There is no common mode error also on high-side current sensing. The ADC HX711 offers the advantage that no data input signal is necessary. This simplifies the interface, but despite this it is possible to select the input channel and gain.

Variant 1:
Isolate power supply with transformer and data/clock transmission with optocoupler
The advantage: coupling with low capacitance (several picofarad), the best choice for high voltage, such as mains voltage.
 

Variant 2:
Isolate power and data transmission with capacitors
Advantage: no special components necessary like optocoupler and transformer. 
Suitable for low voltages up to 40V. There are high voltage capacitors available, but if there occur transients (e.g. switching off and on), the circuit can be damaged because of the high coupling capacitance (about 100nF).

A full description can be found in the Attached pdf.