Differential measurement circuit, and balance with force compensation

A differential measurement circuit for a balance with electromagnetic force compensation having a compensation coil, the circuit being arranged to receive, as inputs, two photo currents (I1, I2) generated by two photodiodes (D1, D2) and to generate from the inputs an output signal that is proportional to a difference between the photo currents, the circuit comprising:

• a switch (SW), arranged to be flipped between two states (zt1, zt2) within two phases (t1, t2) of a time period (T) to control the flow of current through a node (KΔ) to which the two photo currents (I1, I2) are directed,
• wherein the switch (SW) is further arranged to be controlled such that a reference current (IRef) delivered to the switch from a voltage or current source (URef) is superimposed alternatingly within the time phases (t1, t2) on one of the two photo currents (I1, I2) which continuously flow into the node (KΔ);
• and
• wherein the node (KΔ) lies at an input of an integrator (INT), which generates an integrator signal (sINT) that is compared, as an input signal to a comparator (CMP) to a cyclically recurring ramp signal (sRAMP) which conforms to the time period (T), such that the comparator generates as an output thereof a rectangular-shaped pulse-width modulated signal (sPWM), a duty cycle ratio of which is defined by the intersection of the integrator signal (sINT) with the ramp signal (sRAMP), with the pulse-width modulated signal, through feedback, compensating two functions:
• when directed to a control input of the switch, the photo current is compensated so that the reference current is alternatingly superimposed on the first and second photo current as dictated by the duty cycle ratio;
• and
• when directed as a compensation current to the compensation coil, the load force is compensated.