4.2 Signal Interference

|

107

Fig. 4.15: Amplifier circuit for biosignals with reference potential control.

in a conductor loop according to

Uind = −^∂Φ

∂t ^,

(4.8)

where the magnetic flux is defined as follows:

Φ = ∬BdA .

(4.9)

A is here the area enclosed by a conductor loop. The two measuring cables, the body

and the measuring amplifier form a conductor loop (cf. Figure 4.16). If this conductor

loop is perpendicular to the alternating magnetic field an induced alternating voltage

is generated at the input of the measuring amplifier, which in turn superimposes the

biosignal. Overall, the strength of the inductive coupling is determined by four quant-

ities, namely, the current I, the frequency (due to the derivative in Equation 4.8), the

distance r between the conductor loop and the mains cable L1 and the area A of the

conductor loop. While it is usually not possible to influence the current I in the power

cable and the frequency with 50 Hz (60 Hz in North America), the remaining ways to

reduce the effect are: i) to keep a large distance r to the current-carrying power cable,

and ii) keep the area A of the conductor loop as small as possible. The latter can be

achieved by twisting the measuring cables together (twisted pair) or at least run close

together. With these two methods, inductive coupling can be effectively avoided.