4.2 Signal Interference

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Amplifier

L1

C1

R1

R3

R2

N

C2

Fig. 4.13: Schematic representation of the capacitive

coupling of mains interference into the body by stray

capacitances.

pressed by a sufficiently high CMRR value to such an extent that it has practically no

significance for the output signal. This situation was clarified in Figure 4.8. In fact, the

conditions are more complex. In reality, the two electrode junction resistances Ru1,2

are never exactly equal. For example, if Ru1 is replaced in the circuit according to Fig-

ure 4.11 by 2 kΩto 4 kΩ, this is reflected in the simulation result shown in Figure 4.14.

In it, the ECG is clearly overlaid by the network disturbance.

Usually the amplitude of the mains disturbance at the two electrodes is also not

identical. This can be explained in principle with the help of of Figure 4.13. Between

the potential of L1 and N the upper stray capacitance C1, the body and the lower stray

capacitance C2 form a voltage divider. Within the body, the electrical behavior can

be described by ohmic resistances: as R1 between the fictitious coupling point at the

head and the first electrode, as R2 between the two electrodes, as well as as R3 between

the second electrode and the junction to the lower stray capacitance. Now the differ-

ential voltage between the both electrodes is to be estimated. For simplification, it is

assumed that the two stray capacitances have the same value C1 = C2 = 10 pF and

the resistances R1 and R3 can be neglected. The impedance of C1,2 at 50 Hz is about

320 MΩ, respectively. For R2 2 kΩis assumed as before, between L1 and N 230 V AC

voltage is applied. According to the voltage divider rule, this results in a differential

voltage between the two electrodes of about 0.7 mV. An ECG signal has an amplitude

of about 1 mV at this point. Accordingly, the biosignal and the superimposed network

disturbance have the same magnitude. Both are transmitted equally amplified to the

output of the amplifier circuit, which makes a medical evaluation of the biosignal im-

possible.