4.1 Measurement of Electrical Biosignals

|

103

OPV2

LT1179

OPV1

LT1179

OPV3

LT1179

R1

22k

R2

22k

R3

220

R4

22k

R5

22k

R6

22k

R7

22k

C1

1µ

R8

1Meg

OPV4

LT1179

R9

100k

R10

22k

C2

2n2

C3

2n2

C4

2n2

Ru1

2k

Ru2

2k

V1

15

V2

-15

C5

10p

C6

10p

Ugl1

Uekg1

R11

2k

R12

200k

R13

200k

+15V

+15V

+15V

-15V

-15V

-15V

+15V

-15V

Ua

+15V

-15V

Fig. 4.11: Amplifier considering the biosignal Uekg1, the common mode signal UGL and the resistors

Ru1,2 representing the interface to skin. The capacitors C5,6 represent the stray capacitance of the

measurement cables to ground.

time t / s

ECG with superinposed common-mode signal

Fig. 4.12: Output signal during simulation of the measuring amplifier after Figure 4.11 with a ECG and

superimposed common-mode signal (50Hz hum with 5 V amplitude voltage at the circuit input). The

common mode disturbance is hardly visible in the output signal because of the high common mode

rejection of the circuit.

which in turn shorts the resistor R9 at high frequency and thereby reducing the gain of

the non-inverting amplifier down to one. The cutoff frequency of the second low-pass

filter is 720 Hz. This measurement amplifier is very suitable for recording ECG, EOG or

the low frequency part of an EMG.

Figure 4.12 shows the output signal of the circuit simulation from Figure 4.11,

where an ECG of amplitude 2.5 mV with a superimposed common-mode signal was

applied. Due to the high common-mode rejection, the output signal hardly shows a