Biosignal Processing Fundamentals and Recent Applications with MATLAB (Stefan Bernhard, Andreas Brensing etc.)

4 Measurement of Biosignals and Analog Signal Processing

Amplifier

UEKG

UGL

Fig. 4.6: Measurement situation with consid-

eration of the common mode signal UGL: UECG

represents the biosignal applied between the

electrodes.

Another requirement for the measurement amplifier is a high input impedance. In the

following equivalent circuit diagrams, the biosignal source is assumed to be in series

with the signal source assumed to be ideal by an internal resistance of RI = 2 kΩ.

A current load on this source would cause the source voltage to collapse. This can

be avoided by using a high input impedance, several orders of magnitude above the

internal resistance of the signal source. In practice, therefore, the input impedance of

the measurement amplifier should be at least ZE = 1 MΩ.

The next consideration is directed to the thermal noise of the amplifier circuit, ex-

pressed by the effective noise voltage Ueff. The cause of thermal noise is fluctuations

in the spatial distribution of free charge carriers within a conductor or semiconductor

due to thermal motion. According to Equation 4.1, Ueff depends on the ohmic resist-

ance R, the absolute temperature T and the bandwidth fB. kB is the Boltzmann con-

stant.

Ueff = ^√4kBTRfB

(4.1)

The strength of the noise in relation to the strength of the wanted signal is often

expressed by the signal-to-noise-ratio (SNR: signal-noise-ratio) in dB. Related to the

thermal noise , the following applies

SNR = 20 log10

Ubiosignal

Ueff

dB .

(4.2)

Here Uuse is the RMS voltage of the biosignal and Ueff is again the effective noise

voltage. Now a calculation example: If the ohmic input resistance of the amplifier is

10 kΩand the bandwidth is limited to 800 Hz, this results in an effective noise voltage

at the input of the measurement amplifier of 0.36 μV at room temperature. Let us as-

sume a useful signal strength of 36 μV, this results in an SNR of 40 dB.

A further consequence of the consideration of the thermal noise in Equation 4.1

is to limit the amplifier to high frequencies by a low-pass filter due to the dependence

of the effective noise voltage on the bandwidth. The cutoff frequency of the low-pass

filter must be designed according to the bandwidth of the biosignal to be measured

in order to ensure both distortion-free transmission of the biosignal and to obtain a

value for fB that is as small as possible.