4.1 Measurement of Electrical Biosignals

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Another noise source, the so-called 1/f noise, makes the limitation of the transmis-

sion band towards low frequencies important. The 1/f noise is particularly strong in

semiconductor devices and is due to the temporal fluctuation of the charge carrier con-

centration caused by generation and recombination processes. As the name implies,

the noise power spectral density³ is not uniformly distributed, but increases towards

low frequencies. Therefore, high-pass filters are used for suppression. The cutoff fre-

quency of the high-pass filter must again be matched to the bandwidth of the biosignal

to avoid signal distortion. Therefore, the cutoff frequency must be well below 1 Hz. A

second positive effect of the high-pass filter is the elimination of a possible DC com-

ponent in the biosignal. The DC component⁴ can be caused, for example, by unequal

electrical charging at the phase boundaries of the electrodes (cf. subsection 4.1.1).

In summary, the amplifier for electrical biosignals must be able to uniformly amp-

lify a differential signal with a bandwidth of up to 800 Hz, as well as have high input

impedance and high common-mode rejection. These requirements can best be met

with a circuit based on an Instrumentation amplifier. Instrumentation amplifiers are

available as integrated devices or can be be built with individual operational amp-

lifiers (OPV). The advantage of integrated circuits is the very low component toler-

ance, which provides a high common mode rejection. In this book, however, the circuit

design with lumped OPVs will be dealt with in detail in order to gain a deeper under-

standing of the individual circuit elements. The following equivalent circuit diagrams

were created and simulated with the freely available software LTspice. The exercises

for this chapter assume application knowledge in LTspice.

In Figure 4.7 a two-stage instrumentation amplifier consisting of three operational

amplifiers is shown. OPV1,2 together with resistors R1,2,3 form the first stage. The bi-

osignal is fed as a differential voltage is supplied via the terminals UE1 and UE2. The

required high input impedance is realized by the high input resistance of the two of the

two OPVs. To calculate the gain of this stage, recall that for negative feedback OPVs,

the potential difference between the two inputs of an OPV is exactly 0 V. Accordingly,

the voltage drop across R3 is

UR3 = Ue1 −Ue2 .

(4.3)

Furthermore, let Ua1.2 be the output voltage of OPV1 and OPV2, respectively, refer-

enced to ground. Then, according to the voltage divider rule:

UR3 =

R3

R1 + R2 + R3

(Ua1 −Ua2) .

(4.4)

3 The noise power spectral density describes, how much noise power is contained per frequency in-

terval. The calculation of the power spectral density is presented in Equation 6.1.

4 The DC component is a DC voltage superimposed on the useful signal. The DC component is not to

be confused with the common-mode signal.