4.6 Post-Reading and Exercises

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145

Fig. 4.48: Circuit of a 2nd order RLC Bessel-filter (left) and associated frequency response by mag-

nitude and phase (right).

3.

What is a Helmholtz layer?

4.

Why is an electrode gel required at the junction between skin and electrode?

5.

For what reason does a amplifier for biosignals require a high input impedance?

6.

What is a common mode signal and what does common mode rejection mean?

7.

What does the signal-to-noise ratio describe and what is it used for?

8.

What is the magnitude and phase frequency response of a amplifier?

9.

What is the magnitude of the internal resistance of the body and that of the elec-

trodes?

10. What characteristics especially qualify an instrumentation amplifier for amplify-

ing biosignals?

11. Enter the instrumentation amplifier presented in subsection 4.1.2 into LTspice.

12. Check the frequency response of the measurement amplifier after the subtractor,

after the high-pass filter and at the circuit output in the frequency range 0.1 Hz to

10 kHz. To do this, apply an AC voltage of amplitude 1 mV between the two inputs,

and simulate the frequency range.

13. Which components of the measurement amplifier would have to be changed and

how, so that the high pass has a cutoff frequency of 1.6 Hz and the second low

pass has a cutoff frequency of 360 Hz?

14. Check the calculated gain of the measurement amplifier after the instrumentation

amplifier and at the circuit output.

15. Add a common mode signal source in the LTspice model.

16. At the circuit output of the instrumentation amplifier, calculate the ratio of the

useful signal to the common-mode signal when at the circuit input the amp-

litude of the useful signal is 100 μV and the common-mode signal is 1 V. For this

purpose, it should be taken into account that the operational amplifiers have a

common-mode rejection of 100 dB.