Example Problem You are measuring the EEG of a patient and accidently choose two different types of electrodes for EEG lead. One of them has a source impedance of 1500 Ω while the other has 4700 Ω source impedance. You also chose a ground electrode with impedance of 2500 Ω source impedance. The input impedance of each differential input of the EEG machine to ground was 10 MΩ, and the instrument had a CMRR of 80 dB. The power-line displacement current to the patient was measured at 400 nA. The amplitude of the patient’s EEG was 12 uV. How much common-mode voltage will be seen on this patient ? Will it significantly interfere with EEG signal? How much power-line interference will be seen on the patient’s EEG?

Example Problem 2 Silicone diodes having a forward resistance of 2 Ω are to be used as voltage-limiting devices in the protection circuit of an electrocardiograph. They are connected as shown in Figure 6.14 (b). The protection circuit is shown in Figure 6.13. If voltage transients as high as 500 V can appear at the electrocardiograph input during defibrillation. What is minimal value of R that the designer can choose so that the voltage at the preamplifier input does not exceed 800 mV? Assume that the silicon diodes have a breakdown voltage of 600 mV.

Example Problem 3 Design a driven-right-leg circuit, and show all resistor values. For 1 uA of 60 Hz current flowing through the body, the common-mode voltage should be reduced to 2 mV. The circuit should supply no more than 5 uA when the amplifier is saturated at ±13 V.

Amplifier for glass micropipette intracellular electrodes Measure potential around 50 to 100 mV, across the cell membrane Small size and small effective surface-contact area gives very high source impedance Geometry results in large shunting capacitance affecting the frequency response Positive-feedback schemes are used to provide an effective negative capacitance to compensate for high shunt capacitance

Example Problem 4 For Figure 6.17, assume Cs = 10 pF and Cf = 20 pF. Design an amplifier circuit to replace the triangle containing Av. Use an op amp and passive components to achieve an ideal negative-input capacitance amplifier. There can be common-mode noise in the electrode system as large as 100 mV. What must the minimum CMRR of the micropipette electrode system be so that an intracellular signal of 50 mV amplitude has no more than 1% common-mode noise?

Biopotential Pre-amplifier Preamplifier stage should have a low noise Directly coupled with the electrodes to provide optimal low-frequency response and minimize charging of capacitors Low voltage gains to prevent amplification of electrodes overpotential Capacitively-coupled to remaining amplifier stages to prevent saturation from dc potential Very high input impedance to prevent loading of electrodes Electrically isolated for safety

ECG Amplifier fig_06_18

Example Problem 5 For the ECG amplifier shown in Figure 6.18. What is the gain for the dc-coupled preamplifier stage? What is the frequency response for the amplifier? What is the gain for the second stage? If the peak-to-peak noise level is 40 mV at 100 Hz, what is the CMRR for of this amplifier at 100 Hz?

Example Problem 6 In a cardiac pacemaker, we only need to know that a heart beat has occurred. We do not need to know the complete diagnostic electrocardiogram. For the ECG amplifier from Figure 6.18, show the changes you would make to reduce the cardiac pacemaker frequency bandwidth to 4 Hz to 10 Hz.

Example Problem 7 Design a circuit that uses one op amp plus other passive components that will detect QRS complexes of the ECG even when the amplitude of the T wave exceeds that of the QRS complex and provides output signals suitable for counting these complexes on a counter.

Example Problem 8 In an evoked-response experiment in which the EEG is studied after a patient is given the stimulus of a flashing light, the experimenter finds that the response has approximately the same amplitude as the random noise of the signal. If a signal averager is used, how many samples must be averaged to get an SNR of 20 dB? If we wanted SNR of 40 dB, would it be practical to use this technique?