A. Develop a Thévenin equivalent circuit (calculate VTH in series with ZTH) modelling the source of the common-mode interference voltage, VCM, found on the human body resulting from the capacitance Ca = 2 pF between the nearby 120 VRMS 60 Hz mains supply and the body, and the capacitance Cb = 3 pF between the body and the earth (0 V).
B. Figure 4 shows the Thévenin equivalent model of the mains supply, human body, and earth, which causes a common-mode voltage on the body, VCM. Also shown, the first two stages of an instrumentation amplifier with its input terminals connected to the body using two electrodes. RL = 15 kΩ models the impedance of the right-leg electrode and right leg.
Alter the circuit in Figure 4 to include a driven right leg (DRL) circuit to improve the overall common-mode rejection performance of the instrumentation amplifier by encouraging Vg to follow VCM as closely as possible. Include any alterations to the design of Stage 1 of the instrumentation amplifier which are required to extract an estimate of the common-mode input voltage, VCM, to be used as input to the DRL circuit.
C. i. For your new circuit, write down an expression for VCM - Vg in terms of VTH, ZTH, RL, and the magnitude of the gain of your DRL circuit.
ii. If the capacitance between the ground (Vg) of the amplifier circuit and earth (0 V) is 1 pF, calculate the impedance Zg of this capacitance at the frequency of 60 Hz.
iii. Assuming VTH and ZTH are the same as in part A above, that RL = 15 kΩ, and using your DRL design parameters from part B above, calculate the magnitude of the voltage difference, VCM - Vg, between the voltage on the body (VCM) and the voltage of the amplifier circuit ground (Vg).