The Thevenin resistance will change the effective value of the input resistance in the inverting configuration.
Hint for P14-1
Refer to the weighted adder circuit. Detemine the variation needed in each input resistor to obtain the desired effect.
Hint for P14-2
Part 1. For the differential configuration,
vo = Rf/Ri (v1 - v2)
If vo = v1 + 5V is desired, what should Rf/Ri and v2 be?
Part 2. Use a voltage divider formed from two resistors R1 and R2 and one of the available power supply voltages to craft a v2 input with the same Thevenin equivalent as the answer to Part 1.
Hint for P14-3
Derive the gain of 100 from a differential configuration. The input impedance of this part of the circuit will be Ri+Rf. How can the remaining two op amps be used to increase the effective impedance seen by the electrodes?