6.4 A thermocouple giving a 10 mV d.c. output voltage is connected to a high impedance digital voltmeter some distance away. A difference in potential exists between earth at the thermo- couple and earth at the voltmeter. Using the equivalent circuit given in Figure Prob. 4. (a) Calculate the r.m.s. values of the series mode and common mode interference voltages at the voltmeter input. (b) If the digital voltmeter has a common mode rejection ratio of 100 dB, find the minimum and maximum possible measured voltages. Cable ww R 100 Digital 10 ΜΩ voltmeter (d.v.m.) S 1000 pF leakage capacitance Thermocouple 10 mV d.c. ww 100 Ω Leakage resistance 10 2 U ww www 102 100 V r.m.s. Thermocouple earth Voltmeter 50 Hz earth