Question

In DVM the common mode rejection noise is eliminated by ________

(a) increasing the signal amplitude

(b) making use of a resistance

(c) using a transformer

(d) guarding

The question was asked during an interview.

My doubt is from Performance Parameters of Digital Voltmeters in chapter Electronic Instruments of Electrical Measurements

Answer 1

Right answer is (d) guarding

To explain I would say: In a DVM, common mode noise can be eliminated through guarding. A guard is basically a sheet metal box around the circuit. This is made available to the circuit being measured by making use of a terminal at the front panel.