To minimise the loading effect of circuit under test, the input impedance of the device must:

Loading effect:

• When a voltmeter having an internal resistance of Rm is connected in parallel with load resistance RL of circuit under test, the circuit conditions will be altered.
• The effective resistance will be the parallel combination of RL and Rm. The voltmeter indicates the voltage across this effective resistance, where the indicated voltage will always be less than true voltage. This is known as loading effect.
• Hence the instrument must possess high input impedance to reduce loading effect.