Technical data

34405A User’s and Service Guide 49

Measurement Tutorial 3

Noise Rejection

Rejecting Power–Line Noise Voltages

A desirable characteristic of integrating analog–to–digital (A/D) converters

is their ability to reject power–line related noise present with DC input

signals. This is called normal mode noise rejection, or NMR. The

multimeter achieves NMR by measuring the average DC input by

"integrating" it over a fixed period.

Common Mode Rejection (CMR)

Ideally, a multimeter is completely isolated from earth–referenced circuits.

However, there is finite resistance between the multimeter's input LO

terminal and earth ground, as shown below. This can cause errors when

measuring low voltages which are floating relative to earth ground.

Refer to “Measurement Noise Rejection” on page 131 for the multimeter’s

NMR and CMR characteristics.

Noise Caused by Magnetic Loops

If you are making measurements near magnetic fields, take caution to

avoid inducing voltages in the measurement connections. You should be

especially careful when working near conductors carrying large currents.

Use twisted–pair connections to the multimeter to reduce the noise pickup

loop area, or dress the test leads as close together as possible. Loose or

Ideal

Meter

= Float Voltage

= DUT Source Resistance

Imbalance

= Multimeter Isolation Resistance

(LO-Earth)

= Multimeter Input Capacitance:

x R

Error (v) =

+ R

test