Rejecting Power-Line Noise Voltages; Common Mode Rejection (Cmr) - DATATEC EDU34450A Guia Del Usuario

DC Measurement Considerations
Thermal EMF Errors
Thermoelectric voltages are the most common source of error in low-level DC voltage measurements.
Thermoelectric voltages are generated when you make circuit connections using dissimilar metals at different
temperatures. Each metal-to-metal junction forms a thermocouple, which generates a voltage proportional to the
junction temperature. You should take the necessary precautions to minimize thermocouple voltages and
temperature variations in low-level voltage measurements. The best connections are formed using copper-to-
copper crimped connections, as the instrument's input terminals are a copper alloy. The table below shows common
thermoelectric voltages for connections between dissimilar metals.
Copper-to-
Cadmium-Tin Solder
Copper
Gold
Silver
Brass
Beryllium Copper
Aluminum
Tin-Lead Solder
Kovar or Alloy 42
Silicon
Copper-Oxide
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 instrument 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 instrument'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.
Keysight EDU34450A User's Guide
Approx. mV / °C
0.2
<0.3
0.5
0.5
3
5
5
5
40
500
1000
99