Chapter 2 NI 4050 Operation
NI 4050 User Manual 2-4 www.ni.com
Thermal EMF
Thermal EMFs, or thermoelectric potentials, are voltages generated at the
junctions of dissimilar metals and are functions of temperature. Thermal
EMFs in a circuit under test can cause higher than expected offsets that
change with temperature.
Noise Rejection
The NI 4050 filters out AC voltages in the DC voltage measurement
ranges. However, if the amplitudes of the AC voltages are large compared
to the DC voltages, or if the peak value (AC + DC) of the measured voltage
is outside the overrange limits, the NI 4050 may exhibit additional errors.
To minimize these errors, keep the NI 4050 away from strong AC magnetic
sources and minimize the area of the loop formed by the test leads.
Choosing the 5 1/2 digit resolution will also help minimize noise from
AC sources. If the peak value of the measured voltage is likely to exceed
the selected input range, select the next highest input range.
Normal Mode Rejection
Normal mode rejection (NMR) is the ability of the NI 4050 to reject a
normally (differentially) applied signal. The ability is quantified in the
normal mode rejection ratio (NMRR) specification, which indicates the
capability of the NI 4050 to reject 50 or 60 Hz and is valid only at the
specified frequency and useful only when taking DC measurements. The
NMRR is specified at the powerline frequency because this is typically
where most measurement noise occurs.
Figure 2-2 shows a 60 Hz signal connected differentially to the NI 4050 in
DC Volts mode. V
m
is the voltage that will be measured after the signal is
rejected. NMR is very useful when trying to measure DC voltages in the
presence of large powerline interference.
