Chapter 2 NI4050 Operation
© National Instruments Corporation 2-3 NI 4050 User Manual
Input Ranges
The NI40 50 has five input ranges available for measuring DC voltages.
These ranges are ±20 mV, ±200 mV, ±2.0 V, ±25V, and ±250 V. Each
range has a 10% overrange, except for the 250 V range. The 250 V and
25 V in put ranges have a 1 MΩ input imped ance; the 2 V, 200 mV, and
20 mV rang es hav e an input impedance greater than 1 GΩ. Take these
values into consideration when measuring high-impedance sources. When
the NI4 050 is powered off, the 250 V and 25 V input range have a 1 MΩ
input impedance and the 2 V, 200 mV, and 20 mV ranges have an input
impedance of 100 kΩ.
If you are taking measurements that require a high degree of accuracy, you
should consider problems associated with input impedance, noise effects,
and thermal electromotive forces (thermal EMFs). These effects are
discussed in the Measurement Considerations section.
Measurement Considerations

Input Impedance

Figure2 -1 illustrates the input impedance of an NI40 50 and its effect on
the measurement of a circuit under test. If you know the source impedance
of the circuit being tested, you can correct for the attenuation caused by the
NI4 050 in software. Since Rin is large, at least 1 MΩ, it will require a large
source impedance, Rs, to cause a large change in the measured voltage, Vm.
Figure 2-1. Effect of Input Impedance on Signal Measurements
Source
Voltage Vs
External Source
Impedance Rs
+
+
–
–
Input
Impedance
Rin
Measured
Voltage
Vm
VsRin
Rs+ Rin
Vm---------------------------=
HI
LO
Input
VΩ