National Instruments NI 4050 用户手册
Chapter 2
NI 4050 Operation
© National Instruments Corporation
2-5
Figure 2-2. Normal Mode Measurement Effects
If you are measuring signals in the presence of large normal mode voltages,
consult Appendix A,
consult Appendix A,
, to calculate the additional error to your
system. Use the equation in Figure 2-2 to calculate the voltage error due to
normal mode voltage.
normal mode voltage.
Common Mode Rejection
Common mode rejection (CMR) is the ability of the NI 4050 to reject
signals that are common to both input terminals. The ability is quantified in
the common mode rejection ratio (CMRR) specification. Theoretically, the
floating measurement circuitry of the NI 4050 should have an infinite
CMRR. Parasitic resistances and capacitances to earth ground limit the
CMR of the NI 4050. This effect is most noticeable when measuring small
signals in the presence of a large common mode voltage, as shown in
Figure 2-3.
signals that are common to both input terminals. The ability is quantified in
the common mode rejection ratio (CMRR) specification. Theoretically, the
floating measurement circuitry of the NI 4050 should have an infinite
CMRR. Parasitic resistances and capacitances to earth ground limit the
CMR of the NI 4050. This effect is most noticeable when measuring small
signals in the presence of a large common mode voltage, as shown in
Figure 2-3.
Source
Voltage
V
s
at 60 Hz
+
–
Measured
Voltage
V
m
V
m
V
s
10
N MRR
–
20
--------------------
×
=
HI
LO
Input
V
Ω