National Instruments NI 4050 用户手册

Figure 2-3.  Common Mode Measurement Effects

Using the equation in Figure 2-3, you can calculate the voltage error due to 
the common mode voltage. If you are measuring signals in the presence of 
large common mode voltages, consult Appendix A, 

, to 

calculate the additional error to your system.

Effective common mode rejection is the sum of the CMRR and the NMRR 
at a given frequency. It is the effective rejection on a given noise signal that 
is applied to both input leads as it gets rejected first by the CMR capability 
of the instrument then again by its NMR capability. This specification is 
most useful at the powerline frequency where most of the noise resides and 
is only valid for DC measurements.

In the AC voltage ranges, the NI 4050 measures the AC-coupled RMS 
value of a signal. The RMS value of a signal is a fundamental measurement 
of the magnitude of an AC signal. The RMS value of an AC signal can be 
defined mathematically as the square root of the average of the square of 
the signal. 

In practical terms, the RMS value of an AC signal is the DC value required 
to produce an equivalent amount of heat in the same resistive load. The 
NI 4050 first AC-couples the measured signal to remove any DC 

HI

Source

Voltage

 V

s

Common

Voltage

 V

c

+

–

LO

Input

Measured

Voltage

V

m

V

Ω

 

+
–

+
–

V

error

V

2

------

V

c

+ 

10

CMRR

–

20

--------------------









×

=

s

V

m

V

s

V

error

+

=



