Chapter 3
Using the NI PXI-4204
3-6
ni.com
can be approximated by taking the ratio of the source impedance and the
NI PXI-4204 input impedance, and combining it with the minimum CMRR
of the NI PXI-4204.
For example, consider the following ratio of signal source and input
impedance:
Converting the NI PXI-4204 60 db CMRR specification from decibels to a
ratio yields:
The two terms are then added together to yield 0.002. Expressing this result
in decibels yields:
Normal-Mode Rejection
Normal-mode rejection (NMR) refers to the ability of the NI PXI-4204 to
reject a differentially (normally) applied signal when the NI PXI-4204
filters are set to 6 Hz. The NMR is a function of the lowpass filter
characteristics and is quantified in the normal-mode rejection ratio
(NMRR) specification, which specifies the capability of the NI PXI-4204
to reject a differentially applied unwanted 60 Hz signal. In the case of the
NI PXI-4204, this specification is 40 db at 60 Hz with a 6 Hz filter setting.
This NMR is usually only applicable when taking DC measurements. The
NMRR is specified at the powerline frequency because this is typically
where most measurement noise arises.
Effective CMR
When the frequency of a common-mode signal is known and outside of the
measurement frequency band of interest, you can use an analog or digital
filter, or both, to further reduce the residual error left from the finite CMRR
of the instrument. The combined CMR of the instrument and the filter
attenuation results in an effective CMR. When expressed in decibels, the
effective CMR is equal to the sum of the CMRR and the attenuation due to
the filter at a specified frequency.
1 k
Ω
1 M
Ω
---------------
0.001
=
1
10
60
20
------
----------
0.001
=
20log
10
0.002
(
)
54dB
–
=
