Chapter 3
NI 4350 Operation
©
National Instruments Corporation
3-3
In single-channel measurements, the reading rate is the same as the
notch filter frequency — 10, 50, or 60 readings/s. In multiple-channel
measurements, the reading rates adjust to allow the analog and digital
filters to settle to the specified accuracy.
Note:
To determine the reading rate per channel when scanning multiple
channels, divide the multiple-channel measurement reading rate by the
number of channels in the scan.
In certain applications, such as resistance measurements above 25 k
Ω
or voltage measurements with more than 25 k
Ω
of source resistance,
you should measure the same channel for up to 1 s, then switch to
another channel to achieve the specified accuracy.
To optimize measurement accuracy and minimize the noise level,
choose the 10 Hz notch filter setting.
In practice, most of the noise encountered in measurements occurs
at harmonics (multiples) of the local power line frequency (PLF).
Table 3-1 shows which programming settings to use to reject harmonics
of particular frequencies.
Table 3-1.
Filtering and Sample Rates
LabVIEW
NI435X Instrument Driver
VirtualBench-Logger
Equivalent
Filter Setting
Harmonics of
Noise
Frequencies
Rejected (Hz)
Single-Channel
Measurement
Reading Rate
(readings/s)
Multiple-Channel
Measurement
Reading Rate
(readings/s)
Notch Filter
Frequency
Setting (Hz)
PLF† (Hz)
Reading Rate
PLC*
PLF†
(Hz)
10
50
or
60
slow
5
6
40
50
60
400
10, 50, 60,
and 400
10
2.8
1.4‡
50
50
fast
1
8
50
400
50 and 400
50
8.8
2.1‡
60
60
fast
1
60
60
60
9.7
2.1‡
*Number of power-line cycles used for filtering
†Power line frequency
‡For resistance ranges of 50 k
Ω
and higher
