National Instruments NI 435 Series, User Manual, Page: 21 / 77

Chapter 2 Operating the NI 435x Device
© National Instruments Corporation 2-3 NI 435x User Manual
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 in Table 2-1 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. This extra time allows the input
filter capacitors of the NI 435
x devices to fully charge or discharge.
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 powerline frequency (PLF). Table 2-1
shows which programming settings to use to reject harmonics of particular
frequencies.
Note These rates were obtained without auto-zeroing and cold-junction compensation.
Table 2-1. Filtering and Sample Rates
NI-DAQ or
VirtualBench-
Logger
NI 435 x Instrument
Driver
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 ***
* Powerline frequency
** Number of powerline cycles used for filtering
*** For resistance ranges of 50 k
Ω
and higher