National Instruments Corporation SC-207 Series, Руководство пользователя

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SC-2070 Board Chapter 2
SC-207X Series User Manual 2-10 © National Instruments Corporation
Building Lowpass Filters
Simple, RC lowpass filters are easily installed in the SC-2070 board on any differential input channel.
The filters are useful for accurate measurement and noise rejection. By substituting resistance and
capacitance values into the following Formula (hereafter referred to as Formula 2-1), you can calculate
a simple, one-pole RC filter to have a -3 dB point (cutoff frequency):
fc =
1
(2
π
RC)
(Formula 2-1)
The frequency response rolls off at a rate of -20 dB per decade of increase thereafter. A Bode plot of
the amplitude versus normalized frequency is shown in Figure 2-8.
Amplitude
Normalized Frequency
1
dB
| | | | | |
0.1 1 10 100 1,000 10,000
(f )
0.1
0.01
0.001
0.0001
0
-20
-40
-60
-80
c
Figure 2-8. Normalized Frequency Response of Lowpass Filter
When measuring low-frequency signals (about 4 Hz), if you have 400-Hz noise on your inputs, you
can add a lowpass filter with a cutoff frequency of 4 Hz. The 400-Hz noise then attenuates by 40 dB.
Notice that your 4-Hz signal also attenuates, but by only 3 dB. Do not neglect any potential
attenuation of signals of interest by this low-order filter.
You must also choose the filter component values. The resistance or the capacitance can be selected
arbitrarily; one value determines the other. Picking the capacitor first and letting its value determine
the resistance required is preferable because more standard resistor values are available. If a
capacitance of 1
µ
F is available, the resistance is (by substitution into Formula 2-1) 39,789
Ω
, or about
39.8 k
Ω
. Divide this resistance by two to get the resistor value on each input of a differential channel.
Therefore, in this example, each input has a 19.89-k
Ω
resistor (or closest standard value) in its series
positions, E and F. The closest standard 5% tolerance resistors are 20 k
Ω
. The closest standard 0.5%
resistors are 19.8 k
Ω
. National Instruments recommends using 1% or better tolerance resistors in this
application because differences between the resistor values degrade the common-mode rejection ratio.
Figure 2-9 shows both the schematic and the component placement for a 4-Hz lowpass filter placed on
differential input channel 1. If the input signal source is floating, place a bias return resistor in the D
position (R10 in this case).