National Instruments Corporation SC-207 Series, Руководство пользователя

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SC-2070 Board Chapter 2
SC-207X Series User Manual 2-12 © National Instruments Corporation
Building Highpass Filters
Simple, RC highpass filters are easily installed in the SC-2070 board on any differential input channel.
The filters are useful for accurate high-frequency measurement and low-frequency noise rejection. By
substituting resistance and capacitance values into the following formula, you can calculate a simple,
one-pole RC filter to have a -3 dB point (cutoff frequency):
fc =
1
(2
π
RC)
(Formula 2-2)
The frequency response rolls off at a rate of -20 dB per decade decrease thereafter. A Bode plot of the
amplitude versus normalized frequency is shown in Figure 2-10.
Amplitude
Normalized Frequency
dB
| | | | | |
(f )
0.0001 0.001 0.01 0.1 1 10
0
-20
-40
-60
-80
1
0.1
0.01
0.001
0.0001
c
Figure 2-10. Normalized Frequency Response of Highpass Filter
When measuring high-frequency signals (about 50 kHz), if you have 50-Hz noise on your inputs, you
can add a highpass filter with a cutoff frequency of 50 kHz. The 50-Hz noise then attenuates by 60 dB.
Notice that your 50-kHz signal also attenuates, but by only 3 dB. Do not neglect any potential
attenuation of signals of interest if you add a 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. The filter
circuit has one series capacitor on each input of the differential channel. Because the two capacitors
are in series, the capacitance value that must be substituted into Formula 2-2
is the series capacitance
of the two capacitors in series. For two capacitors in series, the net capacitance is the reciprocal of the
sum of the reciprocals of the two capacitances. For example, two 0.001
µ
F capacitors in series have a
net capacitance of 0.0005
µ
F. The two capacitors should be the same value, or the common mode
rejection is degraded. If capacitors of 0.001
µ
F are available, the resistance is (by substitution into
Formula 2-2) 6,366
Ω
, or about 6.4 k
Ω
. Therefore, in this example, the input channel has a 6.37-k
Ω
resistor (or closest standard value) in its capacitor position, G. The closest standard 5% tolerance