Chapter 4
Signal Conditioning
4-8
©
National Instruments Corporation
Building Lowpass Filters
You can easily install simple resistance-capacitance (RC) lowpass filters
in the AMUX-64T 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 equation (hereafter
referred to as equation 4-1), you can calculate a simple, one-pole RC filter
to have a –3 dB point (fc or cutoff frequency):
(4-1)
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 4-3.
Figure 4-3.
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. You can select the
resistance or the capacitance 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.
f
c
1
2
π
RC
(
)
-------------------
=
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
