Chapter 4
Signal Conditioning
4-10
©
National Instruments Corporation
Building Highpass Filters
You can easily install simple RC highpass filters in the AMUX-64T 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 equation,
(hereafter referred to as equation 4-2), you can calculate a simple, one-pole
R-C filter to have a –3 dB point (fc or cutoff frequency):
(4-2)
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 4-5.
Figure 4-5.
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. 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
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
