Chapter 3
BNC-2081 Board
© National Instruments Corporation
3-11
BNC-208X Series User Manual
Building Highpass Filters
Simple, R-C highpass filters are easily installed in the BNC-2081 board on any 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 (hereafter referred to as Formula 3-2), you
can calculate a simple, one-pole R-C filter to have a -3-dB point fc:
fc =
1
(2
π
RC)
(Formula
3-2)
The response rolls off at a rate of -20 dB per decade drop in frequency thereafter. A Bode plot of the
amplitude versus normalized frequency is shown in Figure 3-9.
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 3-9. Normalized Frequency Response of Highpass Filter
When measuring high-frequency signals (about 20 kHz), if you have 20-Hz noise on your inputs, you
can add a highpass filter with a cutoff frequency of 20 kHz. The 20-Hz noise then attenuates by 60 dB.
Notice that your 20-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. If a 0.001-
µ
F
capacitor is available, the resistance is (by substitution into Formula 3-2) 7,958
Ω
, or about 8.0 k
Ω
.
Therefore, in this example, the input channel has a 7.96-k
Ω
resistor (or the closest standard value) in
position
D
and a 0.001-
µ
F capacitor in position
C
. The closest standard 5% tolerance resistor is 8.2 k
Ω
.
Resistor tolerance is not a concern in this application because most capacitors have poor tolerances (for
example, 20% to 30%). Figure 3-10 illustrates both the schematic and the component placement for a
20-kHz highpass filter placed on input Channel 1.
