Chapter 3
SC-2071 Board
© National Instruments Corporation
3-11
SC-207X Series User Manual
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 3-10. 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 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 0.001
µ
F capacitor is available, the resistance is (by substitution into Formula 3-2)
7958
Ω
, or about 8.0 k
Ω
. Therefore, in this example, the input channel has a 7.96 k
Ω
resistor (or
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 3-11 shows
both the schematic and the component placement for a 20-kHz highpass filter placed on input
channel 1.
Summary of Contents for SC-207 Series
Page 15: ......
