Chapter 3
SC-2071 Board
© National Instruments Corporation
3-9
SC-207X Series User Manual
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
Figure 3-8. 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. 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 capacitance of 1
µ
F is available, then the resistance is (by substitution into
Formula 3-1) 39,789
Ω
, or about 39.80 k
Ω
. Therefore, in this example, the input has a 39.80 k
Ω
resistor (or closest standard value) in its series position, C. The closest standard 5% tolerance
resistor is 39 k
Ω
. For a lowpass filter on a single-ended input, 5% tolerance resistors suffice.
Figure 3-9 shows both the schematic and the component placement for a 4-Hz lowpass filter
placed on channel 1.
Summary of Contents for SC-207 Series
Page 15: ......
