Chapter 3 Signal Conditioning Application Examples
National Instruments Corporation
3-5
BNC-2090 User Manual
Building Lowpass Filters
You can install simple, RC lowpass filters in the BNC-2090 on any
differential input channel. The filters are useful for accurate
measurement and noise rejection. By substituting resistance and
capacitance values into the following formula (hereafter referred to as
Formula 3-1), you can calculate a simple, one-pole RC filter to have a
-3 dB point cutoff frequency (f
c
):
(Formula 3-1)
The frequency response rolls off at a rate of -20 dB per decade of
increase thereafter. Figure 3-3 shows a Bode plot of the amplitude
versus normalized frequency.
Figure 3-3.
Normalized Frequency Response of Lowpass Filter
When measuring low-frequency signals (about 4 Hz), if you have
400
Hz noise on your input signals, 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.
f
c
1
(2
π
RC)
-------------------
=
Amplitude
Normalized
Frequency
1
dB
0.1 1 10 100 1,000 10,000
(f
c
)
0.1
0.01
0.001
0.0001
0
-20
-40
-60
-80
