Chapter 3
Signal Conditioning
3-6
ni.com
Figure 3-3.
Normalized Frequency Response of Lowpass Filter
Example
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, by 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.
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, the resistance is (by substitution into
Equation 3-1) about 39.8 k
Ω
. This resistance must be split evenly between
each input of a differential channel. Therefore, in this example, each input
channel has a 19.89 k
Ω
resistor (or the closest standard value) in its series
positions, C and D. NI recommends using 1% or better tolerance resistors
in this application because differences between the resistor values degrade
the common-mode rejection ratio (CMRR).
Figure 3-4 shows both the schematic and the component placement for a
4 Hz lowpass filter placed on differential input channel 1. If the input signal
source is floating, you must place a bias return resistor in positions A and/or
B (R28 and R27, respectively).
–100
–
8
0
–60
–40
–20
0
0.01
0.1
1
10
100
1000
10000
Norm
a
lized Freq
u
ency (fc)
O
u
tp
u
t Amplit
u
de (dB)
