Chapter 3
Signal Conditioning
3-8
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Figure 3-6.
Normalized Frequency Response of Highpass Filter
Example
When measuring high-frequency signals (about 50 kHz), if you have 50 Hz
noise on your input signals, you can add a highpass filter with a cutoff
frequency of 50 kHz. The 50 Hz noise then attenuates by 60 dB. Notice that
your 50 kHz signal also attenuates, by 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. 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.
The filter circuit has one series capacitor on each input signal of the
differential channel. Because the two capacitors are in series, the
capacitance value that must be substituted into Equation 3-1 is the series
capacitance of the two capacitors. For two capacitors in series, the net
capacitance is the reciprocal of the sum of the reciprocals of the
two capacitances. For example, two 0.001
μ
F capacitors in series have a net
capacitance of 0.0005
μ
F. The two capacitors should be the same value,
or the CMRR is degraded. If capacitors of 0.001
μ
F are available, the
resistance is (by substitution into Equation 3-1) about 6.4 k
Ω
. Therefore,
in this example, the input channel has a 6.34 k
Ω
resistor (or the closest
standard value) in position E.
–100
–
8
0
–60
–40
–20
0
0.0001
0.001
0.01
0.1
1
10
100
Norm
a
lized Freq
u
ency (fc)
O
u
tp
u
t Amplit
u
de (dB)
