Chapter 3 Signal Conditioning Application Examples
National Instruments Corporation
3-7
BNC-2090 User Manual
thereafter. Figure 3-5 shows a Bode plot of the amplitude versus
normalized frequency.
Figure 3-5.
Normalized Frequency Response of Highpass Filter
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, but by only 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 Formula 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 Formula 3-1) about 6.4 k
Ω
.
Therefore, in this example, the input channel has a 6.34 k
Ω
resistor (or
Amplitude
Normalized
Frequency
1
dB
0.0001 0.001 0.01 0.1 1 10
(f
c
)
0.1
0.01
0.001
0.0001
0
-20
-40
-60
-80
