BNC-2080 Board
Chapter 2
BNC-208X Series User Manual
2-18
© National Instruments Corporation
Amplitude
Normalized
Frequency
dB
| | | | | |
(f )
0.0001 0.001 0.01 0.1 1 10
0
-20
-40
-60
-80
1
0.1
0.01
0.001
0.0001
c
Figure 2-14. Normalized Frequency Response of Highpass Filter
When measuring high-frequency signals (about 50 kHz), if you have 50-Hz noise on your inputs,
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. 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.
The filter circuit has one series capacitor on each input of the differential channel. Because the two
capacitors are in series, the capacitance value that must be substituted into Formula 2-2
is the series
capacitance of the two capacitors in series. 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 2-2) 6,366
1
, or about 6.4 k
1
. Therefore, in this example, the input
channel has a 6.37-k
1
resistor (or the closest standard value) in its capacitor position,
G
. The
closest standard 5% tolerance resistors are 6.2 k
1
. The closest standard 1% resistors are 6.34 k
1
.
Figure 2-15 shows both the schematic and the component placement for a 50-kHz highpass filter
placed on differential input Channel 1. If the input signal source is floating, a bias return resistor
must be placed in the
D
position (
R15
in this case).
Note
: Highpass filters generally exhibit poorer common-mode rejection characteristics than
lowpass filters because capacitors are in the series input paths. Capacitors have poorer
tolerances than resistors, and matching of the input impedances is crucial for good
common-mode rejection.
The BNC-2080 board open component locations do not facilitate R-C highpass filters with the
MIO-16 board configured for single-ended inputs. Therefore, if the MIO-16 board is configured
for single-ended inputs, highpass filters must be built on the custom breadboard area.
Note
: Due to space constraints, when a resistor is inserted in position
G
(capacitor position) it
must be inserted vertically before bending the other lead to the board.
