National Instruments BNC-208 Series, Руководство пользователя

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BNC-2080 Board Chapter 2
BNC-208X Series User Manual 2-16 © National Instruments Corporation
Amplitude
Normalized Frequency
1
dB
| | | | | |
0.1 1 10 100 1,000 10,000
(f )
0.1
0.01
0.001
0.0001
0
-20
-40
-60
-80
c
Figure 2-12. Normalized Frequency Response of Lowpass Filter
When measuring low-frequency signals (about 4 Hz), if you have 400-Hz noise on your inputs,
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. 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.
If a capacitance of 1
µ
F is available, the resistance is (by substitution into Formula 2-1) 39,789
1
,
or about 39.8 k
1
. This resistance must be divided by two to get the resistor value on each input of
a differential channel. Therefore, in this example, each input has a 19.89-k
1
resistor (or the
closest standard value) in its series positions,
E
and
F
. The closest standard 5% tolerance resistors
are 20 k
1
. The closest standard 0.5% resistors are 19.8 k
1
. National Instruments recommends
using 1% or better tolerance resistors in this application because differences between the resistor
values degrade the common-mode rejection ratio (CMRR). Figure 2-13 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, a bias return resistor must be placed in the
D
position (
R15
in
this case).
The BNC-2080 board open component locations do not facilitate R-C lowpass filters with the
MIO-16 board configured for single-ended inputs. Therefore, if the MIO-16 board is configured
for single-ended inputs, lowpass filters must be built on the custom breadboard area.