Chapter 4
Signal Conditioning
©
National Instruments Corporation
4-9
If a capacitance of 1
µ
F is available, the resistance is (by substitution into
the equation) 39,789
Ω
, or about 39.8 k
Ω
. 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
Ω
resistor (or closest
standard value) in its series positions, E and F.
The closest standard 5% tolerance resistors are 20 k
Ω
. The closest standard
0.5% resistors are 19.8 k
Ω
. National Instruments recommends using 1% or
better tolerance resistors in this application because differences between
the resistor values degrade the common-mode rejection ratio. Figure 4-4
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 the D position
(R8 in this case).
Do not install RC lowpass filters on the AMUX-64T board open
component locations when the MIO board is configured for single-ended
inputs.
Figure 4-4.
Lowpass Filter on Differential Channel 1
Channel 1
(+in)
Channel 33
(–in)
19.8 k
Ω
Resistor 1
µ
F Capacitor
A R5
B R6
C R7
D R8
F R68
E R67
G C4
G 1
µ
F
E 19.8 k
Ω
+ Channel
– Channel
Input Schematic
+
–
To
Input
Multiplexer
F 19.8 k
Ω
