Chapter 4
Signal Conditioning
©
National Instruments Corporation
4-11
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 equation 4-2 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
common-mode rejection is degraded. If capacitors of 0.001
µ
F are
available, the resistance is (by substitution into equation 4-2) 6,366
Ω
, or
about 6.4 k
Ω
. Therefore, in this example, the input channel has a 6.37 k
Ω
resistor (or closest standard value) in its capacitor position, G.
The closest standard 5% tolerance resistors are 6.2 k
Ω
. The closest standard
1% resistors are 6.34 k
Ω
. Figure 4-6 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, you must place a bias
return resistor in the D position (R8 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.
Do not install RC highpass filters on the AMUX-64T board open
component locations when the MIO board is configured for single-ended
inputs.
