Chapter 3 Signal Conditioning Application Examples
BNC-2090 User Manual
3-6
National Instruments Corporation
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 3-1) 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 channel has a 19.89 k
Ω
resistor (or the closest standard value) in its series positions, C and D.
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 (CMRR). Figure 3-4 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 B position (R4 in this case).
Note:
The BNC-2090 open-component locations do not facilitate RC lowpass
filters with the MIO board configured for single-ended inputs. Therefore,
if you configure the MIO board for single-ended inputs, you must build
lowpass filters external to the BNC-2090.
Figure 3-4.
Lowpass Filter on Differential Channel 1
Building Highpass Filters
You can install simple, RC highpass filters in the BNC-2090 on any
differential input channel. The filters are useful for accurate high-
frequency measurement and low-frequency noise rejection. By
substituting resistance and capacitance values into Formula 3-1, you
can calculate a simple, one-pole RC filter to have a -3 dB point f
c
. The
frequency response rolls off at a rate of -20 dB per decade decrease
To Input
Mux
J2
R22 = 19.8 k
Ω
(D)
R21 = 19.8 k
Ω
(C)
+
-
C2 = 1
µ
F(E)
