User manual

System Noise
Electrical noise can present problems even with good equipment; thus, controlling noise is imperative.
Some techniques avoid or prevent noise sources from entering the system; other techniques remove noise
from the signal.
Laboratory and industrial environments often have multiple sources of electrical noise. An AC power line
is a source of 50/60 Hz noise. Heavy equipment (air conditioners, elevators, pumps, etc.) can be a source
of noise, particularly when turned on and off. Local radio stations are a source of high-frequency noise,
and computers and other electronic equipment can create noise in a multitude of frequency ranges. Thus,
an absolute noise-free environment for data acquisition is not realistic. Fortunately, noise-reduction
techniques such as averaging, filtering, differential voltage measurement, and shielding are available to
reduce noise to an acceptable level.
Note: Additional noise-reduction information is contained in the section, “Signal Modes,” especially in the
paragraphs pertaining to connections, signal cables, and ground loops.
Averaging
Averaging is done in software after several samples have been collected. Depending on the nature of the
noise, averaging can reduce noise by the square root of the number of averaged samples. Although
averaging can be effective, it suffers from several drawbacks. Noise in measurements only decreases as
the square root of the number of measurements—reducing RMS noise significantly may require many
samples. Thus, averaging is suited to low-speed applications that can provide many samples.
Note: Only random noise is reduced or eliminated by averaging. Averaging will not reduce or eliminate
any signal that is periodic.
Analog Filtering
A filter is an analog circuit element that attenuates an incoming signal according to its frequency. A low-
pass filter attenuates frequencies above the cutoff frequency. Conversely, a high-pass filter attenuates
frequencies below the cutoff. As frequency increases beyond the cutoff point, the attenuation of a single-
pole, low-pass filter increases slowly. Multi-pole filters provide greater attenuation beyond the cutoff
frequency but may introduce phase (time delay) problems that could affect some applications.
Filter circuits can be active or passive:
• Active. The DBK18 Low-Pass Filter Card has an instrumentation amplifier with variable gain and
filter configurations. The DBK18 uses an active 3-pole filter (mostly contained within the UAFF42
ICs) that can be configured as a Butterworth, Bessel, or Chebyshev filter with corner frequencies up
to 50 kHz. Filter properties depend on the values of resistors and capacitors. These components
can be changed by the user.
• Passive. The DBK11 has a prototype area on the PC board for attaching non-powered components
such as resistors and capacitors. The user chooses component values to produce the desired
properties.
Input and Source Impedance
As shown in the following figure, The input impedance (R
i
) combines with the transducer’s source
impedance (R
s
) forming a voltage divider. This divider distorts the voltage being read at the analog-to-
digital converter. The actual voltage read is represented by the equation:
V
read
= V
T
× [R
i
/ (R
i
+ R
s
)] [ V
t
- I
b
R
s
)
The low source impedance (R
s
) of most signals usually
presents no problem. Some transducers, such as
piezoelectric types, have high source impedance. These
transducer types should be used with a charge-sensitive
amplifier of low output impedance.
DBK Option Cards and Modules 886995 Signal Management 1-5