17.11.3.1 Minimizing electronic noise
Electronic noise can cause significant error in a voltage measurement, especially when measuring
voltages less than 200 mV. So long as input limitations are observed, the PGA ignores voltages,
including noise, that are common to each side of a differential-input pair. This is the common-
mode voltage. Ignoring (rejecting or canceling) the common-mode voltage is an essential
feature of the differential input configuration that improves voltage measurements. The
following image illustrates the common-mode component (V
cm
) and the differential-mode
component (V
dm
) of a voltage signal. V
cm
is the average of the voltages on the V+ and V–
inputs. So, V
cm
= (V+ + V–)/2 or the voltage remaining on the inputs when V
dm
= 0. The total
voltage on the V+ and V– inputs is given as V
H
= V
cm
+ V
dm
/2, and V
L
= V
cm
– V
dm
/2,
respectively.
17.11.4 Filtering to Reduce Measurement Noise
The data logger applies an adjustable filter to analog measurements, reducing signal
components at selected frequencies. The following figures show the frequency response of the
filters applied when the first notch frequency (
fN1
) is set to 4000, 400, or 50/60 Hz, respectively.
Note that the same filter is applied when
fN1
is set to either 50 or 60 Hz, simultaneously filtering
both 50 and 60 Hz signal components. Commonly,
fN1
is set at 50 or 60 Hz in order to filter 50
or 60 Hz signal components, reducing noise from ac power mains.
Filtering comes at the expense of measurement time. The time required for filtering is 0.5 ms
when
fN1
is set to 4000 Hz, 6.226 ms when
fN1
is 400 Hz, and 49.812 ms when
fN1
is set to
either 50 or 60 Hz. Random noise in the measurement results decreases, while measurement time
increases, as
fN1
is set to smaller values. The total time required for a single result includes
se fil overhead.
A faster filter may be preferred to achieve the following objectives:
l
Minimize time skew between successive measurements
l
Maximize throughput rate.
17. Tips and troubleshooting
141