Minimizing Electronic Noise; Filtering To Reduce Measurement Noise - Campbell CR300 Series Product Manual

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
component (V ) of a voltage signal. V
dm
inputs. So, V = (V+ + V–)/2 or the voltage remaining on the inputs when V
cm
voltage on the V+ and V– inputs is given as V
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
settling + filtering + overhead.
A faster filter may be preferred to achieve the following objectives:
Minimize time skew between successive measurements
l
Maximize throughput rate.
l
is the average of the voltages on the V+ and V–
cm
= V + V
H cm dm
) and the differential-mode
cm
= 0. The total
dm
/2, and V = V – V
L cm
Avoiding time skew (p. 123).
17. Tips and troubleshooting
/2,
dm
141