Campbell CR5000, Руководство оператора

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Section 7. Measurement Instructions
7-20
is not recommended for small amplitude sensor signals. For sensor amplitudes
less than 20 mV pk-pk a dc blocking capacitor, see Figure 7.8-1, is
recommended to center the signal at CR5000 ground (threshold = 0) because
of offset voltage drift along with limited accuracy (
±
10 mV) and resolution
(1.2 mV) of a threshold other than 0.
The minimum pulse width requirements increase (maximum frequency
decreases) with increasing gain as shown in range parameter. Signals larger
than the specified maximum for a range will saturate the gain stages and
prevent operation up to the maximum specified frequency. Back-to-back
diodes, Figure 7.8-1, are recommended to limit large amplitude signals to
within the input signal ranges.
Noisy signals with slow transitions through the voltage
threshold have the potential for extra counts around the
comparator switch point. A voltage comparator with 20 mV
of hysteresis follows the voltage gain stages. The effective
input referred hysteresis equals 20 mV divided by the
selected voltage gain. The effective input referred
hysteresis on the
±
200 mV range is 1 mV. Consequently,
1 mV of noise on the input signal could cause extraneous
counts on the
±
200 mV range. For best results, select
the largest input range (smallest gain) that will meet
the minimum input signal requirements.
c
V
o s
Sensor
with
DC
offset
Silicon diodes
such as 1N4001
To single - ended
input
1µF
D1
D2
R
10k
FIGURE 7.8-1. Input conditioning circuit for low-level and high level period averaging.
Figure 7.8-1 shows a circuit that capacitively couples an input signal to center
it around ground and also limits the amplitude of the input to allowable levels.
The capacitor C is a dc blocking capacitor for offset voltage removal. Resistor
R1 is used to bias the datalogger side of the input circuit to ground. The
reactance of the dc blocking capacitor (Xc = (2
⋅π⋅
f
⋅
C)-1) and resistor R1 form
a voltage divider at low frequencies (R1/(R1 + Xc)) that attenuates the applied
input signal. This attenuation sets a lower limit on low-frequency operation
and the minimum size of R1. The circuit attenuates the input signal by a factor
of 2 at 16 Hz.
CAUTION