Campbell CR800 Specifikace stáhnout pdf (Strana 171)

Section 7. Measurement Instructions

recommended to center the signal at CR800 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 7.7-1, 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 ± 25 mV range is 2 mV; consequently,

2 mV of noise on the input signal could cause extraneous

counts. For best results, select the largest input range

(smallest gain) that will meet the minimum input signal

requirements.

CAUTION

o s

ensor

ith

ffset

Silicon diodes

such as 1N4001

To single - ende

input

1µF

10k

FIGURE 7.7-1. Input conditioning circuit for low-level and high level period averaging.

Figure 7.7-1 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.

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