Specifications

Section 7. Measurement Instructions
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 ± 50 mV range is 1 mV; consequently, 1
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
c
V
o s
S
ensor
w
ith
D
C
o
ffset
Silicon diodes
such as 1N4001
To single - ende
d
input
1µF
D1
D2
R
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.
The back-to-back silicon diodes D1 and D2 provide ESD protection of
capacitor C and the sensor, and also limit the amplitude of large amplitude
sensor signals. These diodes clip large amplitude signals to approximately 1.4
V pk-pk which is within the recommended input signal ranges for all range
codes. Diodes D1 and D2 along with resistor R1 are recommended to limit
large amplitude sensor signals, even when dc blocking capacitor C is not used.
Sensors outputting large voltages may cause large currents to flow through
7-18