Campbell Scientific CR800 and CR850 Measurement and Control Systems - Manual (Page 307)

Section 8. Operation
307
8.1.5.4.3 Switch Bounce and NAN
NAN will be the result of a TimerIO() measurement if one of two conditions
occurs:
1. timeout expires
2. a signal on the channel is too fast (> 3 KHz)
When the input channel experiences this type of signal, the CR800 operating
system disables the interrupt that is capturing the precise time until the next scan
is serviced. This is done so that the CR800 does not get bogged down in
interrupts. An small RC filter retrofitted to the sensor switch should fix the
problem.
8.1.6 Period Averaging
The CR800 can measure the period of a signal on any single-ended analog-input
channel (SE1 – 6). The specified number of cycles is timed with a resolution of
136 ns, making the resolution of the period measurement 136 ns divided by the
number of cycles chosen.
Low-level signals are amplified prior to a voltage comparator. The internal
voltage comparator is referenced to the user-entered threshold. The threshold
parameter allows a user to reference the internal voltage comparator to voltages
other than 0 V. For example, a threshold of 2500 mV allows a 0- to 5-Vdc digital
signal to be sensed by the internal comparator without the need of any additional
input conditioning circuitry. The threshold allows direct connection of standard
digital signals, but it is not recommended for small amplitude sensor signals. For
sensor amplitudes less than 20 mV peak-to-peak, a dc blocking capacitor is
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 zero. Figure Input Conditioning Circuit for Period
Averaging
(p. 308)
shows an example circuit.
The minimum pulse-width requirements increase (maximum frequency decreases)
with increasing gain. Signals larger than the specified maximum for a range will
saturate the gain stages and prevent operation up to the maximum specified
frequency. As shown, back-to-back diodes are recommended to limit large
amplitude signals to within the input signal ranges.
Caution 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 meets the
minimum input signal requirements.