Campbell CR800 Series, Operator'S Manual

Page: 318 / 598

Section 8. Operation
318
Ac Noise Rejection on Large Signals
1
Ac-Power Line
Frequency
Measurement
Integration
Time
CRBasic
Integration
Code
Default
Settling
Time
Maximum
Recommended
Settling Time
2
60 Hz 250 μs • 2
_60Hz
3000 μs 8330 μs
50 Hz 250 μs • 2 _50Hz 3000 μs 10000 μs
1
Applies to analog input voltage ranges
mV2500
and
mV5000
.
2
Excitation time and settling time are equal in measurements requiring excitation. The CR800 cannot excite VX
excitation terminals during A-to-D conversion. The one-half-cycle technique with excitation limits the length of
recommended excitation and settling time for the first measurement to one-half-cycle. The CR800 does not
prevent or warn against setting a settling time beyond the one-half-cycle limit. For example, a settling time of up
to 50000 µs can be programmed, but the CR800 will execute the measurement as follows:
1. CR800 turns excitation on, waits 50000 µs, and then makes the first measurement.
2. During A-to-
D, CR800 turns off excitation for ≈170 µs.
3. Excitation is switched on again for one-half cycle, then the second measurement is made.
Restated, when the CR800 is programmed to use the half-cycle 50 Hz or 60 Hz rejection techniques, a sensor does
not see a continuous excitation of the length entered as the settling time before the second measurement — if the
settling time entered is greater than one-half cycle. This causes a truncated second excitation. Depending on the
sensor used, a truncated second excitation may cause measurement errors.
Signal Settling Time
Settling time allows an analog voltage signal to settle closer to the true magnitude
prior to measurement. To minimize measurement error, signal settling is needed
when a signal has been affected by one or more of the following:
• A small transient originating from the internal multiplexing that connects
a CR800 terminal with measurement circuitry
•
A relatively large transient induced by an adjacent excitation conductor
on the signal conductor, if present,because of capacitive coupling during
a bridge measurement
•
Dielectric absorption. 50 Hz or 60 Hz integrations require a relatively
long reset time of the internal integration capacitor before the next
measurement.
The rate at which the signal settles is determined by the input settling time
constant, which is a function of both the source resistance and fixed-input
capacitance (3.3 nfd) of the CR800.
Rise and decay waveforms are exponential. Figure Input Voltage Rise and
Transient Decay
(p. 319)
shows rising and decaying waveforms settling closer to the
true signal magnitude, V
so
. The SettlingTime parameter of an analog
measurement instruction allows tailoring of measurement instruction settling
times with 100 µs resolution up to 50000 µs.