Campbell Scientific CR3000 Micrologger - Manual (Page 289)

Section 8. Operation
289
excitation "on time" for each polarity is exactly the same to ensure that ionic
sensors do not polarize with repetitive measurements.
Read More! A white paper entitled "The Benefits of Input Reversal and
Excitation Reversal for Voltage Measurements" is available at
www.campbellsci.com.
8.1.2.6.2 Ground Reference Offset Voltage
When MeasOff is enabled (= True ), the CR3000 measures the offset voltage of the
ground reference prior to each VoltSe() or TCSe() measurement. This offset
voltage is subtracted from the subsequent measurement.
8.1.2.6.3 Background Calibration
If RevDiff , RevEx , or MeasOff is disabled (= False ) in a measurement instruction,
offset voltage compensation is still performed, albeit less effectively, by using
measurements from automatic background calibration. Disabling
RevDiff , RevEx ,
or MeasOff speeds up measurement time; however, the increase in speed comes at
the cost of accuracy 1) because RevDiff , RevEx , and MeasOff are more effective
techniques, and 2) because background calibrations are performed only
periodically, so more time skew occurs between the background calibration
offsets and the measurements to which they are applied.
Note Disable RevDiff , RevEx, and MeasOff when CR3000 module temperature
and return currents are slow to change or when measurement duration must be
minimal to maximize measurement frequency.
8.1.2.7 Integration
Read More! See White Paper "Preventing and Attacking Measurement Noise
Problems" at www.campbellsci.com.
The CR3000 incorporates circuitry to perform an analog integration on voltages to
be measured prior to the A/D conversion. The magnitude of the frequency
response of an analog integrator is a SIN(x) / x shape, which has notches
(transmission zeros) occurring at 1 / (integer multiples) of the integration
duration. Consequently, noise at 1 / (integer multiples) of the integration duration
is effectively rejected by an analog integrator. table CRBasic Measurement
Integration Times and Codes
(p. 289)
lists three integration durations available in the
CR3000 and associated CRBasic codes. If reversing the differential inputs or
reversing the excitation is specified, there are two separate integrations per
measurement; if both reversals are specified, there are four separate integrations.
Table 56. CRBasic Measurement Integration Times and Codes
Integration Time (ms) CRBasic Code Comments
250 µs 250 Fast integration
16.667 ms _60Hz Filters 60-Hz noise
20 ms _50Hz Filters 50-Hz noise