Campbell CR3000 Micrologger, Operator'S Manual

Page: 348 / 644

Section 8. Operation
348
Voltage offset can be the source of significant error. For example, an offset of 3
μV on a 2500 mV signal causes an error of only 0.00012%, but the same offset on
a 0.25 mV signal causes an error of 1.2%. The primary sources of offset voltage
are ground currents and the Seebeck effect.
Single-ended measurements are susceptible to voltage drop at the ground terminal
caused by return currents from another device that is powered from the CR3000
wiring panel, such as another manufacturer's comms modem, or a sensor that
requires a lot of power. Currents >5 mA are usually undesirable. The error can
be avoided by routing power grounds from these other devices to a power ground
G terminal on the CR3000 wiring panel, rather than using a signal ground (
)
terminal. Ground currents can be caused by the excitation of resistive-bridge
sensors, but these do not usually cause offset error. These currents typically only
flow when a voltage excitation is applied. Return currents associated with
voltage excitation cannot influence other single-ended measurements because the
excitation is usually turned off before the CR3000 moves to the next
measurement. However, if the CRBasic program is written in such a way that an
excitation terminal is enabled during an unrelated measurement of a small voltage,
an offset error may occur.
The Seebeck effect results in small thermally induced voltages across junctions of
dissimilar metals as are common in electronic devices. Differential
measurements are more immune to these than are single-ended measurements
because of passive voltage cancelation occurring between matched high and low
pairs such as 1H/1L. So use differential measurements when measuring critical
low-level voltages, especially those below 200 mV, such as are output from
pyranometers and thermocouples. Differential measurements also have the
advantage of an input reversal option, RevDiff. When RevDiff is True, two
differential measurements are made, the first with a positive polarity and the
second reversed. Subtraction of opposite polarity measurements cancels some
offset voltages associated with the measurement.
Single-ended and differential measurements without input reversal use an offset
voltage measurement with the PGIA inputs grounded. For differential
measurements without input reversal, this offset voltage measurement is
performed as part of the routine auto-calibration of the CR3000. Single-ended
measurement instructions VoltSE() and TCSe() MeasOff parameter determines
whether the offset voltage measured is done at the beginning of measurement
instruction, or as part of self-calibration. This option provides you with the
opportunity to weigh measurement speed against measurement accuracy. When
MeasOff
=
True, a measurement of the single-ended offset voltage is made at the
beginning of the VoltSE() instruction. When MeasOff
=
False, an offset voltage
measurement is made during self-calibration. For slowly fluctuating offset
voltages, choosing MeasOff
=
True for the VoltSE() instruction results in better
offset voltage performance.
Ratiometric measurements use an excitation voltage or current to excite the sensor
during the measurement process. Reversing excitation polarity also reduces
offset voltage error. Setting the RevEx parameter to True programs the
measurement for excitation reversal. Excitation reversal results in a polarity
change of the measured voltage so that two measurements with opposite polarity
can be subtracted and divided by 2 for offset reduction similar to input reversal for
differential measurements. Ratiometric differential measurement instructions
allow both RevDiff and RevEx to be set True. This results in four measurement
sequences: