Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 58)

Section 3. CR5000 Measurement Details
3-4
multiple of 500 µs, the CR5000 will repeat 250 µs integration measurements
every 500
µ
s throughout the integration interval. If the integration time
specified is 100 µs or 200 µs, the CR5000 makes one or two samples in the
integration interval. The average of these measurements is stored as the result
of the measurement.
The random noise level is decreased by the square root of the number of
measurements made. For example, the input noise on the
±
5000 mV range
with one 250
µ
s integration is 120
µ
V RMS; entering 2000
µ
s for the
integration (four measurements) will cut this noise in half (120/(
√
4)=60).
The integration time specified in the measurement instruction is used for each
segment of the measurement. Thus, if reversing the differential input or
reversing the excitation is specified, there will be two integrations per channel;
if both reversals are specified, there will be four integrations.
3.2 Single Ended and Differential Voltage
Measurements
A single-ended voltage measurement is made on a single input which is
measured relative to ground. A differential measurement measures the
difference in voltage between two inputs.
There are two sets of channel numbers on the analog channels.
Differential channels (1-20) have two inputs: high (H) and low
(L). Either the high or low side of a differential channel can be
used for a single ended measurement. The single-ended
channels are numbered 1-40.
Because a single ended measurement is referenced to CR5000 ground, any
difference in ground potential between the sensor and the CR5000 will result in
an error in the measurement. For example, if the measuring junction of a
copper-constantan thermocouple, being used to measure soil temperature, is
not insulated and the potential of earth ground is 1 mV greater at the sensor
than at the point where the CR5000 is grounded, the measured voltage would
be 1 mV greater than the thermocouple output, or approximately 25
o C high.
Another instance where a ground potential difference creates a problem is
where external signal conditioning circuitry is powered from the same source
as the CR5000. Despite being tied to the same ground, differences in current
drain and lead resistance result in different ground potential at the two
instruments. For this reason, a differential measurement should be made on an
analog output from the external signal conditioner. Differential measurements
MUST be used when the inputs are known to be different from ground, such as
the output from a full bridge.
Common mode range
In order to make a differential measurement, the inputs must be within the
CR5000 common mode range of
± 5 V. The common mode range is the
voltage range, relative to CR5000 ground, within which both inputs of a
differential measurement must lie, in order for the differential measurement to
NOTE