Campbell Scientific CR9000X Measurement and Control System - Manual (Page 96)

Section 3. CR9000X Measurement Details
Sustained voltages in excess of ±20 V on the CR9050 Module
inputs or ±150 V on the CR9055 Module inputs will damage the
CR9000X circuitry.
NOTE
3.1.3 Signal Settling Time
Whenever an analog input is switched into the CR9000X measurement
circuitry prior to making a measurement, a finite amount of time is required for
the signal to stabilize at it's correct value. 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 input capacitance. The CR9000X delays after switching
to a channel to allow the input to settle before initiating the measurement. The
default delays used by the CR9000X are 10 µs on the ±5000 and ±1000 mV
ranges and 20 µs on the ±200 and ±50 mV range. This settling time is the
minimum required to allow the input to settle to the resolution specification.
The additional wire capacitance associated with long sensor leads can increase
the settling time constant to the point that measurement errors may occur.
There are three potential sources of error which must settle before the
measurement is made:
1. The signal must rise to its correct value.
2. A small transient caused by switching the analog input into the
measurement circuitry must settle.
3. When a resistive bridge measurement is made using a switched excitation
channel, a larger transient caused when the excitation is switched must
settle.
MINIMIZING SETTLING ERRORS
When long lead lengths are mandatory, the following general practices can be
used to minimize or measure settling errors:
1. When measurement speed is not a prime consideration, additional delay
time can be used to ensure ample settling time.
2 When making fast bridge measurements, use the continuous excitation
channels (1-6) to excite the bridges so the excitation doesn't have to settle
before each measurement.
3. Where possible run excitation leads and signal leads in separate shields to
minimize transients.
4. DO NOT USE WIRE WITH PVC INSULATED CONDUCTORS. PVC
has a high dielectric which extends input settling time.
5. Use the CR9000X to measure the input settling error associated with a
given configuration. Stabilize the sensor so that its output is not
changing. Program the CR9000X to make the measurement with the
delay you would like to use and a second time with a much longer delay
that ensures adequate settling time. The difference between the two
measurements is the error due to inadequate settling time.
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