Download DL2e Data Logger Hardware Reference

Hardware Reference
Sensors and Input Cards
Voltage, differential
Voltage, differential
Differential sensor connection is suitable for measuring DC voltages up to ±2V. Source
impedance of the voltage source should be less than 24kΩ for full logger accuracy.
Fully floating
Fully floating connection is suitable for sensors with an existing connection to the
logger’s earth, or with a low-impedance common-mode voltage less than ±2V.
Bias resistors, earthed sensor
Differential connections with the sensor earthed or with bias resistors fitted, are
superior alternatives to single-ended connection. These schemes are recommended for
inputs susceptible to picking up high-impedance common-mode noise, for example
thermocouples on long leads in electrically noisy environments.
Setting up
LAC1, 15-ch mode
•
Set 15/30 switch to ‘15’.
•
Connect one ribbon cable only, from any terminal group to the position marked
‘differential’ on the card.
ACD1
•
Connect a ribbon cable from any terminal group to the card.
LFW1
•
Set the R/PRT switch to ‘R’ position for optimum common mode rejection (see "4Wire Card, type LFW1" on page 75).
•
Connect one or two ribbon cable(s) from any terminal group to either or both
positions on the card.
LAC1, ACD1 and LFW1
•
Fit bias resistors or earth the sensor, if appropriate. 100kΩ is generally a suitable
value for bias resistors.
Programming Issues
When programming a sensor type, or entering measurement details for a <Custom
sensor type>, proceed as follows in the Measurement tab of the Sensor Type or Channel
Properties dialog:
1.
Select Electrical Measurement ‘DC Voltage’.
2.
If programming a sensor type, also select Connection Requirement ‘Differential,
low CM’ if the sensor output has a negligible common mode (for example a sensor
powered from the logger’s power supply), or ‘Differential, high CM’ if the sensor
output has a significant common mode. The latter setting will exclude the sensor
type from being programmed for a LFW1 card set up to provide a resistance offset,
where common mode can introduce significant reading errors.
Remarks
The logger’s input impedance is only specified for the logger when awake, and may
drop significantly when the logger sleeps. If using a sensor with a high source
impedance, check that it can respond quickly enough (within 250ms) to the change in
the logger’s input impedance as it wakes.
DL2e Hardware Reference
Page 89