Campbell CM3, Instruction Manual, Page: 9 / 16

CM3 Pyranometer
5
If a 21X is used to measure the CM3 and it powers a 12 VDC sensor or 12
VDC radio, the current drawn off the 12 VDC supply may cause a difference in
ground potential between the 21X ground terminals and the reference ground
point in the datalogger. This ground potential results in an offset on single
ended measurements. This offset can be as large as +60 mV. Thus, single
ended measurements should be avoided. The offset does not, however, affect
differential measurements. While the 21X is the logger most susceptible to this
condition, the other dataloggers can be affected if the amount of power is too
large or the power return line is connected to analog ground (AG).
FIGURE 4-1. CM3 Wiring
Color Function CR10(X),
CR510 21X/CR7 CR23X
Red Signal Differential
Channel - H Differential
Channel - H Differential
Channel - H
Blue Signal Reference Differential
Channel - L Differential
Channel - L Differential
Channel - L
White Signal Ground AG
≡ ≡
Clear Shield G
≡ ≡
White = Signal Ground
Blue = Signal Reference
Red = Signal
Clear = Shield