Measurement Computing IOtech 640 Series - Manual (Page 45)

640 & 650 Series 878893 Analog Signals 6-3
Analog Common
The BNC shells for the analog input channels have a 1k ohm resistance from the BNC shell to earth ground
[computer chassis ground]. If a transducer is effectively earth grounded through its connection to a device under
test, there exists the possibility of added measurement noise due to the ground loop that is created. This issue is
minimized by electrically isolating the transducer from the test device.
If the host computer is a desktop PC, then the computer ground will likely connect to the AC power line ground.
If the host computer is a notebook PC, then the computer ground could be: (a) floating, for example, when operating
on batteries, or (b) connected to a vehicle ground, for example, when using an automotive cigarette lighter adapter
in conjunction with the vehicle’s battery.
TIP: Additional measurement noise may be present when using earth grounded transducers.
For best results electrically isolate the input transducers from earth ground.
Current Source (IEPE) with Transducer Fault Detection
If IEPE is selected in software, a constant current is supplied to bias IEPE transducers. The bias current is sourced
through the center conductor of the input channel BNC connector and returns to the conditioning circuit by the outer
conductor. The current source features a voltage operating compliance and is short-circuit and over-voltage
protected as stated in the specifications. Operating compliance refers to the highest voltage that can be sourced
without change of the current source value. In the absence of a transducer, the current source will output a higher
open circuit voltage. For applications that do not require bias, the current source can be disabled from the input via
software control on a per channel basis.
When the current source is enabled, the input voltage is continuously monitored with level detection circuitry for
indicating an open transducer (high voltage) or a transducer short (low voltage). Existence of either condition
triggers a transducer fault for the associated channel. This error is communicated to the user on the monitor via
software and is also available through a software status request at the end of an acquisition. Faults are detected and
communicated when present. Detection of a fault does not alter the acquisition process or its data.
Note that IEPE current source can only be enabled when AC Coupling is selected.
Input Coupling
The analog input channels can be independently set in software to AC Coupling or to DC Coupling. When AC
Coupling is selected, the input signal passes through a high pass filter. When DC Coupling is selected the high-pass
filter is bypassed.