Measurement Computing WavePort rev.3.0 - Manual (Page 38)

4-10 Hardware and Operation Reference 09-29-00 WavePort User’s Manual
Ground Connections
Channel Analog Input, WavePort/PE BNC Signal Connections
The center (+) and shield (-) each connect to ANALOG COMMON through a 5 M Ω resistor, resulting in a
10 M Ω differential input resistance (see above figure). WavePort’s ANALOG COMMON connects to the
computer power supply ground through the TO COMPUTER DB25 connector and cable.
When using a Notebook PC the computer ground could be:
(a) floating, for example, when the Notebook is operating on batteries, or
(b) connected to vehicle ground, for example, when using an automotive cigarette-lighter adapter in conjunction with
the vehicle’s battery.
Note that a pair of Schottky diodes is used in the WBK14 to clamp the ANALOG COMMON to within
0.3V of computer ground (see figure).
WavePort has an isolated power supply. Power input common is isolated from ANALOG COMMON by
>10
9 Ω in parallel with 0.1µF.
For the “WaveBook” module to correctly measure analog signals, each signal must be within
±11 volts of ANALOG COMMON. The following notes provide guidelines on how to achieve this.
Like WavePort, Notebook computers are rarely connected to AC power line ground.
This is true even when these devices are plugged into AC adapters.
Connecting to ANALOG COMMON
If the computer is battery operated and the signal source is floating (such as an ungrounded sensor), then the
internal 5 M
Ω resistors may provide enough of a return path to ANALOG COMMON. If either the
computer or the analog signal source is committed to AC power line ground, then you will require a
direct connection between the signal source and ANALOG COMMON.
When in doubt, connect the signal source common to ANALOG COMMON.
A single-ended signal source needs to have its common connected to ANALOG COMMON.
When connecting several signal source commons to ANALOG COMMON, it is important that there is
no voltage potential [between these signal source commons]. Otherwise, ground currents will circulate,
leading to measurement errors.
If there is a fixed voltage potential between multiple signal source commons, then only one of these signal
source commons needs connected to ANALOG COMMON. This is true as long as the common mode
voltage of any input does not exceed ±11 volts.