Measurement Computing WaveBook rev.5.3 - Manual (Page 82)

Analog Signal & Ground Connections
Channel Analog Input, BNC Signal Connections
For each of the eight channel analog inputs, the BNC center (+) and shield (-) are internally connected to
WaveBook’s binding post labeled, ANALOG COMMON. 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 figures). WaveBook’s ANALOG COMMON connects to the computer power supply ground through
the TO COMPUTER DB25 connector and cable.
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.
Note that a pair of Schottky diodes is used in the WBK10A to clamp the ANALOG COMMON to within
0.3V of computer ground (see figure).
WaveBook and WBK10A both have isolated power supplies. Power input common is isolated from
ANALOG COMMON by >10 9 Ω in parallel with 0.1µF.
For WaveBook/512A, /516, /516A, /516E, or WBK10A 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 WaveBooks, notebook computers are rarely connected to AC power line ground.
This is true even when these devices are plugged into AC adapters.
Floating Grounds: 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.
4-12 WaveBook Operation Reference 897895 WaveBook/512A, /516, /516A, /516E