Measurement Computing PCM-DAS16x/16 - Manual (Page 23)

mode voltage. Depending on the magnitude of this voltage, it may or may not be
possible to connect the PCM-DAS16x/16 directly to your signal source. We will
discuss this topic further in a later section.
3. The PCM-DAS16x/16 and the signal source may already have isolated grounds.
This signal source may be connected directly to the PCM-DAS16x/16.
6.1.5 Which system do you have?
Try the following experiment. Using a battery powered voltmeter
1 , measure the volt-
age (difference) between the ground signal at your signal source and at your PC. Place
one voltmeter probe on the PC ground and the other on the signal source ground.
Measure both the AC and DC Voltages.
If both AC and DC readings are 0.00 volts, you may have a system with common
grounds. However, since voltmeters will average out high frequency signals, there is
no guarantee. Please refer to the section below titled Common Grounds.
If you measure reasonably stable AC and DC voltages, your system has an offset volt-
age between the grounds category. This offset is referred to as a Common Mode Volt-
age. Please be careful to read the following warning and then proceed to the section
describing Common Mode systems.
WARNING
If either the AC or DC voltage is greater than 10 volts, do not con-
nect the PCM-DAS16x/16 to this signal source. You are beyond the
boards usable common mode range and will need to either adjust
your grounding system or add special Isolation signal conditioning
to take useful measurements. A ground offset voltage of more than
30 volts will likely damage the PCM-DAS16x/16 board and possi-
bly your computer. Note that an offset voltage much greater than 30
volts will not only damage your electronics, but it may also be haz-
ardous to your health.
This is such an important point, that we will state it again. If the
voltage between the ground of your signal source and your PC is
greater than 10 volts, your board will not take useful
measurements. If this voltage is greater than 30 volts, it will likely
cause damage, and may represent a serious shock hazard! In this
case you will need to either reconfigure your system to reduce the
ground differentials, or purchase and install special electrical iso-
lation signal conditioning.
19
1
If you do not have access to a voltmeter, skip the experiment and take a look at the following three sec-
tions. You may be able to identify your system type from the descriptions provided.