User`s manual

Which system do you have?

Try the following experiment. Using a battery powered voltmeter*, measure the voltage (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 you do not have access to a voltmeter, skip this experiment and take a look a the following three

sections. You may be able to identify your system type from the descriptions provided.

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 voltage between the

grounds category. This offset is referred to as a Common Mode Voltage. Please to read the following

warning carefully, then proceed to the section describing Common Mode systems.

WARNING

If either the ac or dc voltage is greater than 10 volts, do not connect the

PCI-DAS1602/12 to this signal source. You are beyond the board’s usable common

mode range and will need to either adjust your grounding system or add special isola-

tion signal conditioning to take useful measurements. A ground offset voltage of more

than 30 volts will likely damage the PCI-DAS1602/12 board and possibly your

computer. Note that an offset voltage much greater than 30 volts will not only damage

your electronics, but it may also be hazardous to you.

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 isolation signal conditioning.

If you cannot obtain a reasonably stable dc voltage measurement between the grounds, or the voltage drifts

around considerably, the two grounds are most likely isolated. The easiest way to check for isolation is to

change your voltmeter to it’s ohm scale and measure the resistance between the two grounds. It is recom-

mended that you turn both systems off prior to taking this resistance measurement. If the measured resis-

tance is more than 100 Kohm, it’s a fairly safe bet that your system has electrically isolated grounds.

Systems with Common Grounds

In the simplest (but perhaps least likely) case, your signal source will have the same ground as the

PCI-DAS1602/12. This would typically occur when providing power or excitation to your signal source

directly from the PCI-DAS1602/12. There may be other common ground configurations, but it is important

to note that any voltage between the PCI-DAS1602/12 ground and your signal ground is a potential error

voltage if you set up your system based on a common ground assumption.

As a safe rule of thumb, if your signal source or sensor is not connected directly to an LLGND pin on your

PCI-DAS1602/12, it’s best to assume that you do not have a common ground even if your voltmeter

measured 0.0 volts. Configure your system as if there is ground offset voltage between the source and the

PCI-DAS1602/12. This is especially true if you are using high gains, since ground potentials in the

sub-millivolt range will be large enough to cause A/D errors, yet will not likely be measured by your

handheld voltmeter.

ystems with Common Mode (ground offset) Voltages