Measurement-computing CIO-DAS08-PGH User Manual download pdf (Page 11)

Before moving on to the discussion of grounding and isolation, it is important to explain the concepts of common mode, and

common mode range (CM Range). Common mode voltage is depicted in Figure 3-2 as Vcm. Though differential inputs measure

the voltage between two signals, without (almost) respect to the either signal’s voltages relative to ground, there is a limit to how

far away from ground either signal can go. Though the CIO-DAS08-PGx has differential inputs, it will not measure the difference

between 100V and 101V as 1 Volt (in fact the 100V would destroy the board!). This limitation or common mode range is depicted

graphically in Figure 3-3. The CIO-DAS08-PGx common mode range is +/- 10 Volts. Even in differential mode, no input signal

can be measured if it is more than 10V from the board’s low level ground (LLGND).

Figure 3-3. Common Mode Range Limits

3.1.3 System Grounds and Isolation

There are three scenarios possible when connecting your signal source to your board.

1. The board and the signal source may have the same (or common) ground. This signal source can be connected directly to the

board.

2. The board and the signal source may have an offset voltage between their grounds (AC and/or DC). This offset it commonly

referred to a common mode voltage. Depending on the magnitude of this voltage, it may or may not be possible to connect

the board directly to your signal source. We will discuss this topic further in a later section.

3. The board and the signal source may already have isolated grounds. This signal source may be connected directly to the

board.

3.1.4 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 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 do not have access to a voltmeter, skip the experiment and take a look at the following three sections. You may be able to identify your system type from the

descriptions provided.

+1V

-13V

+2V

-12V

+3V

-11V

+4V

-10V

+5V

-9V

+6V

-8V

+7V

-7V

+8V

-6V

+9V

-5V

+10V

-4V

+11V

-3V

+12V

-2V

+13V

-1V

Gra

area represents common mode ran

Both V+ and V- must alwa

s remain within

the common mode ran

e relative to LL Gnd

Vcm (Common Mode Volta

e) = +5 Volts

Vcm

With Vcm= +5VDC,

+Vs must be less than +5V, or the common mode ran

e will be exceeded (>+10V)

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Measurement-computing CIO-DAS08-PGH User Manual Page 11

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