Measurement-computing CIO-DAS160x/1x Manual de usuario descargar pdf (Pagina 14)

Figure 5-2b. Differential Input Theory

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 the diagram

above 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-DAS1600 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 5-3. The CIO-DAS1600 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 5-3. Common Mode Range

Input

Amp

To A/D

Differential

Input

LL GND

CH High

CH Low

Vcm

Common Mode Voltage (Vcm) is ignored

by differential input configuration. However,

note that Vcm + Vs must remain within

the amplifier’s common mode range of ±10V

Vcm = Vg2 - Vg1

gg1 2

+1V

+2V

+3V

+4V

-10V

+5V

-9V

+6V

-8V

+7V

-7V

+8V

-6V

+9V

-5V

+10V

-4V

+11V

-3V

+12V

-2V

+13V

-1V

Gray area represents common mode ran

Both V+ and V- must always remain withi

the common mode range relative to LL G

Vcm

With Vcm= +5VDC,

+Vs must be less than +5V, or the common mode range will be exceeded (>+10V)

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