Omega Speaker Systems PCI-DAS1001 User Manual download pdf (Page 9)

Differential Inputs

Differential inputs measure the voltage between two distinct input signals. Within a certain range (referred to as the common

mode range), the measurement is almost independent of signal source to PCI-DAS1000 ground variations. A differential

input is also much more immune to EMI than a single-ended one. Most EMI noise induced in one lead is also induced in the

other, the input only measures the difference between the two leads, and the EMI common to both is ignored. This effect is a

major reason there is twisted pair wire as the twisting assures that both wires are subject to virtually identical external influ-

ence. The diagram below shows a typical differential input configuration.

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 PCI-DAS1000 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 the following diagram. The PCI-DAS1000 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).

Input

Amp

To A /D

Differential Input

I/O

Connector

LL GND

CH High

CH Low

Input

Amp

To A/D

Differential

Input

LL GND

CH High

CH Low

Vcm

Common Mode Volta

e (Vcm) is i

nored

differential input confi

uration. However,

note that Vcm + Vs must remain within

the amplifier’s common mode ran

e of ±10V

Vcm = V

2 - V

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Omega Speaker Systems PCI-DAS1001 User Manual Page 9