Omega Speaker Systems PCI-DAS1001, PCI-DAS1002 manual Differential Inputs

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.

CH High

CH Low

LL GN D

I/O

Connector

+

Inp ut	To A /D
A m p

-

Differential Input

~	CH High
	Vs

+

Inp ut

Vs

CH Low

Vcm

LL GND

A m p

-

To A/D

g1	Vcm = Vg2 - Vg1
	g 2

Common Mode Voltage (Vcm) is ignored by differential input configuration. However, note that Vcm + Vs must remain within

the am plifier’s comm on mode range of ±10V

Diffe rential Inp ut

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).

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