2: ANALOG CONNECTIONS
2.1 ANALOG INPUTS
Before making actual connections, you may want to review the basic concepts of single-ended vs. differential inputs, and system
grounding and isolation.
2.1.1 Single-Ended Inputs
The input amplifier amplifies the voltage between the channel input line and LLGND. The single-ended input configuration actually
requires only one physical connection (wire) per channel and allows monitoring more channels than the (2-wire) differential configura-
tion using the same connector and on-board multiplexer.
However, since the input amplifier is amplifying the input voltage relative to its own low level ground, single-ended inputs are more
susceptible to both EMI (electromagnetic interference) and any ground noise at the signal source. Figures 2-1 and 2-2 show the single-
ended input configuration.
Figure 2-1. Single-Ended Input
Figure 2-2. Common-Mode voltage on Single-Ended Input
2.1.2 Differential Inputs
Differential amplifiers amplify the voltage between two distinct input signals. Within a certain range (referred to as the common mode
range), the amplified value is almost independent of signal source to board 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. Since the amplifier only
amplifies the difference between the two leads, and the EMI common to both inputs is ignored. This effect is a major reason for using
twisted-pair wire as the twisting assures that both wires are subject to virtually identical external influences. Figure 2-3 below shows
the basic differential input configuration.
4
+
-
Input
Amp
To A/D
I/O
Connector
LL GND
CH IN
Note: Input MUX omitted for
clarity.
+
-
Inp ut
A m p
To A /D
LL GN D
CH IN
~
1
2
V s
V s + V g2 - V g1
A n y vo ltag e diffe re ntia l b etw ee n groun ds
g1 a nd g 2 sho w s up a s a n e rror sign al
at th e inpu t am p lifier
S in g le -e n d e d in p u t w ith C o m m o n M o d e Vo lta g e
g
g
