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Single-Ended vs. Differential Analog Signals

This section discusses and compares Single-Ended and Differential Input analog signals. We start with a description of each type and discuss when to use one type versus the other. We then explore the issue of connecting a Single-Ended analog signal to a Differential Input and the opposite case of connecting a Differential analog signal to a Single-Ended analog input. The concept of Single-Ended (SE) signals is generally well understood and used. However, this is generally not the case with Differential signals, and if you find its concept a little vague and confusing you are not alone as the concept of Differential Analog Signals is a topic that is often misunderstood. The purpose of this discussion is to make both SE and DI understandable and clear up any confusion.

Single-Ended Analog Signal:

An analog signal that is Single-Ended (often abbreviated as SE) consists of two components:

The Analog Signal componet
The Ground Reference for the Analog Signal.

Both aforementioned components of the SE signal are required to properly represent it. The Analog Signal without a Ground reference is not very useful, and likewise the Ground without the Analog Signal is not very useful. Typical type of connectors used for SE analog inputs for data acquisition systems are BNC connections, 3.5mm Minijacks and twisted pair discrete wires.

When a SE signal is connected to the single-ended input of a data acquisition system, the Ground of the SE signal is connected to the analog Ground of the data acquisition system. It is the SE Analog Signal as referenced to ground that is converted with the Analog-to-Digital converter.

Advantages of Single-Ended signals:
(1). single two conductor cable to carry the signal
(2). easy to understand
(3). Most data acqusition systems have twice the number of Single-Ended Analog Input signals versus Differential Input Signals

Disadvantages
(1). No common-mode noise rejection (explained in the discussion about Differential Analog Signals).

Differential Analog Signal:

An analog signal that is Differential (often abbreviated DI, or Differential Input) consists of three components:

The (+) Analog Signal component
The (-) Analog Signal component
The Ground Reference of the Analog Signal

All three of the aforementioned components of the DI signal are required to properly represent it. This is important to understand, as one of the misconceptions is that the DI signal is just the (+) and the (-) components, neglecting the Ground. The Ground reference is part of the DI signal.

When a Differential signal is connected to the Differential Input of a data acquisition system, there are three connections made:
The (+) Analog Signal to the (+) Differential Input connection
The (-) Analog Signal to the (-) Differential Input connection
The Ground reference of the Analog Signal to the Analog Ground of the Differential Input.

A Differential signal is created from a Single-Ended signal as follows:

(1). The Single-Ended signal goes into SE to DI amplifier
(2). The amplifier creates two signals
The SE signal with a gain of positive 1/2
The SE signal with a gain of negative 1/2

In this mode what is converted from Analog-to-Digital is actually the "difference" between two inputs.

For example,
Differential Input 0 is created from Analog Inputs 0 and 8 as follows:
DI(0) = Ain0 - Ain8
That is the DI(0) is made from subtracting the voltage on Ain8 from the voltage on Ain0.

Here are some conversion examples:
AIN0 AIN8 DI(0)
0      0     0
+2    +2     0
+2    -2    +4
0     -2    +2

AIN0 - AIN8 = DI(0)

A "true" differential signal consists of THREE signals, called
Signal+, Signal-, Ground.

When in SE mode you have 16 Analog Inputs, labeled Ain_0 to Ain_15.
When in DI mode you have 8 Analog Inputs, Ain_0 to Ain_7, which are made up of Analog Input pairs.

Single_Ended....Differential Pair
Ain_0...........Ain_0+
Ain_8...........Ain_0-

Ain_1...........Ain_1+
Ain_9...........Ain_1-

Ain_2...........Ain_2+
Ain_10..........Ain_2-

Ain_3...........Ain_3+
Ain_11..........Ain_3-

Ain_4...........Ain_4+
Ain_12..........Ain_4-

Ain_5...........Ain_5+
Ain_13..........Ain_5-

Ain_6...........Ain_6+
Ain_14..........Ain_6-

Ain_7...........Ain_7+
Ain_15..........Ain_7-

Thus the above listing shows how you using the 16 SE Analog Inputs to create the 8 DI inputs. Jumpers on the LabMaster External ADC card configure the Analog inputs for SE or DI.

If you are running in DI mode, and if you only put a single in 1 of the differential pair (either the Signal+ OR the Signal-) then the other Signal MUST be grounded.

For example:
If you are using Differential Input Ain6 and happen to only connect a signal to Ain6+ (Ain6) leaving Ain6- (Ain14) un-connected and let's say the signal in Ain6+ is +1 volt

IF Ain6- (which is Ain14) is GROUNDED, then
DI:Ain6 = (Ain6+) - (Ain6-)
DI:Ain6 = 1volt - 0 volt
DI:Ain6 = 1 volt
Which is correct! You put in 1 volt and measured 1 volt.

IF Ain6- (which is Ain_14) is FLOATING, then
DI:Ain6 = (Ain6+) - (Ain6-)
DI:Ain6 = 1 volt - floating
DI:Ain6 = floating
Which is wrong! You put in 1 volt and measured a floating signal that can be anything.

I suspect that you are really working with Single-Ended signals (and not true Differential Input signals) and this means that you probably connect to the Signal+ of the Differential pair which means the Signal- of the pair needs to be grounded.

Specifications subject to change without notice.

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Scientific Solutions ® Inc.

On-Line Reference:

Single-Ended vs. Differential Analog Signals

Single-Ended Analog Signal:

Differential Analog Signal:

Scientific Solutions ^® Inc.