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A/D Board Tutorial
Slideshow Contents
Introduction
Typical A/D Board Architecture
A/D Chip Technology
Resolution and Input Range
Input Bandwidth
Input Channel Configurations
Differential Input Simulation
A/D Sampling Methods
A/D Triggers
Sampling Sequences
Data Transfer to Memory
Interrupt Timing
Source Impedance
Source Impedance - Solution
Calibration
Autocalibration
Comparison Test: Autocalibration vs. Manual Calibration
Results: Autocalibration vs. Manual Calibration
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Input Channel Configurations


The analog inputs on an A/D board can usually be configured as either Single-Ended or Differential.

A single-ended input has one signal wire and one ground wire. The ground wire may be shared across multiple inputs. The A/D board measures the difference between the signal input and the ground, or reference.

A differential input has two signal wires and a ground wire. The A/D board measures the difference between the two signal wires, regardless of their voltage relative to ground.

Type Single-Ended Differential
Wires2: Input and Reference3: Input+, Input-, and Reference
Noise immunityLessMore
Input channelsMoreLess
Wiring costLowerHigher

Differential inputs are used in two situations: (1) There is a possibility that electrical noise will adversely affect the quality of the signal, for example a low-voltage signal in a high-noise environment; and (2) The ground at the signal source is at a different potential than the ground at the A/D board, making it difficult to connect the two grounds together.

Differential inputs usually take up 2 input channels on an A/D board. This has a limiting effect on the number of channels the board can accept. Therefore most boards provide a means to select whether you are using single-ended or differential. Usually all the inputs must be of the same type. For example, a board with 32 single-ended inputs would let you configure it also for 16 differential inputs.

On the Diamond Systems Diamond-MM-32 board, however, you can select a "mixed mode" configuration, with 16 single-ended inputs and 8 differential inputs at the same time. This provides 24 total input channels, 8 more than you would get if you had to configure all inputs as differential.