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Table of Contents

Introduction

Connecting to the 24/7 Data Acquisition Module

Hardware

On-Board Reference

Digital Inputs

Analog Inputs

Programmable Gain

Input Protection

Allowable Input Impedance

Converter Noise and Effective Resolution

Other Sources of Noise and Offset Error

Tips For Measuring Small Voltages

Addressing the 24/7 WildCard Using Module Select Jumpers

Software

Initializing the 24/7 Data Acquisition Wildcard

Specifying the Reference Voltage

Starting a Conversion

Calibration Options
Choosing the Sample Rate
Setting the Gain
Choosing Bipolar or Unipolar Conversion
Specifying 16- or 24-bit Resolution
Synchronizing Conversions
Specifying the Source Channels

Obtaining A Sample

Single- and Multi-Channel Sampling

Continuous Sampling from a Single Channel
Sequential Sampling from Several Channels

C Example Listing (pdf)

Forth Example Listing (pdf)

Glossary

Appendix A:24/7 Data Acquisition Wildcard Pin-outs

Appendix B:24/7 Data Acquisition Wildcard Schematics (pdf)

24/7 Data Acquisition Wildcard Users Guide

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Introduction

The 24/7 Data Acquisition Wildcard gives you the ability to amplify and sample low level signals at various conversion rates directly from transducers using a sigma-delta technique. Its state-of-the-art analog-to-digital converter, Analog Devicesí AD7714, provides 24 bits of resolution with no missing codes performance. The 24/7 Data Acquisition Wildcard performs all signal conditioning and conversion for a system of up to seven input channels that are configurable in 10 different ways. This Wildcard also provides three general purpose digital inputs. Specifications are summarized in the following table.

A/D Type: 24-bit sigma-delta A/D converter
Programmable Gain: Precisely 1, 2, 4, 8, 16, 32, 64 or 128, software selectable
Input Range: Differential: ±2.5V with each input within -0.03 V to +5.03 V
Common Mode: -30 mV to 5.0 V unbuffered,
50 mV to 3.5V buffered
Analog Inputs: 4 fully differential or 7 pseudo-differential, protected to ± 70 volts continuous
Conversion Rate: Programmable to 1010 Hz, determines the effective resolution and filter cut-off frequency. For example:
Data
Rate
Filter
Cutoff
Effective
Resolution
10Hz2.6Hz21.5 bits
30Hz7.9 HZ20
100 Hz26 Hz18.5
500 Hz131 Hz13
Monotonicity: 24 bits at up to 60 Hz data rate
Linearity: 0.0015% FS at up to 60 Hz data rate
Noise Rejection: DC Common Mode: >90 dB
Normal Mode at 50/60 Hz: >100 dB
Common Mode at 50/60 Hz: >150 dB
Filter Setting: < 4x the data conversion period
Power: 5.5 to 14 v at 70 ma.
Size: 2.0 x 2.5 in.

High-level software routines allow you to initialize, calibrate, configure, and control the 24/7 Data Acquisition Module. Once commanded to start converting, the analog to digital converter continually samples and converts at a fixed rate. You can read the output once, or store each sequential conversion to a memory buffer.

You must sample one channel at a time; changing channels requires restarting the conversion process with a different channel specified.

data acquisition board: diagram

The above figure diagrams the WildCard. Signals are taken from the Field Bus on the left. The eight inputs may be configured in four pairs, for four differential input channels, or they may be configured as seven pseudo-differential inputs with the eighth line acting as a common return. Power and reference voltages are provided to the field header for your use. You can configure the converter to use the 2.5 volt internal reference, or to take its reference from the field header.

Many sensors and measurement methods produce an output that is ratiometric; that is, an output proportional to a reference signal or that is accompanied by its own full scale reference signal. You can accurately measure these by providing an appropriate reference voltage to the reference inputs on the field header. Communication with the WildCard is via the SPI serial lines and is handled transparently by the supplied driver routines.

This document describes, step by step, how to use the 24/7 Data Acquisition Module. It will show you how to:

  • Power up and connect to the module;
  • Configure and use the available hardware; and,
  • Call the pre-coded software drivers to initialize, calibrate, configure, and obtain samples from the module.
The last sections of this document provide an example code listing, a glossary, and the complete schematics of the 24/7 Data Acquisition Module. If after reading this document you have further questions, please consult the data sheet for the AD7714 that is provided with the distribution diskette or located at http://www.analog.com.

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