Single Chip Data Acquisition System Simplifies Analog To Digital Conversion AN 0193
User Manual: AN-0193
Open the PDF directly: View PDF 
.
Page Count: 4
| Download | |
| Open PDF In Browser | View PDF |
National Semiconductor Application Note 193 July 1977 Until recently, building an analog data acquisition system required a hardy cross-breed of both analog design and digital design. Now National Semiconductor has simplified the design problem of a data acquisition system with the introduction of the ADC0816 (MM74C948). This CMOS device incorporates many of the standard features of a data acquisition system onto a single chip. Included on-chip is an 8-bit analog-to-digital converter with bus oriented outputs, a 16channel expandable multiplexer, provisions for external signal conditioning, and logic control for systems interface. This chip marks the advent of a new generation in A/D converters, bringing versatility, performance, and economy using a technology ideally suited to data acquisition systems. structure of a data acquisition system while relieving the user from multichip interface and compatibility problems. A wide range of functional options allows extremely versatile operation of the device in a wide range of applications. The ADC0816 uses National’s low voltage, metal gate technology. The device operates from a single a 5V supply and features a 16-channel multiplexer with address input latches, latched TRI-STATEÉ outputs and a true eight-bit-accurate analog-to-digital converter. It consumes only 20 mW of power. Total conversion time of an analog signal is 100 ms. By using a patented A/D conversion technique the converter is guaranteed to have no missing codes and to be monotonic. The internal chopper stabilized comparator is the key element in minimizing both long term drift and temperature coefficients of other error terms. Figure 1 shows a block diagram of the functions provided within a single package. The chip duplicates the classical Single Chip Data Acquisition System Simplifies Analog-to-Digital Conversion Single Chip Data Acquisition System Simplifies Analog-to-Digital Conversion TL/H/7207 – 1 FIGURE 1. ADC0816/MM74C948 Block Diagram AN-193 TRI-STATEÉ is a registered trademark of National Semiconductor Corporation C1995 National Semiconductor Corporation TL/H/7207 RRD-B30M115/Printed in U. S. A. Figure 2 shows a typical application employing the ADC0816 for use in a microprocessor-based environmental control system. In this system the microprocessor can select a channel, monitor a particular sensor reading, convert that signal to a digital word, and make a system decision based upon that input. Many other areas of process control, machine control, or multi-input analog system can utilize this basic configuration. The bottom resistor and the top resistor of the ladder network in Figure 4 are not the same value as the remainder of the network. The difference in these resistors causes the output characteristic to be symmetrical with the zero and full-scale points of the transfer curve. The first output transition occurs when the analog signal has reached a 1/2 LSB and succeeding output transitions occur every 1 LSB later up to full-scale. The successive approximation register (SAR) performs eight iterations to approximate the input voltage. For any SAR-type converter, n iterations are required for an n-bit converter. Figure 4 shows a typical example of a 3-bit converter with an input voltage of 1/4 full-scale. Since the initial approximation at 7/16 of full-scale is too high, a zero is posted for the most significant bit (MSB). The second approximation is too low, therefore a one is posted for the second bit. The final approximation is determined to be too high, so a zero is posted for the least significant bit (LSB). In the ADC0816/MM74C948 the approximation technique is extended to eight bits using the 256R network. The most important section of the A/D converter is the comparator. It is this section which is responsible for the ultimate accuracy of the entire converter. It is also the comparator drift which has the greatest influence on the respectability of the device. A chopper stabilized comparator provides the most effective method of satisfying all the converter requirements. THE CONVERTER The heart of this single-chip data acquisition system is its 8-bit analog-to-digital converter. The converter is designed to give fast, accurate, and repeatable conversions over a wide range of temperatures. The converter is partitioned into three major sections: the 256R ladder network, the successive approximation register, and the comparator. The 256R ladder network approach was chosen over the conventional R/2R ladder because of its inherent monotonicity. Monotonicity is particularly important in closed-loop feedback control systems. A non-monotonic relationship can cause oscillations that could be catastrophic. Additionally, the 256R network does not cause load variations on the reference voltage. Figure 3 shows a comparison of the output characteristic for the two approaches with a variation in the ladder resistance. In the 256R approach with unequal or shorted resistors the slope of the output transfer function cannot be different from the slope of the analog input. For the R/2R ladder network, mismatches in the resistor values can cause the slope of the output digital code to be different from the analog input signal. TL/H/7207 – 2 FIGURE 2. Remote Environmental Control System 2 The design of this A/D converter has been optimized by incorporating the most desirable aspects of several conversion techniques. The ADC0816 offers high speed, high accuracy, low temperature dependence, excellent long-term accuracy and repeatability, and consumes minimal power. These features make this device ideally suited to applications such as process control, industrial control, and machine control. The chopper stabilized comparator converts the DC input signal into an AC signal. This signal is then fed through a high gain AC amplifier and has the DC level restored. This technique limits the drift component of the amplifier since the drift is a DC component which is not passed by the AC amplifier. This makes the entire A/D converter extremely insensitive to temperature, long-term drift, and input offset errors. TL/H/7207 – 3 FIGURE 3. 2nR and R2R Ladder Transfer Curves. In a 2nR ladder the most unequal resistors can do is cause a nonuniform voltage step. Since a single voltage is across the ladder it must be monotonic. In a R2R ladder unequal resistors may cause a sign change in the transfer curve, causing it to be nonmonotonic. TL/H/7207 – 4 FIGURE 4. Offset-Adjusted 8R Ladder gives g (/2 LSB quantizing error of 3 bits with three comparisons. The output code is derived by posting a one when upward arrows are followed and a zero when downward arrows are followed to the input voltage. 3 Single Chip Data Acquisition System Simplifies Analog-to-Digital Conversion AN-193 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. National Semiconductor Corporation 1111 West Bardin Road Arlington, TX 76017 Tel: 1(800) 272-9959 Fax: 1(800) 737-7018 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. National Semiconductor Europe Fax: (a49) 0-180-530 85 86 Email: cnjwge @ tevm2.nsc.com Deutsch Tel: (a49) 0-180-530 85 85 English Tel: (a49) 0-180-532 78 32 Fran3ais Tel: (a49) 0-180-532 93 58 Italiano Tel: (a49) 0-180-534 16 80 National Semiconductor Hong Kong Ltd. 13th Floor, Straight Block, Ocean Centre, 5 Canton Rd. Tsimshatsui, Kowloon Hong Kong Tel: (852) 2737-1600 Fax: (852) 2736-9960 National Semiconductor Japan Ltd. Tel: 81-043-299-2309 Fax: 81-043-299-2408 National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
Source Exif Data:
File Type : PDF File Type Extension : pdf MIME Type : application/pdf PDF Version : 1.3 Linearized : Yes Create Date : 1995:10:18 18:46:04 Producer : Acrobat Distiller 2.0 for Windows Title : Single Chip Data Acquisition System Simplifies Analog-to-Digital Conversion Subject : AN-193 Author : Keywords : Application Notes, Analog-To-Digital Modify Date : 2001:11:23 11:40:08+05:30 Page Count : 4EXIF Metadata provided by EXIF.tools