Lab 8 Analog-to-Digital Converters, Part I The analog-to-digital converter, known as the A/D converter (read as A-to-D converter) or the ADC, is the second key component to bridging the analog and digital worlds. The ADC is the basis of digital voltmeters, digital multimeters, multichannel analyzers, oscilloscopes, and many other instruments. There are many different ADC designs, of which the ramp, tracking, and successive approximation converters are common. This lab looks at the ramp and tracking A/D converters.
Purpose of the Analog-to-Digital Converter The purpose of an ADC converter is to produce a digital binary number that is proportional to an analog input signal. The fundamental conversion process is shown in the following diagram.
Input Voltage
+ C –
Test Voltage
DAC b7
b0 Counter
Reset
Figure 8-1. Symbolic Design for an 8-Bit Analog-to-Digital Converter
A counter creates a test binary sequence, and its digital output is converted into an analog voltage using a digital-to-analog converter. The DAC is a basic element of many ADC circuits and was discussed in Lab 7. (This is a good time to review its operation if you are not familiar with the DAC.) The test voltage is then compared with the input signal. If the input signal is larger than the test signal, the counter is increased to bring the test signal closer to the input level. If the input signal is smaller than the test signal, the counter is decreased to bring the test signal closer to the input level. The
Š National Instruments Corporation
8-1
Fundamentals of Digital Electronics