TABLE OF CONTENTS Tim Williams
The two main problems that face designers who have to integrate analog and digital circuits on the same PCB are:
preventing digital switching noise from contaminating the analog signal, and
interfacing the wide range of analog input voltages to the digital circuit.
Generating analog outputs from digital signals is not usually a problem. Generating digital inputs from analog signals is.
The high-frequency switching noise that was discussed earlier must be kept out of analog circuits at all costs. An analog-to-digital interface quantizes a variable analog signal into a digital word, and the number of bits in the word determines the resolution that can be achieved of the signal. Assuming a full-scale voltage range of 0 to 10V, which is typical of many analog-digital converters (ADCs), Table 15.1 shows the voltage levels that correspond to one bit change in the digital word.
| Word length | Resolution voltage |
|---|---|
| 8 bit | 39 mV |
| 10 bit | 10 mV |
| 12 bit | 2.4 mV |
| 14 bit | 0.6 mV |
| 16 bit | 0.15 mV |
You can see that the more resolution is demanded of the interface, the smaller the voltage change that will cause one bit change. 8 bits is regarded as commonplace in ADC circuits, 12 bits as reasonably high resolution (0.025%) and 16 bits as precision.
The significance of these diminishing voltage levels is that any noise that is coupled into the analog input will cause...
