The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Using Sensors to Detect and Measure
Choosing a Converter
The ADC0848 is a good, general-purpose chip, but you may want to look at other converters,
depending on your application. Below are some things to consider when choosing an A/D
converter. Example answers describe the ADC0848, using information from its data sheet:
• What is the analog input range? (0V to V+)
• How many analog channels are there? (8)
• What is the converter’s resolution? (8 bits)
• How fast is the conversion? (30 microseconds typical, 60 microseceonds maximum)
• How accurate is the conversion? (±.1 LSB (least significant bit), 1/2 LSB version
available)
• What are the power-supply requirements and power consumption? (+4.5 to +6V, 15
milliwatts)
• What input modes are available? (single-ended, differential, pseudo-differential)
• How is the converter controlled and interfaced? (control signals are WE, OE, CS
• Are there any special features on-chip (sample-and-hold, voltage reference, etc.)? (an
internal clock times the conversions)
• What package types are available? (24-pin 0.3" DIP, 28-lead chip carrier)
Sample and Hold Circuits
An additional component that you may need for rapidly changing analog inputs is a
sample-and-hold circuit. To ensure correct conversions, the analog input must not change
in value while the conversion is taking place.
A sample-and-hold circuit ensures that the analog signal is stable by sampling the signal at
the desired measurement time and storing it, usually as a charge on a capacitor. The converter
uses this stored signal as the input to be converted.
When do you need a sample-and-hold? The ADC0848 requires 60 microseconds or less to
convert, so you should get good results with inputs that do not vary more than 1 bit in this
amount of time. When a rapidly changing input does require one, sample-and-hold ICs like
the LF398 are available, or you can use a converter like the ADC0820, which has the
sample-and-hold on-chip.
The Microcontroller Idea Book 169