microchip_adc.pdf

Analog-to-Digital

Converters

Getting Started - Analog-to-Digital Converters

In this presentation we will look at the Analog-to-Digital Converter Peripherals with

Microchip’s midrange PICmicro ® Microcontrollers series.

Analog- to

to- Digital Converters

Digital Converters

Analog

Analog-to-Digital Converters

l Topics Covered:

Terminology

Configuration

Usage

Differences between 8- and 10- or 12-bit A/D

Additional Resources

Getting Started - Analog-to-Digital Converters

Basic analog-to-digital converter terminology will be covered first, followed by

configuration of the analog-to-digital converter peripheral. Next, information on the

usage of the peripheral will be presented, initially focusing on the 8-bit analog-to-

digital converter. Then the differences between the 8-bit and the 10-or 12-bit

converters will be discussed. Finally, some additional reference resources will be

highlighted.

Analog- to

to- Digital Converter

Digital Converter

Analog

Analog-to-Digital Converter

l Converts an analog voltage level to a digital

number

l Converts an analog voltage level to a digital

number

Digital Numbers can be effectively handled by

microcontrollers, analog levels cannot

Digital Numbers can be effectively handled by

microcontrollers, analog levels cannot

Digital numbers are non-fractional

A/D

Converter

2.343 volts

Getting Started - Analog-to-Digital Converters

Microcontrollers are very efficient at processing digital numbers, but they cannot

handle analog signals directly. An analog-to-digital converter, converts an analog

voltage level to a digital number. The microcontroller can then efficiently process

the digital representation of the original analog voltage. By definition, digital

numbers are non-fractional whole numbers.

In this example, an input voltage of 2.343 volts is converted to 87. The user’s

software can use the value 87 as the representation of the original input voltage. At

this point, the number 87 is only used for discussion purposes as a typical output.

Input Range

l The analog input voltage must be within the

valid input range of the A/D for an accurate

conversion

l Input range set by reference voltages

l The analog input voltage must be within the

valid input range of the A/D for an accurate

conversion

l Input range set by reference voltages

Power and Ground

External References

Internal References

Getting Started - Analog-to-Digital Converters

The analog-to-digital converter is only capable of performing an accurate

conversion if the analog input voltage is within the valid input range of the

converter. If the input voltage falls outside this range, the conversion value will be

inaccurate. The input range is set by high and low voltage references. These define

the upper and lower limits of the valid input range. In many cases, the high and low

voltage references are selected as the microcontroller supply voltage and ground, at

other times an external reference or references are used.

In addition, some devices have internal voltage references that can be used. The

source or sources for these voltage references are a configuration option when

setting up the analog-to-digital converter in the PICmicro microcontroller (MCU).

Note that there are restrictions on the voltage reference levels, for example: the

reference voltages generally shouldn’t be less than Vss or greater than V DD . There

is also a minimum difference that is required between the high and low reference

voltages. Please consult your data sheet for the voltage reference requirements.

Quantization

l Refers to subdividing a space into small but

measurable increments.

l The maximum quantization error is 1/2 the

increment size

Quantization

Limits

Result

2.5V

2.4V

2.3V

2.2V

Input Voltage

2.343V

Getting Started - Analog-to-Digital Converters

The output of an analog-to-digital converter is a quantized representation of the

original analog signal. The term quantization refers to subdividing a range into

small but measurable increments. The total allowable input range is divided into a

finite number of regions with a fixed increment. The analog-to-digital converter

determines the appropriate region to assign the given input voltage.

In this example, the step or increment is one-tenth of a volt and the input voltage is

2.343 volts. The appropriate result would be assigned as a digital value of 87,

because 2.343 volts fits between the quantization limits of 2.3 volts and 2.4 volts.

Any input voltage between the 2.3 and 2.4 volt quantization limits will be assigned

a digital value of 87.

The process of quantization has the potential to introduce an inaccuracy known as

quantization error, which can be viewed as being similar to a rounding error. In the

above example, the 2.343 volt input is in effect rounded to the nearest tenth of a

volt. The maximum quantization error in this case would be five hundredths of a

volt or one-half of the increment size. It should be noted that the minimum

quantization error for the analog-to-digital converter peripheral in the PICmicro

devices is 500 micro volts. Therefore, the smallest step size for each state cannot be

less than one milli-volt.

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