 2001 Microchip Technology Inc.

DS00753A-page 1

M

AN753

OVERVIEW

An Analog-to-Digital (A/D) converter translates an ana-

log input signal into a discrete digital code. This digital

representation of the “real world” signal can be manip-

ulated in the digital domain for the purposes of informa-

tion processing, computing, data transmission or

control system implementation. In any application

where a converter is used, it is advantageous to have

the code structure complement the microcontroller’s

operands.

This application note describes the straight binary and

binary two’s complement code schemes that are out-

putted by Microchip’s Analog-to-Digital (A/D) convert-

ers.

All code examples given in this application note are for

a 4-bit conversion. The median analog voltages in the

tables are the equivalent analog voltages that are at the

center of the digital code.

STRAIGHT BINARY CODE

The straight binary code is more accurately called uni-

polar straight binary. This digital format for an A/D con-

version is the simplest to understand. As the name

implies, this coding scheme is used only when positive

voltages are converted. An example of this type of cod-

ing is shown in Table 1.

When this scheme is used to represent a positive ana-

log signal range, the digital code for zero volts is equal

to zero (0000 per Table 1). Given an ideal converter

with no offset, gain, INL or DNL error, the code transi-

tion from 0000 to 0001 occurs at the analog value of:

where:

where:

n is equal to the number of bits in the converter

+FS is equal to the analog full-scale range.

The A/D converters from Microchip that produce a

straight binary output code are from the MCP320X (12-

bit) and the MCP300X (10-bit) families.

These devices can be operated in a single ended, pos-

itive voltage input mode or a pseudo-differential input

mode, but in both cases the digital output represents a

positive input voltage. In the pseudo-differential mode,

the IN- input is limited to ±100 mV. This can be used to

cancel small noise signals present on both the IN+ and

IN- inputs. This provides a means of rejecting noise

when the IN- input is used to sense a remote signal

ground. The converter will produce digital code that

represents the analog input when the IN+ input range

IN+ is less than IN-, the resultant code for the family of

devices will be still be ‘0’, which does not represent a

negative voltage.

Author:

Bonnie C. Baker

Microchip Technology Inc.

First Code Transition

0

1

2

---LSB

+





=

Second Code Transition

1LSB

1

2

---LSB

+





=

LSB

+FS

2

n

-----------

=

Median Analog Voltage

(V)

Digital Code

1111

1110

1101

1100

1011

1010

1001

1000

0111

0110

0101

0100

0011

0010

0001

0

0000

TABLE 1:

The

unipolar

straight

binary

code

representation of zero volts is equal to a digital (0000).

The analog full-scale minus one LSB digital

representation is equal to (1111). With this code, there

is no digital representation for analog full-scale.

Digital Coding Schemes for Mixed Signal Communication