Microchip MCP3301 Handleiding

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 2002 Microchip Technology Inc.DS00842A-page 1

MAN842

INTRODUCTION

True differential converters can offer many advantages

over single-ended input A/D Converters (ADC). In addi-

tion to their common mode rejection ability, these con-

verters can also be used to overcome many DC biasing

limitations of common signal conditioning circuits.

Listed below are some typical application issues that

can be solved with proper biasing of a differential con-

verter:

•Limited output swing of amplifiers

•Unwanted DC-bias point

•Low level noise riding on ground

•Unwanted or changing common mode level of

input signal

This application note discusses differential input config-

urations and their operation, circuits to implement

these input modes and techniques in choosing the cor-

rect voltage levels to overcome the previously

mentioned challenges.

DIFFERENTIAL AND SINGLE-ENDED

INPUT CONFIGURATIONS

Before discussing biasing solutions, it is important to

understand the functionality of differential A/D convert-

ers. The true differential A/D converter outputs a digital

representation of a differential input signal, typically a

two’s complement binary formatted output. The con-

verter output can be either signed positive or negative,

depending on the voltage level of the differential pair.

The following equation expresses this relationship for

the MCP330X devices:

EQUATION:

The binary output for the MCP330X is a 13-bit output

(12-bit plus sign output).

It is important to note that the converter output is zero

when the inputs are equal. As the voltage difference

between IN+ and IN- increases, the output code also

increases. The maximum voltage at which digital code

saturation will occur is VREF. The differential conver-

sion of the MCP330X converters will reject any DC

common mode signal at the inputs. For the MCP330X

converters, the common mode input range is rail-to-

rail, VSS-0.3V to VDD+0.3V.

The circuit in Figure 1 shows a differential signal being

applied to the IN+ and IN- pins of the converter. This

method is referred to as full differential operation of the

converter. The graph below the circuit shows possible

voltage levels for a differential application. The inputs

are centered around a common mode voltage, VCM.

VREFis equal to the maximum input swing, shown here

as VDD. By setting V

REF equal to the maximum input

swing of the signal, the full range of the A/D converter

is being used.

FIGURE 1:Driving a true differential

converter with a true differential input.

Author:Craig L. King

Microchip Technology Inc.

Digital Code2n( )IN+IN-

–( )

2V REF

--------------------------------------=

VDD

1 µF

Input Signal

Differential VREF

p-p

VREF

p-p

VCM

-4096 +4095

Output Code

IN+

IN-

VREF

VDD

1/2VDD

GND

Voltage Levels (V)

VCM

IN+

IN-

VREF VDD

VSS

Differential ADC Biasing Techniques, Tips and Tricks

Product specificaties