MCP3301-CISLASHP datasheet(18/32 Pages) MICROCHIP

18 / 32 page

MCP3301

DS21700C-page 18

FIGURE 6-5:

Adding an amplifier allows

for gain and also buffers the input from any high

impedance sources.

This circuit shows that some headroom will be lost due

to the amplifier output not being able to swing all the

way to the rail. An example would be for an output

swing of 0V to 5V. This limitation can be overcome by

supplying a VREF that is slightly less than the common

mode voltage. Using a 2.048V reference for the A/D

converter, while biasing the input signal at 2.5V solves

the problem. This circuit is shown in Figure 6-6.

FIGURE 6-6:

Circuit solution to overcome

amplifier output swing limitation.

6.4

Common Mode Input Range

The common mode input range has no restriction and

is equal to the absolute input voltage range: V

SS -0.3V

to V

DD +0.3V. However, for a given V

REF, the common

mode voltage has a limited swing if the entire range of

the A/D converter is to be used. Figure 6-7 and

Figure 6-8 show the relationship between VREF and the

common mode voltage. A smaller VREF allows for wider

flexibility in a common mode voltage. VREF levels down

to 400 mV and exhibits less than 0.1 LSB change in

DNL and INL. See Figure 2-9 and Figure 2-12 for char-

acterization graphs that illustrate this performance

relationship.

FIGURE 6-7:

Common Mode Range of

Full Differential input signal versus V

REF.

FIGURE 6-8:

Common Mode Range

versus VREF for Pseudo Differential Input.

DD = 5V

0.1 µF

MCP6022

IN+

IN-

VREF

MCP3301

1µF

MCP1525

VIN

VOUT

0.1 µF

10 k

1µF

VIN

2.048V

DD = 5V

0.1 µF

MCP606

IN+

IN-

VREF

MCP3301

1µF

MCP1525

VIN

VOUT

0.1 µF

10 k

1µF

VIN

10 k

REF (V)

0.25

DD = 5V

5.0

1.0

2.5

4.0

-1

4.05V

2.8V

2.3V

0.95V

REF (V)

0.25

DD = 5V

2.5

0.5

1.25

2.0

-1

4.05V

2.8V

2.3V

0.95V

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