MCP3302 datasheet(21/46 Pages) MICROCHIP

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DS21697E-page 21

MCP3302/04

Using an external operation amplifier on the input

allows for gain and also buffers the input signal from the

input to the ADC allowing for a higher source

impedance. This circuit is shown in Figure 5-5.

FIGURE 5-5:

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 5-5.

FIGURE 5-6:

Circuit solution to overcome

amplifier output swing limitation.

5.4

Common Mode Input Range

The common mode input range has no restriction and is

equal to the absolute input voltage range: VSS -0.3V to

VDD + 0.3V. However, for a given VREF, the common

mode voltage has a limited swing, if the entire range of

the A/D converter is to be used. Figure 5-7 and

Figure 5-8 show the relationship between VREF and the

common mode voltage swing. A smaller VREF allows for

wider flexibility in a common mode voltage. VREF levels,

down to 400 mv, exhibit less than 0.1 LSB change in

DNL and INL. For characterization graphs that show

this performance relationship, see Figure 2-9 and

Figure 2-12.

FIGURE 5-7:

Common Mode Input Range

of Full Differential Input Signal versus VREF.

FIGURE 5-8:

Common Mode Input Range

versus VREF for Pseudo Differential Input.

VDD = 5V

MCP6021

IN+

IN-

VREF

MCP330X

1µF

MCP1525

VIN

VOUT

10 k

VIN

0.1 µF

1µF

2.048V

VDD = 5V

0.1 µF

MCP606

IN+

IN-

VREF

MCP330X

1µF

MCP1525

VIN

VOUT

0.1 µF

10 k

1µF

VIN

10 k

VREF(V)

0.25

VDD = 5V

5.0

1.0

2.5

4.0

-1

4.05V

2.8V

2.3V

0.95V

VREF (V)

0.25

VDD = 5V

2.5

0.5

1.25

2.0

-1

4.05V

2.8V

2.3V

0.95V

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