MCP3302 datasheet(20/46 Pages) MICROCHIP

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MCP3302/04

DS21697E-page 20

FIGURE 5-3:

Analog Input Model.

5.2.1

MAINTAINING MINIMUM CLOCK

SPEED

When the MCP3302/04 initiates, charge is stored on

the sample capacitor. When the sample period is

complete, the device converts one bit for each clock

that is received. It is important for the user to note that

a slow clock rate will allow charge to bleed off the

sample cap while the conversion is taking place. For

the MCP330X devices, the recommended minimum

clock speed during the conversion cycle (TCONV) is

105 kHz. Failure to meet this criteria may induce

linearity errors into the conversion outside the rated

specifications. It should be noted that during the entire

conversion cycle, the

A/D converter does not have

requirements for clock speed or duty cycle, as long as

all timing specifications are met.

5.3

Biasing Solutions

For pseudo-differential bipolar operation, the biasing

circuit (shown in Figure 5-4) shows a single ended

input AC coupled to the converter. This configuration

will give a digital output range of -4096 to +4095. With

the 2.5V reference, the LSB size equal to 610 µV.

Although the ADC is not production tested with a 2.5V

reference as shown, linearity will not change more than

0.1 LSB. See Figure 2-2 and Figure 2-9 for DNL and

INL errors versus VREF at VDD = 5V. A trade-off exists

between the high-pass corner and the acquisition time.

The value of C will need to be quite large in order to

bring down the high-pass corner. The value of R will

need to be 1 kΩ, or less, since higher input

impedances require additional acquisition time. Using

the RC values in Figure 5-4, we have a 100 Hz corner

frequency. See Figure 2-12 for relation between input

impedance and acquisition time.

FIGURE 5-4:

Pseudo-differential biasing

circuit for bipolar operation.

CPIN

RSS CHx

7pF

VT = 0.6V

ILEAKAGE

Sampling

Switch

RS = 1 kΩ

CSAMPLE

= DAC capacitance

VSS

VDD

= 25 pF

±1 nA

Legend

= signal source

RSS = source impedance

CHx

= input channel pad

CPIN = input pin capacitance

VT = threshold voltage

ILEAKAGE = leakage current at the pin

due to various junctions

= sampling switch

RS = sampling switch resistor

CSAMPLE = sample/hold capacitance

VDD = 5V

0.1 µF

IN+

IN-

VREF

MCP330X

1µF

MCP1525

VIN

VOUT

0.1 µF

10 µF

VIN

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