MCP3021 datasheet(18/26 Pages) MICROCHIP

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MCP3021

DS21805A-page 18

 2003 Microchip Technology Inc.

6.0

APPLICATIONS INFORMATION

6.1

Driving the Analog Input

The MCP3021 has a single-ended analog input (AIN).

For proper conversion results, the voltage at the AIN

pin must be kept between VSS and VDD. If the converter

has no offset error, gain error, INL or DNL errors and

the voltage level of AIN is equal to or less than

VSS + 1/2 LSB, the resultant code will be 000h. Addi-

tionally, if the voltage at AIN is equal to or greater than

VDD - 1.5 LSB, the output code will be 1FFh.

The analog input model is shown in Figure 6-1. In this

diagram, the source impedance (RSS) adds to the inter-

nal sampling switch (RS) impedance, directly affecting

the time required to charge the capacitor (CSAMPLE).

Consequently, a larger source impedance increases

the offset error, gain error and integral linearity errors of

the conversion. Ideally, the impedance of the signal

source should be near zero. This is achievable with an

operational amplifier such as the MCP6022, which has

a closed-loop output impedance of tens of ohms.

FIGURE 6-1:

Analog Input Model, AIN.

6.2

Connecting to the I2C Bus

The I2C bus is an open collector bus, requiring pull-up

resistors connected to the SDA and SCL lines. This

configuration is shown in Figure 6-2.

FIGURE 6-2:

Pull-up Resistors on I2C

Bus.

The number of devices connected to the bus is limited

only by the maximum bus capacitance of 400 pF. A

possible configuration using multiple devices is shown

in Figure 6-3.

FIGURE 6-3:

Multiple Devices on I2C Bus.

CPIN

RSS AIN

7pF

VT = 0.6V

ILEAKAGE

Sampling

Switch

RS = 1 kΩ

CSAMPLE

= DAC capacitance

VSS

VDD

= 20 pF

±1 nA

Legend

VA = signal source

RSS = source impedance

AIN

analog input pad

CPIN = analog input pin capacitance

VT = threshold voltage

ILEAKAGE = leakage current at the pin

due to various junctions

sampling switch

sampling switch resistor

CSAMPLE = sample/hold capacitance

SDA

SCL

VDD

RPU

RPU is typically: 10 kΩ for fSCL = 100 kHz

Ω for fSCL = 400 kHz

MCP3021

Analog

Input

Signal

AIN

SDA SCL

PIC16F876

Microcontroller

MCP3021

10-bit ADC

TC74

Temperature

Sensor

24LC01

EEPROM

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