MCP3421 datasheet(21/30 Pages) MICROCHIP

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DS22003B-page 21

MCP3421

6.0

BASIC APPLICATION

CONFIGURATION

The MCP3421 device can be used for various precision

analog-to-digital converter applications. The device

operates with very simple connections to the

application circuit. The following sections discuss the

examples of the device connections and applications.

6.1

Connecting to the Application

Circuits

6.1.1

INPUT VOLTAGE RANGE

The fully differential input signals can be connected to

the VIN+ and VIN- input pins. The input range should be

within absolute common mode input voltage range:

VSS - 0.3V to VDD + 0.3V. Outside this limit, the ESD

protection diode at the input pin begins to conduct and

the error due to input leakage current increases rapidly.

Within this limit, the differential input VIN (= VIN+-VIN-)

is boosted by the PGA before a conversion takes place.

The MCP3421 can not accept negative input voltages

on the input pins. Figures 6-1 and 6-2 show typical con-

nection examples for differential inputs and a single-

ended input, respectively. For the single-ended input,

the input signal is applied to one of the input pins

(typically connected to the VIN+ pin) while the other

input pin (typically VIN- pin) is grounded. The input

signal range of the single-ended configuration is from

0V to 2.048V. All device characteristics hold for the

single-ended configuration, but this configuration loses

one bit resolution because the input can only stand in

positive half scale. Refer to Section 1.0 “Electrical

Characteristics”.

6.1.2

BYPASS CAPACITORS ON VDD PIN

For accurate measurement, the application circuit

needs a clean supply voltage and must block any noise

signal to the MCP3421 device. Figure 6-1 shows an

example of using two bypass capacitors (a 10 µF

tantalum capacitor and a 0.1 µF ceramic capacitor) in

parallel on the VDD line. These capacitors are helpful to

filter out any high frequency noises on the VDD line and

also provide the momentary bursts of extra currents

when the device needs from the supply. These

capacitors should be placed as close to the VDD pin as

possible (within one inch). If the application circuit has

separate digital and analog power supplies, the VDD

and VSS of the MCP3421 should reside on the analog

plane.

6.1.3

CONNECTING TO I2C BUS USING

PULL-UP RESISTORS

The SCL and SDA pins of the MCP3421 are open-drain

configurations. These pins require a pull-up resistor as

shown in Figure 6-1. The value of these pull-up resis-

tors depends on the operating speed (standard, fast,

and high speed) and loading capacitance of the I2C bus

line. Higher value of pull-up resistor consumes less

power, but increases the signal transition time (higher

RC time constant) on the bus. Therefore, it can limit the

bus operating speed. The lower value of resistor, on the

other hand, consumes higher power, but allows higher

operating speed. If the bus line has higher capacitance

due to long bus line or high number of devices

connected to the bus, a smaller pull-up resistor is

needed to compensate the long RC time constant. The

pull-up resistor is typically chosen between 1 k

Ω and

10 k

Ω ranges for standard and fast modes, and less

than 1 k

Ω for high speed mode in high loading

capacitance environments.

FIGURE 6-1:

Typical Connection Example

for Differential Inputs.

FIGURE 6-2:

Typical Connection Example

for Single-Ended Input.

The number of devices connected to the bus is limited

only by the maximum bus capacitance of 400 pF. The

bus loading capacitance affects on the bus operating

speed. For example, the highest bus operating speed

for the 400 pF bus capacitance is 1.7 MHz, and

3.4 MHz for 100 pF. Figure 6-3 shows an example of

multiple device connections.

MCP3421

VIN+

VIN-

VDD

VSS

SCL

SDL

10 µF

0.1 µF

Input Signals

VDD

TO MCU

(MASTER)

Note: R is the pull-up resistor.

MCP3421

VIN+

VIN-

VDD

VSS

SCL

SDL

10 µF

0.1 µF

Input Signals

VDD

TO MCU

(MASTER)

Note: R is the pull-up resistor.

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