MCP3421 datasheet(11/42 Pages) MICROCHIP

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DS22003E-page 11

MCP3421

4.0

DESCRIPTION OF DEVICE

OPERATION

4.1

General Overview

The MCP3421 is a low-power, 18-Bit Delta-Sigma A/D

converter with an I2C serial interface. The device

contains an on-board voltage reference (2.048V),

programmable gain amplifier (PGA), and internal

oscillator. When the device powers up (POR is set), it

automatically resets the configuration bits to default

settings.

Device default settings are:

• Conversion bit resolution: 12 bits (240 sps)

• PGA gain setting: x1

• Continuous conversion

Once the device is powered-up, the user can

reprogram the configuration bits using I2C serial

interface any time. The configuration bits are stored in

volatile memory.

User selectable options are:

• Conversion bit resolution: 12, 14, 16, or 18 bits

• PGA Gain selection: x1, x2, x4, or x8

• Continuous or one-shot conversion

In the Continuous Conversion mode, the device

converts the inputs continuously. While in the One-Shot

Conversion mode, the device converts the input one

time and stays in the low-power standby mode until it

receives another command for a new conversion.

During the standby mode, the device consumes less

than 1 µA maximum.

4.2

Power-On-Reset (POR)

The device contains an internal Power-On-Reset

(POR) circuit that monitors power supply voltage (VDD)

during operation. This circuit ensures correct device

start-up at system power-up and power-down events.

The POR has built-in hysteresis and a timer to give a

high degree of immunity to potential ripples and noises

on the power supply. A 0.1 µF decoupling capacitor

should be mounted as close as possible to the VDD pin

for additional transient immunity.

The threshold voltage is set at 2.2V with a tolerance of

approximately ±5%. If the supply voltage falls below

this threshold, the device will be held in a reset

condition. The typical hysteresis value is approximately

200 mV.

The POR circuit is shut-down during the low-power

standby mode. Once a power-up event has occurred,

the

device

requires

additional

delay

time

(approximately 300 µs) before a conversion can take

place. During this time, all internal analog circuitries are

settled before the first conversion occurs. Figure 4-1

illustrates the conditions for power-up and power-down

events under typical start-up conditions.

When the device powers up, it automatically resets

and sets the configuration bits to default settings. The

default configuration bit conditions are a PGA gain of

1 V/V and a conversion speed of 240 SPS in

Continuous Conversion mode. When the device

receives an I2C General Call Reset command, it

performs an internal reset similar to a Power-On-Reset

event.

FIGURE 4-1:

POR Operation.

4.3

Internal Voltage Reference

The device contains an on-board 2.048V voltage

reference. This reference voltage is for internal use

only and not directly measurable. The specifications of

the reference voltage are part of the device’s gain and

drift specifications. Therefore, there is no separate

specification for the on-board reference.

4.4

Analog Input Channel

The differential analog input channel has a switched

capacitor structure. The internal sampling capacitor

(3.2 pF for PGA = 1) is charged and discharged to

process a conversion. The charging and discharging of

the input sampling capacitor creates dynamic input

currents at each input pin. The current is a function of

the

differential

input

voltages,

and

inversely

proportional to the internal sampling capacitance,

sampling frequency, and PGA setting.

VDD

2.2V

2.0V

300 µS

Reset Start-up

Normal Operation

Reset

Time

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