MCP3425 datasheet(20/30 Pages) MICROCHIP

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MCP3425

DS22072A-page 20

6.0

BASIC APPLICATION

CONFIGURATION

The MCP3425 device can be used for various precision

analog-to-digital converter applications. The device

operates

with

very

simple

connections

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 MCP3425 can not accept negative input voltages

on the input pins. Figure 6-1 and Figure 6-2 show

typical connection 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 MCP3425 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 MCP3425 should reside on the analog

plane.

6.1.3

CONNECTING TO I2C BUS USING

PULL-UP RESISTORS

The SCL and SDA pins of the MCP3425 are open-drain

configurations. These pins require a pull-up resistor as

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

resistors 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 kΩ 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.

MCP3425

VIN+

VIN-

VDD

VSS

SCL

SDL

10 µF

0.1 µF

Input Signals

VDD

TO MCU

(MASTER)

Note: R is the pull-up resistor.

MCP3425

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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