MCP3464T-ESLASHST datasheet(78/108 Pages) MICROCHIP

78 / 108 page

MCP3461/2/4

DS20006180D-page 78

 2019-2021 Microchip Technology Inc.

For measurement of voltages that can reach AVDD or a

few mV higher, a gain setting of 0.33x is useful since it will

increase the input range to 3 x VREF value, so a 1.2V

VREF will allow a theoretical input range of 3.6V. The

maximum voltage that can be measured is always

bounded by AVDD + 0.1V in order to limit excess leakage

current at the input pins created by the ESD structures.

Therefore, in order to properly measure 3.6V with a 1.2V

voltage reference, it is recommended to use an AVDD

supply voltage as close as possible to 3.6V.

7.1.2

THERMOCOUPLE CONNECTION

One of the most used temperature transducers in the

industry is the thermocouple. The thermocouples

provide a voltage dependent on the temperature differ-

ence between cold junction and hot junction. This

voltage is in the order of magnitude of few tens of

µV/°C, which require amplification that can be provided

by the internal gain stage of the ADC.

FIGURE 7-2:

Thermocouple Connection

to MCP3461.

The connection of the thermocouple to the ADC requires

minimal extra components and it’s recommended to

use a differential input structure. The cold junction can

be measured using a digital temperature sensor, such

as MCP9804 connected to the MCU. If high accuracy

is not required, the cold junction temperature can be

estimated directly with the internal temperature sensor

of the ADC (Figure 7-2).

7.2

Typical Application for

Ratiometric Voltage Measurement

A wide range of sensors provides an output voltage

directly related to the power supply of the sensors.

These sensors are known as ratiometric output. These

sensors often have Wheatstone bridge structure, like

pressure sensors or load cells (Figure 7-3).

FIGURE 7-3:

Wheatstone Bridge

Ratiometric Connection.

Others act as a single resistor with a value dependent

on temperature (pure metal Resistance Thermometer,

RTD, and Negative Temperature Coefficient resistor,

NTC). To accurately measure the signal from these

sensors, most often the REFIN+ is connected to the

same power supply of the sensor (Figure 7-4) as long

as this will respect the Electrical Characteristics table.

FIGURE 7-4:

RTD Ratiometric

Connection.

7.3

Power Supply Design and

Bypassing

In any system, the analog ICs (such as references or

operational amplifiers) are always connected to the

analog ground plane. The MCP3461/2/4 should also be

considered as sensitive analog components and

connected to the analog ground plane. The ADC

features two pairs of power supply voltage pins: AGND

and AVDD, DGND and DVDD. For best performance, it is

recommended to keep the two pairs of pins connected

to two different networks (Figure 7-5). This way, the

design will feature two ground traces and two power

supplies (Figure 7-6).

The analog circuitry (including MCP3461/2/4) and the

digital circuitry (MCU) should have separate power

supplies and return paths to the external ground

reference, as described in Figure 7-5. An example of a

typical power supply circuit, with different paths for

analog and digital return circuit, is shown in Figure 7-6.

A possible split example is shown in Figure 7-7, where

the ground star connection can be located underneath

the device with the exposed pad. The split here,

between analog and digital, can be done under the

device, and AVDD and DVDD can be connected

together with lines coming under the ground plane. The

two separate return paths eventually share a unique

connection point (star connection) in order to minimize

coupling between the two power supply domains.

Another possibility, sometimes easier to implement in

terms of PCB layout, is to consider the MCP3461/2/4

as an analog component, and connect both AVDD and

DVDD together, and AGND and DGND together, with a

star connection. In this scheme, the decoupling capac-

itors may be larger, due to the ripple on the digital

power supply (caused by the digital filters and the SPI

interface of the MCP3461/2/4), now causing glitches on

the analog power supply.

VIN+

VIN-

24-Bit ADC

I2C

MCP3461

Anti

Aliasing

Filter

Sensor

Vin+

Vin-

REFIN+

REFIN-

MCP3461

AGND

DGND

Vin +

Vin -

RE F IN+

RE F IN-

M C P 3461

R 1

Inpu t

Sign al

RT D

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