MCP3564-ESLASHST datasheet(78/108 Pages) MICROCHIP

78 / 108 page

MCP3561/2/4

DS20006181C-page 78

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7.1.1

HIGH-SIDE AND LOW-SIDE

CURRENT SENSING

The ADC has the ability to perform differential

measurements with analog input Common-mode equal

to or slightly larger than AVDD, or equal to or slightly lower

than AGND (see the Electrical Characteristics table).

A differential input structure and a Kelvin connection

are required in order to achieve the most accurate

measurements. An anti-aliasing filter is required to

avoid aliasing of the oversampling frequency (DMCLK)

back into the baseband of the input signal and possible

corruption of the output data. Figure 7-1 provides an

example of an anti-aliasing filter.

For the measurement of voltages that can reach AVDD

or a few mV higher, a gain setting of 0.33x is useful

since it increases the input range to a 3 x VREF value,

so a 1.2V VREF will allow a theoretical input range of

3.6V. However, 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 rec-

ommended 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. Thermocouples provide a

voltage dependent on the temperature difference

between cold junction and hot junction. This voltage is

in the order of magnitude of tens of µV/°C, which

requires amplification that can be provided by the

internal gain stage of the ADC.

FIGURE 7-2:

Thermocouple Connection

to MCP3561.

The connection of the thermocouple to the ADC

requires minimal extra components. A differential input

structure is recommended. The cold junction can be

measured by using a digital temperature sensor like

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 (see 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 a Wheatstone bridge structure,

such as 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, REFIN+ is usually connected to the same

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

respects the specified voltage range on the REFIN+ pin

(see the Electrical Characteristics table.

FIGURE 7-4:

RTD Ratiometric Connection.

VIN+

VIN-

24-Bit ADC

I2C

Anti-aliasing

Filter

Sensor

VIN+

VIN-

MCP3561

AGND

DGND

VIN+

VIN-

MCP3561

Input

Signal

RTD

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