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2019 Microchip Technology Inc. DS00003183A-page 1

AN3183

INTRODUCTION

This application note describes a weight scale design

using Microchip's MCP3564 24-bit Delta-Sigma

Analog-to-Digital Converter (ADC) and PIC24

microcontroller. The weight scale uses an automatic

calibration process and techniques to minimize power

consumption. The design takes advantage of the

ADC's dynamic reconfigurability features of

oversampling ratio (OSR) and Gain settings to

demonstrate the impact of certain configurations on the

precision and the accuracy of the device. These

settings can be made from a dedicated graphical user

interface (GUI) which also allows the calibration of the

weight scale and much more.

Starting from the sensor (the load cell) and its

specifications and ending with the digital filter

implementation, this application note provides

information about each feature of the weight scale and

their importance in the application.

WEIGHT SCALE SYSTEM DIAGRAM

Figure 1 shows the weight scale demo using the

MCP3564 24-bit Delta-Sigma ADC. Figure 2 shows an

example of weight scale system diagram, containing

the front end load cell circuit, gain amplifier, MCP3564

Delta-Sigma ADC and PIC24 16-bit microcontroller

(MCU).

FIGURE 1: Photo of the MCP3564

Weight Scale Demo, P/N ARD00906.

FIGURE 2: Weight Scale Block Schematic.

Author: Dana Diaconu,

Microchip Technology Inc.

AVDD

AGND

OUT+

OUT- MCP3564 MCU

(PIC24FJ256GB410)

CH0

CH1 SPI

CH2

CH3

Low Noise

Differential

Amplifier

VREF+

VREF-

/RDG&HOO

Weight ale Application using Sc

MCP3564 24- it Delta-Sigma ADCB

AN3183

DS00003183A-page 2  2019 Microchip Technology Inc.

Load Cell

The most popular type of sensor for weight scales is

the strain gauge load cell. The load cell is a transducer

which converts an applied mechanical force into an

electrical signal directly proportional with the magni-

tude of the force. The resistive load cell used in this

application is composed of four strain gauges: two that

measure tension and two that measure compression.

These four strain gauges correspond to four resistors

organized in a Wheatstone Bridge type of circuit,

shown in Figure 3.

FIGURE 3: Wheatstone Bridge

Resistive Load Cell.

The four strain gauges sense the bending contortion of

the load cell under load translating to changes in the

electrical resistances.

Power to the Wheatstone Bridge is supplied by a

known excitation voltage (VE) and its output voltage is

defined by Equation 1.

EQUATION 1: WHEATSTONE BRIDGE

OUTPUT VOLTAGE

The bridge reaches a balanced state (V

OUT = 0) when

no strain is applied (i.e. no load conditions). In this

case, Equation 1 shows that the output voltage is null

if: (R1/R2) = (R4/R3).

Any resistance variation in any arm of the bridge leads

to an unbalanced state and to a nonzero output

voltage. Ideally, if no strain is applied there is no

variation in the resistances (R1-4 = 0). In practice,

these variations can occur due to changes in

temperature load cell manufacturing imperfections and

aging. These changes can lead to an offset output

voltage which needs to be calibrated out of the system.

Other types of load cells have different strain gauge

configurations. The one presented in Figure 3 is called

a Full Bridge Configuration because it has four active

elements (the strain gauges) which form the entire

Wheatstone Bridge. There are two other types: quarter

bridge and half bridge. The quarter bridge configuration

has a single active strain gauge element, while the half

bridge has two elements. The downside of these

configurations is that additional passive elements

(resistors) are required in order to complete the bridge.

The full bridge configuration load cell is more sensitive

to bending contortion and less sensitive to temperature

changes. The temperature change effect is more visi-

ble in the quarter and half bridge designs because the

additional resistors used to complete the bridge will

react to the temperature variation differently than the

strain gauges, as they are made of different materials

with different temperature tolerances.

One method of minimizing temperature effects is using

the full bridge configuration where the strain gauges

respond to temperature variations with the same

change in resistance. This means that the ratios of the

resistances are kept constant. Some load cells even

have an additional resistive element which is not used

for measuring the applied force. However, it is used in

an instrumentation amplifier configuration as a gain

setting resistor. This resistor will have a similar thermal

coefficient as the resistive bridge and the temperature

variation effect on output voltage will be minimized.

This method was not investigated in this application

note.

Another solution is to implement temperature drift

correction using software, but the necessity of this

software correction depends on the chosen load cell's

specifications and the conditions in which the sensor is

used. For instance, it is important to know the

temperature operating range of the load cell and how

thermally unstable the environment in which it is

used/integrated will be. Also, the temperature effect on

output is often specified in the sensor's specifications

table and this gives the maximum difference in the

output caused by a change in temperature of ±1°C,

when the load remains constant. In addition, even

using a high excitation voltage for the load cell could

cause the sensor to overheat and affect the accuracy

of the measurement. These aspects should be taken

into consideration when deciding if a temperature drift

software correction is required.

Table 1 shows the key specifications of the load cell

used for this application.

Ͳ+

VOUT

R1- Rȴ1

R3- Rȴ3

R4+ Rȴ4

R2+ Rȴ2

(tension)

(tension)(compression)

(compression)

VOUT

R3R4

-------------------

R2R1

-------------------–VE



 2019 Microchip Technology Inc. DS00003183A-page 3

AN3183

The output of the Wheatstone Bridge composed of

strain gauges is a very low-level voltage signal

(typically in the range of µV to mV) which needs to be

gain amplified before feeding into the ADC.

Equation 2 can be used to estimate the load cell output

voltage range using Table 1. For example, the output

sensitivity of the load cell used in this application is

1 mV/V. This means that when the excitation voltage is

3.3V, the output signal will be 3.3 mV at full load. Full

load is the maximum capacity of the sensor, in this case

2 kg. Equation 2 shows that for a 1 gram load change

the output will vary with 1.65 µV. This is a quick

estimation before selecting a right load cell based on

system requirements.

EQUATION 2: LOAD CELL OUTPUT

VOLTAGE

Also, the capacity of the load cell should be selected

carefully. If the maximum overload limit of the sensor is

exceeded, it can be permanently damaged.

As mentioned before, a balanced state of the bridge is

achieved when an excitation voltage is applied and

there is no load. Under these conditions the output

voltage of the bridge would ideally be 0V. In practice,

the maximum output voltage deviation of the load cell

from 0 is given by the Zero Balance and the

Temperature Effect on Zero specifications of the sensor

(See Table 1). This deviation can be corrected through

offset calibration.

Sensor temperature drift can be corrected in software

using a digital temperature sensor and a compensation

algorithm which accounts for variation of the sensor

output voltage over temperature. The Temperature

Effect on Zero and Temperature Effect on Output spec-

ifications are key to implementing a compensation

algorithm for the output voltage variation over tempera-

ture.

For adding extra precision and accuracy to a weight

scale, a thermocouple can be used together with the

digital temperature sensor. The thermocouple provides

a voltage which is dependent on the temperature

difference between its hot and its cold junction, but its

output is in the µV range. Therefore, it requires

amplification before converting the output to digital

data. The digital temperature sensor can be used to

measure the cold junction temperature. There are

ADCs which provide both an analog gain stage and an

internal temperature sensor, such as the MCP3564

ADC used in this application. The thermocouple can be

connected directly to the converter without additional

circuitry and the cold junction temperature can be

estimated with this internal temperature sensor.

However, an external digital temperature sensor, such

as Microchip’s MCP9800, is highly recommended as it

increases the accuracy of the measurement.

TABLE 1: LOAD CELL SPECIFICATIONS

Specification Description Specification Value Specification Value for a Load Cell

with a 2 kg Capacity

Output Sensitivity 1.0 ± 15% mV/V

Safe Overload 150 %F.S( )13 kg

Maximum Overload 200 %F.S 4 kg

Nonlinearity ±0.05 %F.S ±1g

Non-repeatability

Hysteresis

Creep (5 min.)

Temperature Effect on Output ±0.05 %F.S/°C ±1g/°C

Temperature Effect on Zero ±2.0 %F.S/°C ±40g/°C

Zero Balance ±10 %F.S/°C ±0.2 kg

Input/Output Resistance 1000 ± 10

Excitation Voltage 6V

Operating Temperature Range -10°C to +40°C

Note 1: Where F.S is full scale.

VOUT

Output Sensitivity mV V



  Excitation Voltage V 



Maximum Capacity g 

---------------------------------------------------------------------------------------------------------------------------- Load g 



VOUT

1 mV/V 3.3V



2000g

----------------------------------- 1g



1.65 µV= =

Product specificaties

Merk:	Microchip
Categorie:	Niet gecategoriseerd
Model:	MCP3461R

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