Microchip MCP3461R Manual

Microchip Ikke kategoriseret MCP3461R

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Side 1/42
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.
VE
Ͳ+
VOUT
R1- Rȴ1
Ͳ
+
R3- Rȴ3
R4+ Rȴ4
R2+ Rȴ2
(tension)
(tension)(compression)
(compression)
VOUT
R3
R3R4
+
-------------------
R2
R2R1
+
-------------------–VE
=

Produkt Specifikationer

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Kategori: Ikke kategoriseret
Model: MCP3461R

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