MCP6N11 and MCP6V2x
Wheatstone Bridge
Reference Design
User’s Guide
2012 Microchip Technology Inc.
DS52031A
Note the following details of the code protection feature on Microchip devices:
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Microchip is willing to work with the customer who is concerned about the integrity of their code.
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Trademarks
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PIC32 logo, rfPIC and UNI/O are registered trademarks of
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Analog-for-the-Digital Age, Application Maestro, chipKIT,
chipKIT logo, CodeGuard, dsPICDEM, dsPICDEM.net,
dsPICworks, dsSPEAK, ECAN, ECONOMONITOR,
FanSense, HI-TIDE, In-Circuit Serial Programming, ICSP,
Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB,
MPLINK, mTouch, Omniscient Code Generation, PICC,
PICC-18, PICDEM, PICDEM.net, PICkit, PICtail, REAL ICE,
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© 2012, Microchip Technology Incorporated, Printed in the
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Printed on recycled paper.
ISBN: ISBN: 978-1-61341-925-0
Microchip received ISO/TS-16949:2009 certification for its worldwide
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and India. The Company’s quality system processes and procedures
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devices, Serial EEPROMs, microperipherals, nonvolatile memory and
analog products. In addition, Microchip’s quality system for the design
and manufacture of development systems is ISO 9001:2000 certified.
DS52031A-page 2
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Table of Contents
Preface ........................................................................................................................... 5
Introduction............................................................................................................ 5
Document Layout .................................................................................................. 6
Conventions Used in this Guide ............................................................................ 7
Recommended Reading........................................................................................ 8
The Microchip Web Site ........................................................................................ 8
Customer Support ................................................................................................. 9
Document Revision History ................................................................................... 9
Chapter 1. Product Overview
1.1 Introduction ................................................................................................... 11
1.2 Purpose ........................................................................................................ 11
1.3 Description ................................................................................................... 12
1.4 What Does this Kit Contain? ........................................................................ 15
Chapter 2. Installation and Operation
2.1 Introduction ................................................................................................... 17
2.2 Required Tools ............................................................................................. 17
2.3 Basic Configurations .................................................................................... 18
2.4 Configurations with Modifications ................................................................. 19
2.5 Configuring the PCB ..................................................................................... 20
Chapter 3. Analog Circuit
3.1 Introduction ................................................................................................... 23
3.2 Bridge With PWM Interference ..................................................................... 23
3.3 Signal Processing Circuitry .......................................................................... 24
3.4 Test Points ................................................................................................... 28
3.5 PIC Microcontroller ....................................................................................... 28
Chapter 4. Firmware
4.1 Introduction ................................................................................................... 31
4.2 Analog Signals at the ADC Inputs ................................................................ 31
4.3 Data Conditioning ......................................................................................... 32
4.4 Temperature Calculations ............................................................................ 33
4.5 Resistance Calculations ............................................................................... 34
4.6 Calibrating the MCP6N11 ............................................................................. 34
4.7 Temperature Calibration ............................................................................... 35
4.8 PWM Output ................................................................................................. 35
4.9 USB Communications .................................................................................. 35
2012 Microchip Technology Inc.
DS52031A-page 3
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
Chapter 5. Software GUI
5.1 Introduction ................................................................................................... 37
5.2 Platform Requirements ................................................................................. 37
5.3 USB Communications .................................................................................. 37
5.4 Display Data ................................................................................................. 38
5.5 Calibrate Temperature ................................................................................. 38
5.6 Calibrate the MCP6N11 ............................................................................... 38
5.7 Configuration Options ................................................................................... 38
5.8 Export Data to a File ..................................................................................... 39
5.9 Screen Captures .......................................................................................... 39
Appendix A. Schematics and Layouts
A.1 Introduction .................................................................................................. 51
A.2 Board – Schematic ....................................................................................... 52
A.3 Board – Top Silk Screen and Pads .............................................................. 53
A.4 Board – Top Metal Layer .......................................................................... 54
A.5 Board – Ground Plane (Second layer) ......................................................... 55
A.6 Board – Power Plane (Third layer) ............................................................... 56
A.7 Board – Bottom Metal Layer (top view) ........................................................ 57
A.8 Board – Bottom Silk and Pads (top view) .................................................... 58
Appendix B. Bill Of Materials (BOM)
Appendix C. Conversion Polynomials
C.1 Circuit Response ......................................................................................... 63
C.2 RTD Temperature and Resistance .............................................................. 65
C.3 INA Voltage to Temperature ........................................................................ 66
Appendix D. Board Validation Summary
D.1 Thermal Steady State Response ................................................................. 67
D.2 Thermal Impulse Response ......................................................................... 67
Worldwide Sales and Service .....................................................................................70
DS52031A-page 4
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Preface
NOTICE TO CUSTOMERS
All documentation becomes dated, and this manual is no exception. Microchip tools and
documentation are constantly evolving to meet customer needs, so some actual dialogs
and/or tool descriptions may differ from those in this document. Please refer to our web site
(www.microchip.com) to obtain the latest documentation available.
Documents are identified with a “DS” number. This number is located on the bottom of each
page, in front of the page number. The numbering convention for the DS number is
“DSXXXXXA”, where “XXXXX” is the document number and “A” is the revision level of the
document.
For the most up-to-date information on development tools, see the MPLAB® IDE online help.
Select the Help menu, and then Topics to open a list of available online help files.
INTRODUCTION
This chapter contains general information that will be useful to know before using the
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design User’s Guide. Items
discussed in this chapter include:
•
•
•
•
•
•
Document Layout
Conventions Used in this Guide
Recommended Reading
The Microchip Web Site
Customer Support
Document Revision History
2012 Microchip Technology Inc.
DS52031A-page 5
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
DOCUMENT LAYOUT
This document describes how to use the MCP6N11 and MCP6V2x Wheatstone Bridge
Reference Design. The manual layout is as follows:
• Chapter 1. “Product Overview” – Important information about the MCP6N11
and MCP6V2x Wheatstone Bridge Reference Design.
• Chapter 2. “Installation and Operation” – Covers the initial set-up of this board,
required tools, board setup and lab equipment connections.
• Chapter 3. “Analog Circuit” – Discusses the analog functionality of the circuit on
this board.
• Chapter 4. “Firmware” – Discusses the firmware on the PIC® device.
• Chapter 5. “Software GUI” – Discusses the GUI on the PC.
• Appendix A. “Schematics and Layouts” – Shows the schematic and board
layouts for the MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design.
• Appendix B. “Bill Of Materials (BOM)” – Lists the parts used to populate the
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design. Also lists alternate components.
• Appendix C. “Conversion Polynomials” – Gives background information on
the polynomials used in firmware.
• Appendix D. “Board Validation Summary” – Summarizes analog performance
of two boards.
DS52031A-page 6
2012 Microchip Technology Inc.
Preface
CONVENTIONS USED IN THIS GUIDE
This manual uses the following documentation conventions:
DOCUMENTATION CONVENTIONS
Description
Arial font:
Italic characters
Initial caps
Quotes
Underlined, italic text with
right angle bracket
Bold characters
N‘Rnnnn
Text in angle brackets < >
Courier New font:
Plain Courier New
Represents
Referenced books
Emphasized text
A window
A dialog
A menu selection
A field name in a window or
dialog
A menu path
MPLAB® IDE User’s Guide
...is the only compiler...
the Output window
the Settings dialog
select Enable Programmer
“Save project before build”
A dialog button
A tab
A number in verilog format,
where N is the total number of
digits, R is the radix and n is a
digit.
A key on the keyboard
Click OK
Click the Power tab
4‘b0010, 2‘hF1
Italic Courier New
Sample source code
Filenames
File paths
Keywords
Command-line options
Bit values
Constants
A variable argument
Square brackets [ ]
Optional arguments
Curly brackets and pipe
character: { | }
Ellipses...
Choice of mutually exclusive
arguments; an OR selection
Replaces repeated text
Represents code supplied by
user
2012 Microchip Technology Inc.
Examples
File>Save
Press <Enter>, <F1>
#define START
autoexec.bat
c:\mcc18\h
_asm, _endasm, static
-Opa+, -Opa0, 1
0xFF, ‘A’
file.o, where file can be
any valid filename
mcc18 [options] file
[options]
errorlevel {0|1}
var_name [,
var_name...]
void main (void)
{ ...
}
DS52031A-page 7
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
RECOMMENDED READING
This user's guide describes how to use MCP6N1X. Other useful documents are listed
below. The following Microchip documents are available and recommended as supplemental reference resources.
• MCP6001 Data Sheet (DS21733)
Provides detailed information on the op amp that is used for VREF.
• MCP6N11 Data Sheet – “500 kHz, 800 µA Instrumentation Amplifiers”
(DS25073)
Provides detailed information on the INA that is used for INA #1.
• MCP6V26/7/8 Data Sheet – “620 µA, 2 MHz Auto-Zeroed Op Amps”
(DS25007)
Provides detailed information on the auto-zeroed op amps used for INA #2.
• PIC18F2455/2550/4455/4550 Data Sheet – “28/40/44-Pin, High-Performance,
Enhanced Flash, USB Microcontrollers with nanoWatt Technology”
(DS39632)
Provides information on the USB PIC MCU family.
• PIC18F2458/2553/4458/4553 Data Sheet – “28/40/44-Pin High-Performance,
Enhanced Flash, USB Microcontrollers with 12-Bit A/D and nanoWatt
Technology” (DS39887)
Provides additional information on the USB PIC MCU devices with 12-bit ADCs.
• AN1258 Application Note – “Op Amp Precision Design: PCB Layout
Techniques” (DS01258)
Discusses methods to minimize thermo-junction voltage effects in a PCB design.
THE MICROCHIP WEB SITE
Microchip provides online support via our web site at www.microchip.com. This web
site is used as a means to make files and information easily available to customers.
Accessible by using your favorite Internet browser, the web site contains the following
information:
• Product Support – Data sheets and errata, application notes and sample programs, design resources, user’s guides and hardware support documents, latest
software releases and archived software
• General Technical Support – Frequently Asked Questions (FAQs), technical
support requests, online discussion groups, Microchip consultant program
member listing
• Business of Microchip – Product selector and ordering guides, latest Microchip
press releases, listing of seminars and events, listings of Microchip sales offices,
distributors and factory representatives
DS52031A-page 8
2012 Microchip Technology Inc.
Preface
CUSTOMER SUPPORT
Users of Microchip products can receive assistance through several channels:
•
•
•
•
•
Distributor or Representative
Local Sales Office
Field Application Engineer (FAE)
Technical Support
Development Systems Information Line
Customers can contact their distributor, representative or field application engineer for
support. Local sales offices are also available to help customers. A listing of sales
offices and locations is included in the back of this document.
Technical support is available on the web site at: http://www.microchip.com/support.
DOCUMENT REVISION HISTORY
Revision A (January 2012)
The initial release of this document.
2012 Microchip Technology Inc.
DS52031A-page 9
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
NOTES:
DS52031A-page 10
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Chapter 1. Product Overview
1.1
INTRODUCTION
The MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design is described by
the following:
• Assembly #: 114-00354-R2
• Order #: ADM00354
• Name: MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
This board uses the following Microchip ICs:
•
•
•
•
•
MCP6001 (op amp)
MCP6N11-100 (INA)
MCP6V27 (dual auto-zeroed op amp)
MCP6V26 (single auto-zeroed op amp)
PIC18F2553 (USB PIC® microcontroller)
Items discussed in this chapter include:
• Purpose
• Description
• What Does this Kit Contain?
1.2
PURPOSE
This board demonstrates the performance of Microchip’s MCP6N11 instrumentation
amplifier (INA) and a traditional three op amp INA using the MCP6V26 and MCP6V27
auto-zeroed op amps from Microchip. The input signal comes from an RTD temperature sensor in a Wheatstone bridge. Real world interference is added to the bridge’s
output, to provide realistic performance comparisons.
Data is gathered and displayed on a PC, for ease of use. The USB PIC microcontroller
and included Graphical User Interface (GUI) show how to implement these functions.
2012 Microchip Technology Inc.
DS52031A-page 11
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
1.3
DESCRIPTION
1.3.1
Block Diagram
Figure 1-1 shows the overall functionality of this demo board. Detailed information is
available in Appendix A. “Schematics and Layouts” and Appendix B. “Bill Of
Materials (BOM)”.
Sensor
• RTD
• Bridge
INA #1
• I/O Filters
• INA with mCal
MCP6N11-100
VREF
INA #2
• I/O Filters
• Traditional INA
- MCP6V27
- MCP6V26
FIGURE 1-1:
Interference
PWM at 25 kHz
Miscellaneous
• Test Points
• Power Supply
Mixed Signal
• USB PIC MCU
- PIC18F2553
- PWM output
- 12-bit ADC
- USB
- ISSP
• Firmware
- Averaging
- Calculations
- Communications
Digital
• PC
- USB
- Display
- Memory
• GUI
- Strip Chart
- User Inputs
- Save Results
- Communications
Overall Block Diagram.
This board uses an RTD temperature sensor, in a Wheatstone bridge, to measure the
board’s temperature. The bridge’s output is a small Differential mode (DM) signal with
a large Common mode (CM) interference signal superimposed. The CM signal is a
PWM waveform generated by the microcontroller, which simulates real world
interference.
VREF (not a fixed reference) outputs a voltage level proportional to the analog supply
voltage, keeping the INAs’ outputs in range.
The two INAs process the bridge’s output, rejecting the CM interference and providing
large gains for the DM signal. There are R-C low-pass filters at the input and output of
each INA.
A pulse-width modulation (PWM) signal at 25 kHz is an optional output of the microcontroller. It shows how the two INAs perform with one type of real world interference. This
signal can represent, for instance, interference from an H-bridge motor controller.
Test points simplify bench setups.
The power supply can be provided by the USB or by a lab power supply.
A 12-bit ADC inside the microcontroller converts analog signals to digital data.
The microcontroller’s firmware averages the data to achieve noise and data rate reduction. It then subtracts the VREF voltage from the INA outputs (correcting any slow
VREF errors), calculates the voltages and temperatures, calibrates the temperature
and sends data to the PC via the USB.
The ICSP header allows the user, if desired, to re-program the microcontroller. In many
cases, the installed firmware will provide the necessary functionality for an evaluation
of this design.
The GUI (Thermal Management Utility) runs on the PC. It handles communications
through the USB, displays results on a strip chart, stores single point calibration coefficients on the PIC MCU, tells the PIC MCU how to configure the averaging filter, initiates offset calibration events for the MCP6N11 and exports data to CSV files.
DS52031A-page 12
2012 Microchip Technology Inc.
Product Overview
1.3.2
Sensor
The RTD converts board temperature to resistance with a nearly linear response. It is
placed in a Wheatstone bridge to convert its resistance to a (small) DM voltage. The
bridge’s characteristics are:
• Ratiometric current (reducing hardware and firmware complexity)
• Sets the CM voltage at mid-supply (2.5V) for the greatest headroom
1.3.3
Interference
The PIC device outputs a PWM waveform at 25 kHz (when it is enabled by the GUI and
by JP2), which can be coupled onto the top of the bridge using a jumper (JP1); this is
a CM interference signal (about 0.2 VP-P at the bridge). The coupling mechanism can
be either a capacitor or a resistor with a DC blocking capacitor; the bridge’s DC bias
point is not affected. This interference demonstrates how bridges are susceptible to CM
noise.
The following are options selected with a jumper or in the GUI:
• GUI option:
- Enable or Disable the PIC MCU’s PWM output
• JP1 options:
- Couple=C uses a capacitor to put the PWM waveform on the bridge
- Couple=RC uses resistive coupling (with a DC blocking capacitor) to put the
PWM waveform on the bridge
- Couple=Open disconnects the PWM signal from the bridge
• JP2 options:
- PWM=PIC connects the PIC MCU’s PWM signal onto the bridge
- PWM=EXT connects an external (PWM) signal onto the bridge
1.3.4
VREF
VREF (not a fixed voltage reference) provides a buffered ratiometric level (proportional
to VDD) that keeps the amplifiers in their normal operating range. When at the design
point of VDD = 5.0V, its output is 1.8V.
It has a low-pass filter at the output, whose pole is the same as the DM pole for the
INA’s input and output filters. This minimizes mismatches in the outputs, due to unequal
settling after a VDD disturbance.
1.3.5
INA #1
The input filter provides a low-pass function for both CM and DM signals. They are fast
enough to follow supply variations and to let the INA reject CM mains noise (e.g.,
harmonics of 50 or 60 Hz).
This INA is the MCP6N11-100. It is set at a high DM gain (200 V/V) and has good
CMRR at 25 kHz (92 dB, at the PWM frequency). Its VOS is calibrated using the internal
mCal function, which has the following (selectable) control options:
• The DUT is calibrated at power up
• The GUI sends a signal to the MCP6N11 via the USB PIC device
• The user presses SW1 (mCalSw)
The output filter provides a low-pass function for both CM and DM signals out of the
INA. It is slow to minimize noise and interference.
2012 Microchip Technology Inc.
DS52031A-page 13
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
1.3.6
INA #2
The input and output filters are the same as for the INA #1. This input filter is isolated
from INA #1’s input filter by the input resistors.
The INA is a traditional three op amp instrumentation amplifier. It uses Microchip’s
auto-zeroed MCP6V27(dual) and MCP6V26 (single). Its performance can be compared to INA #1; it is also set at a high DM gain (200 V/V). It has good CMRR at DC
(>100 dB). Its CMRR at 25 kHz (e.g., 96 dB) is also good.
1.3.7
Mixed Signal
The USB PIC microcontroller performs the following functions:
• Interfaces with the analog circuitry:
- 12-bit ADC
- PWM output for interference
• Does calculations in firmware:
- Averages data from the ADC
- Converts voltages to temperature
- Corrects the RTD temperature using stored calibration coefficients
- Calculates the RTD resistance from temperature
• Communicates with the PC:
- Uses the USB port to upload data (averaged voltages, temperatures and
resistances)
- Uses the USB port to download configuration options (calibrate INA #1, number of averages, temperature calibration coefficient)
• Provides easy programing for user designs, if desired:
- ICSP interface
1.3.8
Digital
The PC is a convenient interface for the user. The GUI provides:
•
•
•
•
Communications with the demo board (via the USB PIC device)
Data display (strip chart)
Data storage (graphics and text files)
Configuration inputs
- Temperature calibration (an input sent to the USB PIC MCU)
- Send an mCal calibration signal (to the USB PIC MCU) now
- Number of averages
1.3.9
Miscellaneous
The Test Points are for evaluating analog performance and for connecting a lab power
supply:
• The +5.0V power supply is for all components on the PCB, when not connected to
the USB
• A remote RTD can be connected via wires (for measuring elevated temperatures)
• The INA outputs (filtered and unfiltered) are available
• An external PWM signal provides for alternate interfering signals
The power supply provides a 5V rail from either lab equipment or from the USB.
DS52031A-page 14
2012 Microchip Technology Inc.
Product Overview
1.4
WHAT DOES THIS KIT CONTAIN?
This MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design kit includes:
• MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design, 102-00354
• USB cable
• Important Information Sheet
FIGURE 1-2:
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design Kit Contents.
2012 Microchip Technology Inc.
DS52031A-page 15
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
NOTES:
DS52031A-page 16
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Chapter 2. Installation and Operation
2.1
INTRODUCTION
This chapter shows how to set up and operate the MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design. Items discussed in this chapter include:
•
•
•
•
2.2
Required Tools
Basic Configurations
Configurations with Modifications
Configuring the PCB
REQUIRED TOOLS
2.2.1
Setup With the PC
The default setup uses the USB bus to connect to the PC. The following is required:
• MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
• USB 2.0 Cable, A to Mini B
• PC
- Microsoft Windows XP SP3, Vista, and 7 (64 and 32 bit)
- .NET2 Framework
• Thermal Management Utility
- Microchip’s GUI for thermal demo boards
- Version 1.4.0.0, or later
CAUTION
When using the PC and USB, do not connect lab supplies to test points TP1 (+5.0V) and
TP2 (GND). This avoids contention with the USB.
2.2.2
Bench Setup
Measurements on the bench focus on analog performance; they only use the PIC
microcontroller to generate the 25 kHz interference signal. The USB is not connected.
• MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
• Lab Power Supply with single output
- Generates +5.0V (TP1) and ground (TP2)
• Voltmeter
• Signal Analyzer (oscilloscope, network analyzer, spectrum analyzer, …)
- High input impedance (e.g., 1 M || 10 pF)
CAUTION
When using the bench setup (5V supply at TP1 (+5.0V) and TP2 (GND)), do not connect
a USB cable to the demo board. This avoids contention with the lab supply.
2012 Microchip Technology Inc.
DS52031A-page 17
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
2.3
BASIC CONFIGURATIONS
The following sections discuss various configurations supported by this board.
2.3.1
Out of the Box Setup
The setup for these boards when they are shipped is as follows:
• R2 (RTD) is mounted on the PCB
• JP1 is at position #1 (Couple=C); the PWM signal capacitively couples to the
bridge
• JP2 is at position #1 (PWM=PIC); the PIC MCU generates the PWM interference
signal
- The user must enable the PWM output (see #26 in Section 5.9.2 “Additional
Configuration and RTD Temp Tabs”)
• USB cable connects the PCB and PC and provides power to the PCB
• No external power supply is connected to TP1 and TP2
• Thermal Management Utility (GUI) needs to be running before use
• ICSP connector not used
This is the most convenient setup for the user. Most of the work is handled by the GUI.
2.3.2
Other PWM Setups
2.3.2.1
RESISTIVE COUPLING
Changing JP1 to position #2 (Couple=RC) gives a basically resistive coupling into the
bridge. For convenience, C2 blocks any change in DC bias point.
2.3.2.2
NO COUPLING
Changing JP1 to position #3 (Couple=Open) produces no coupling (no PWM signal at
the bridge); it serves as a baseline that other options are compared to.
2.3.2.3
EXTERNAL PWM
Changing JP2 to position #2 (PWM=EXT) allows the user to input an arbitrary PWM
(interference) signal onto the board (using TP16).
Note:
2.3.3
Changing JP2 to a vertical connection between pins 1 and 3 allows the PIC
MCU’s PWM signal to be sent off board, via TP16 (Vpwmx). This is useful
for connecting to other PCBs.
Initiating VOS Calibration Events
At power up, the MCP6N11 internally self-calibrates. The GUI allows the user to send
an mCal event signal to the PIC microcontroller, which uses an open drain connection
to toggle the MCP6N11’s (U2) EN/CAL pin. The mCalSw switch (SW1) also toggles this
pin.
2.3.4
Other Power Supply Setup
The USB can be left open (don’t use the PC) and a lab power supply connected to TP1
and TP2. This makes analog measurements easier and cleaner, but does not have the
convenience of the GUI.
The effects of crosstalk, INA CMRR and analog filters can be examined in detail, using
bench equipment.
DS52031A-page 18
2012 Microchip Technology Inc.
Installation and Operation
2.4
CONFIGURATIONS WITH MODIFICATIONS
2.4.1
External RTD Setup
R2 can be external to the PCB, if desired, for sensing temperatures over a wider range.
De-solder R2 from the PCB. Connect wires from TP3 and TP4 to the RTD (preferably
the RTD that was de-soldered from the board).
This modification has the advantage of allowing a wider temperature range (you may
need to change the INAs’ gains). The main disadvantages come from the wiring
resistance and connectors:
• The wiring resistance adds directly to the RTD resistance
- The apparent RTD temperature increases
- The increase has a significant random component to it
- Accommodating different wire lengths and gauges in firmware is cumbersome
and error prone
- Wires can change resistance with aging
• Connectors can be less than ideal, due to:
- Junctions between different metals (creating thermocouple voltages)
- Vibration (wires become brittle and connections loosen over time)
- Corrosion (resistance increases plus a DC potential appears over time)
Other circuits take advantage of 3-wire and 4-wire RTDs to solve some of these
problems. The MCP6V26 data sheet’s Typical Applications section shows one
example.
2.4.2
Modified Firmware
The PIC microcontroller’s firmware can be re-programmed via the ICSP connector.
Modifications, that a user might decide to code, include:
• Different averaging scheme (for speed or noise performance)
• Convert VINA to RRTD, then RRTD to TRTD (if RRTD is a required output)
2.4.3
Component Substitutions
The following component substitutions may be of interest to the user:
•
•
•
•
Replace the MCP6V26 with either the MCP6V06 or the MCP6V01 (lower power)
Replace the MCP6V27 with either the MCP6V07 or the MCP6V02 (lower power)
Replace the MCP6N11-100 with another MCP6N11 GMIN option (need lower gain)
Replace the RTD (requires a change in firmware) to evaluate another RTD
2012 Microchip Technology Inc.
DS52031A-page 19
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
2.5
CONFIGURING THE PCB
2.5.1
Test Points
Table 2-1 lists the test points and describes their functionality.
TABLE 2-1:
TEST POINTS
Test Point
Comments
Ref. Des.
Label
Connector I/O A/D
TP1
+5.0V
5.0V
I
A
Power supply (also labeled VDD) (Note 1)
TP2
GND
—
I
A
(for TP1)
TP3
Rtd1
Vrtd
I
A
Connected to external RTD (R2)
TP4
Rtd2
VL
I
A
TP5
Vo1
Vo1
O
A
Un-filtered output of U2 (INA #1)
TP6
Vad1
Vad1
O
A
Filtered signal output of INA #1
TP7
Vad2
Vad2
O
A
Filtered reference output of INA #1
TP8
GND
—
I
A
(for TP5, TP6, TP7)
TP9
Vo2
Vo2
O
A
Un-filtered output of U3, Side A
TP10
Vo3
Vo3
O
A
Un-filtered output of U3, Side B
TP11
Vo4
Vo4
O
A
Un-filtered output of U4
TP12
GND
—
I
A
(for TP9, TP10, TP11)
TP13
Vad3
Vad3
O
A
Filtered signal output of INA #2
TP14
Vad4
Vad4
O
A
Filtered reference output of INA #2
TP15
GND
—
I
A
(for TP13, TP14, TP16)
TP16
Vpwms
Vpwmx
I
D
External PWM signal (couples onto bridge)
Note 1:
2.5.2
Use this test point for only one of two purposes: to measure power supplied by the
USB or to provide 5V power when the USB is not connected. Do not provide power
to the USB and this test point at the same time.
Jumper and Switch Settings
Table 2-2 gives the jumper settings.
TABLE 2-2:
JUMPER SETTINGS
Jumper
Identity
Comments
Ref.
Des.
Label
No.
JP1
Couple=
1
C
Uses capacitive coupling between PWM source and bridge
2
RC
Uses resistive coupling between PWM source and bridge;
a DC-blocking capacitor avoids DC bias shifts and has low
step response droop
3
Open
No coupling of PWM source to bridge
1
PIC
Use PIC MCU as source for PWM signal
2
EXT
Use external source (via TP16) for PWM signal
JP2
PWM=
Note 1:
DS52031A-page 20
Position
Label
Changing JP2 to a vertical connection between pins 1 and 3 allows the PIC
microcontroller’s PWM signal to be sent off board, via TP16 (Vpwmx). This is useful
for connecting to other PCBs.
2012 Microchip Technology Inc.
Installation and Operation
2.5.3
Schematic Connectors
Table 2-3 shows the connector labels used in the schematic.
TABLE 2-3:
CONNECTOR LABELS
External Signals
Label
Test
Point
Description
Internal Signals
Label
Description
5.0V
TP1
Power Supply
Vdd1
Filtered lab power supply;
analog supply (PCB’s power plane)
Vrtd
TP3
Connection to RTD
Vdd2
Filtered USB power; digital power
VL
TP4
Connection to RTD and
negative output of bridge
Vdd3
USB power
Vad1
TP6
INA #1’s signal output
VR
Positive output of bridge
Vad2
TP7
INA #1’s ref. output
Vpwm
PWM output from PIC MCU
Vad3
TP13
INA #2’s signal output
Vref
Buffered 1.8V reference
Vad4
TP14
INA #2’s ref. output
E/C
mCal signal (EN/CAL) sent by PIC
MCU
Vo1
TP5
U2’s output
Vo2
TP9
U3, Side A’s output
Vo3
TP10
U3, Side B’s output
Vo4
TP11
U4’s output
Vpwmx
TP16
External PWM input
2012 Microchip Technology Inc.
DS52031A-page 21
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
2.5.4
Connection and Configuration Points on the PCB
Figure 2-1 shows the points on the PCB where equipment is connected, and where the
configuration is altered.
8
7
6
1
5
2
3
4
Legend:
1 = ICSP connector for re-programming the PIC MCU’s firmware.
2 = USB connector (bottom layer) for the USB cable from the PC (when TP1 and TP2 are not
connected; see item #5).
3 = Test Points for evaluating INA #2’s performance on the bench.
a) Auto-zeroed op amp output voltages (Vo2, Vo3 and Vo4)
b) Filtered output voltages sent to the ADC (Vad3 and Vad4)
4 = PWM signal configuration:
a) Test Points for inputting an external PWM signal
b) Jumper for selecting between PWM sources (PIC microcontroller and external)
5 = Test Points for inputting power (5V) from a lab supply (when the USB is not connected; see item #2)
6 = Test Points for evaluating INA #1’s performance on the bench
a) INA output voltage (Vo1)
b) Filtered output voltages sent to the ADC (Vad1 and Vad2)
7 = Manually input a VOS calibration for INA #1’s (mCal event)
8 = Jumper for configuring the PWM signal’s coupling (capacitive only, series R-C or no coupling (open))
FIGURE 2-1:
DS52031A-page 22
Equipment Connection Scheme.
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Chapter 3. Analog Circuit
3.1
INTRODUCTION
This chapter discusses the performance of the analog circuitry. Items discussed in this
chapter include:
• Bridge With PWM Interference
• Signal Processing Circuitry
• PIC Microcontroller
3.2
BRIDGE WITH PWM INTERFERENCE
Figure 3-1 shows the Wheatstone bridge with one RTD element (R2) and three other
resistors (R3, R4 and R5). R1 and R6 set the current, which is ratiometric with the 5.0V
supply. R1 and R3-R6 are 0.1% resistors to minimize the gain error (they can be lower
precision, if the user modifies the firmware to calibrate offset and gain errors).
Vpwm is a 5 VP-P PWM signal generated by the PIC MCU (U3), which is set at 25 kHz
and 50% duty cycle. JP1 selects a capacitive feedthrough (C1), a resistive feedthrough
(R7, with C2 for DC-blocking) or no feedthrough (open). The first two methods produce
roughly 0.2 VP-P at the top of the bridge (Vrtd), while the last produces no PWM signal
at Vrtd. Inputting an external PWM signal (JP2, position #2) will give different results.
FIGURE 3-1:
Bridge with PWM Interference.
The RTD’s resistance is a quadratic polynomial in temperature (see
Appendix C. “Conversion Polynomials”). It can be approximated as a linear polynomial, with a temperature error less than ±1.3°C, but that is not accurate enough for
applications requiring an RTD. The point is that the resistance to temperature
conversion is a non-linear relationship.
The RTD’s resistance (R2) appears in the denominator of the equation describing the
bridge’s differential output voltage (VR – VL) as a function of Vdd1 and the resistors R1
to R6. This means that the relationship between R2 and VR – VL is also non-linear.
Fortunately, as will be discussed later, the firmware does the necessary calculations to
overcome these non-linear relationships.
2012 Microchip Technology Inc.
DS52031A-page 23
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
3.3
SIGNAL PROCESSING CIRCUITRY
This circuitry includes two INAs, a VREF and corresponding filters. The MCP6N11’s
performance is contrasted with a three op amp INA based on the MCP6V27 and
MCP6V26 auto-zeroed op amps.
3.3.1
Voltage Reference
Figure 3-2 shows the VREF block (not a fixed reference), which produces a buffered
ratiometric (proportional to VDD) voltage (Vref). Vref keeps the INAs within their range,
even when VDD changes. Errors in Vref are subtracted out later in firmware.
The pole set at VREF’s input matches Input Filter #1 and #2’s Differential Mode (DM)
poles so that power supply disturbances will be treated the same. Its output is 1.80V,
which gives good output headroom for both INAs. The filter on the supply helps keep
Vref quiet. R8 and R9 are 1% resistors to minimize circuit cost; the error in Vref is
corrected in firmware (subtraction of Vref from INA output voltages).
FIGURE 3-2:
DS52031A-page 24
Voltage Reference.
2012 Microchip Technology Inc.
Analog Circuit
3.3.2
First INA
Figure 3-3 shows INA #1, which includes the Input Filter #1, mCal #1 and Output Filter
#1 blocks.
Input Filter #1 is a Differential Mode (DM) and Common Mode (CM), low-pass filter. The
CM pole is at 800 Hz, so that INA #1 will reject mains interference (at 50 Hz or 60 Hz);
its CMRR is very good at low frequencies. The DM pole is at 38 Hz, for low CM-to-DM
conversion error, at a reasonable price. R11 and R12 are small enough to have good
EMI performance; C5-C7 are sized for the desired poles.
INA #1 (U2) is set to a DM gain of 201 V/V, and has its output shifted up by 1.80V (Vref),
producing an output that uses most of the ADC’s input range. INA #1’s CMRR at 25
kHz is excellent (e.g., 92 dB). INA #1’s PSRR rejects mains interference (e.g., harmonics of 50 Hz or 60 Hz). R17 and R18 are 0.1% resistors to minimize gain error (they can
be lower precision, if the user modifies the firmware to calibrate offset and gain errors).
To minimize thermo-junction voltage effects, at INA #1’s input, R11 and R12 are close
together and in parallel. See AN1258 (“Op Amp Precision Design: PCB Layout
Techniques”) for more information on this topic.
Output Filter #1 is a DM and CM, low-pass filter. The CM pole is at 8 Hz, for further
rejection of CM interference. The DM pole is at 2.7 Hz; it is faster than the RTD, but
slow enough to limit the output noise. C12 is large enough to cause minimal gain error
(it causes a voltage division as it interacts with the ADC’s input sampling capacitor,
CHOLD = 25 pF). R19 and R20 are small enough to have good EMI performance;
C11-C13 are sized for the desired poles.
mCal #1 triggers an internal calibration event in U1 (MCP6N11), which corrects its VOS;
the pole is set for SW1’s maximum bounce time (10 ms). R16 makes the EN/CAL pin
of INA #1 act as a wired-OR logic input; an E/C signal can be sent from the PIC MCU
independent of mCalSw’s state.
FIGURE 3-3:
First INA.
2012 Microchip Technology Inc.
DS52031A-page 25
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
3.3.3
Power Supply
Figure 3-4 shows the power supply connections and filters. Power is provided at either
the Lab Power Supply or by the USB connection. The R-C low-pass filters have a pole
at 0.80 kHz, which reduces ripple and crosstalk from the PWM interference; they are
symmetrically placed so that applying power at either end works well. L1 isolates the
analog and digital supplies, to prevent digital signals from interfering with the analog
signals; it produces an L-C pole at 23 kHz (a compromise between performance and
cost). R23 is a bleed-off resistor that discharges the bypass capacitors when no power
supply voltage is present; its power drain is not significant. The signal connectors
shown are:
• 5.0V = An external connection to the analog power supply voltage
- When the USB powers the board (no lab supply connected), it provides a
convenient point to measure the USB’s 5V power
- When a lab supply powers the board (USB is not connected), it is connected
here (at TP1)
• Vdd1 = Analog power supply voltage
• Vdd2 = Digital power supply voltage
• Vdd3 = USB 5V power
FIGURE 3-4:
Power Supply.
CAUTION
When using the PC and USB, do not connect lab supplies to test points TP1 (+5.0V) and
TP2 (GND). This avoids contention with the USB.
When using the bench setup (5V supply at TP1 (+5.0V) and TP2 (GND)), do not connect
USB to the demo board. This avoids contention with the lab supply.
DS52031A-page 26
2012 Microchip Technology Inc.
Analog Circuit
3.3.4
Second INA (Traditional Three Op Amp Implementation)
Figure 3-5 shows INA #2, which provides a means of comparing the MCP6N11 to a
traditional INA.
Input Filter #2 is the same as Input Filter #1, for ease of comparison. R24 and R25 provide a separate signal path, so that U3’s input bias currents and U2’s input bias currents
don’t interact.
INA #2 (U3, U4) is set to a DM gain of 201 V/V, and has its output shifted up by 1.80V
(Vref), producing an output that uses most of the ADC’s input range. INA #2’s CMRR
at 25 kHz is excellent (e.g., 96 dB). INA #2’s PSRR rejects mains interference (e.g.,
harmonics of 50 Hz or 60 Hz). R27-R30 are 0.1% resistors to minimize gain error (they
can be lower precision, if the user modifies the firmware to calibrate offset and gain
errors). R31-R34 are 0.1% resistors to minimize gain error and maximize CMRR.
To minimize thermo-junction voltage effects, at INA #2’s input, R24 and R25 are close
together and in parallel. The gain resistors for U3 (R27-R30) are also close together
and in parallel; R28 and R29 have not been combined into one resistor, to make this
possible. The gain resistors for U4 (R31-R34) are also close together and in parallel.
See AN1258 (“Op Amp Precision Design: PCB Layout Techniques”) for more
information on this topic.
FIGURE 3-5:
Second INA.
2012 Microchip Technology Inc.
DS52031A-page 27
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
3.4
TEST POINTS
Figure 3-6 shows the test points available to the user. See Table 2-1 for a list of their
functions.
FIGURE 3-6:
3.5
Test Points.
PIC MICROCONTROLLER
Figure 3-7 shows the PIC microcontroller (USB PIC MCU) with analog connectors to
the blocks previously shown, local supply bypassing, a 20 MHz ceramic resonator,
ICSP header and USB header.
The four ADC inputs (Vad1 to Vad4) are multiplexed into the USB PIC MCU’s internal
12-bit ADC. As discussed later on, the firmware takes the differences Vad1 – Vad2 and
Vad3 – Vad4, which corrects VREF’s output error (present at Vad2 and Vad4). Since
the analog signal processing circuitry does not totally eliminate interference from the
25 kHz PWM signal or the mains (50 Hz or 60 Hz), some will appear at the ADC inputs.
The 12-bit ADC supports a typical temperature resolution better than 0.09°C.
The PWM output (Vpwm) is a 5 VP-P digital signal at 25 kHz and 50% duty cycle. It emulates the CM interference from a motor control application.
The 20 MHz ceramic resonator runs the USB PIC MCU at a rate that supports the firmware tasks’ overhead and the USB. An internal PLL Prescaler divides by 5, to produce
a 4 MHz clock. The internal PLL then locks onto this clock and produces a 96 MHz
output, which is divided down for the USB module’s operation.
Use a low ESR capacitor for C27.
The ICSP header makes it possible to program the USB PIC MCU in circuit, for further
user experiments.
The USB header provides the data link to the PC, where data is displayed on the
included GUI.
DS52031A-page 28
2012 Microchip Technology Inc.
Analog Circuit
FIGURE 3-7:
PIC Microcontroller.
Section Chapter 4. “Firmware” and Section Chapter 5. “Software GUI” discuss in
detail how the digitized data is processed, displayed and stored.
2012 Microchip Technology Inc.
DS52031A-page 29
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
NOTES:
DS52031A-page 30
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Chapter 4. Firmware
4.1
INTRODUCTION
This chapter discusses the PIC microcontroller’s firmware. Items discussed in this
chapter include:
•
•
•
•
•
•
•
•
4.2
Analog Signals at the ADC Inputs
Data Conditioning
Temperature Calculations
Resistance Calculations
Calibrating the MCP6N11
Temperature Calibration
PWM Output
USB Communications
ANALOG SIGNALS AT THE ADC INPUTS
As discussed in Section 3.2 “Bridge With PWM Interference”, the relationship
between the RTD temperature and RTD resistance is non-linear, as is the relationship
between the RTD resistance and the bridge’s differential output voltage. These
non-linearities are corrected in firmware, because that minimizes the design cost.
Section 3.3 “Signal Processing Circuitry” mentions that interference from the 25
kHz PWM signal and the mains (50 Hz or 60 Hz) is reduced significantly, but not totally
eliminated, at the ADC inputs. The firmware uses averaging to reduce the effect of
these interfering signals.
The 12-bit ADC supports a temperature resolution that could be as good as 0.05°C.
Since the analog inputs do not go rail-to-rail, for a more robust solution, the resolution
is reduced somewhat. The non-linear relationships mentioned above make the resolution change, depending on the RTD temperature. The firmware solution gives a resolution between 0.06°C and 0.09°C. Obviously, the ADC is not ideal, so the temperature
error can be somewhat larger.
2012 Microchip Technology Inc.
DS52031A-page 31
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
4.3
DATA CONDITIONING
4.3.1
Basic Digital Data
The firmware on the PIC microcontroller inputs data for key analog DC voltages (Vad1
to Vad4), then converts them to digital (VAD1 to VAD4), with a 12-bit representation:
EQUATION
VAD1 = INA #1’s signal (Vad1)
VAD2 = INA #1’s VREF (Vad2)
VAD3 = INA #2’s signal (Vad3)
VAD4 = INA #2’s VREF (Vad4)
4.3.2
Data Sampling
The DC analog voltages are sampled in this order: VAD1, VAD2, VAD3, and VAD4. Each
voltage is sampled every 50 ms (a 20 SPS rate). Since the ADC samples all four
voltages, it takes a total of 80 samples per second.
4.3.3
Data Averaging and Output Data Rate
The GUI tells the microcontroller the number of averages (n) or whether averaging is
turned off. The GUI collects the averaged data from the microcontroller at adjustable
time intervals.
When averaging is off, the firmware makes the latest sample available to the GUI. The
samples are updated every 50 ms.
When averaging is on, the firmware calculates an exponential moving average, which
is updated every 50 ms. These samples are made available for collection by the GUI.
A higher number of averages (n) reduces noise more, but has a slower response. The
following equation shows how VAD1, VAD2, VAD3, and VAD4 (shown as xk) are averaged
(shown as yk):
EQUATION
yk = α xk + (1 – α) yk–1
Where:
n = 2, 4, 8, 16, 32, 64 or 128
α = 2 / (n + 1)
xk = current sample
yk–1 = previous average
yk = current average
DS52031A-page 32
2012 Microchip Technology Inc.
Firmware
4.4
TEMPERATURE CALCULATIONS
It would be possible to convert the measured voltages into RTD resistance, then into
RTD temperature. This approach adds overhead to the PIC MCU, so it was not done
on this design; the firmware converts directly from INA voltage to temperature.
The firmware gives two estimates of the same RTD temperature, based on the outputs
of INA #1 and INA #2:
EQUATION
TRTD1 = RTD temperature from INA #1 (based on VAD1 – VAD2)
TRTD2 = RTD temperature from INA #2 (based on VAD3 – VAD4)
4.4.1
Polynomial Estimate of Temperature
Section C.3 “INA Voltage to Temperature” discusses the conversion of VINA to TRTD.
This section summarizes those results. To accommodate fixed point arithmetic, TRTD
and VINA are scaled as follows:
EQUATION
v = VINA / (VDD/2)
w = TRTD / TS1
Where:
VINA = VAD1 – VAD2,
for INA #1
= VAD3 – VAD4,
for INA #2
VDD/2 = 2.5V
TS1 = 256°C
The firmware uses a cubic polynomial for the conversion. Its accuracy supports the
12-bit ADC we are using (1 LSB is between 0.055°C and 0.088°C, nominally).
EQUATION
w = K0 + v (K1 + v (K2 + v (K3))) ± εW
TRTD = w TS1 ± εT
Where:
K0 = 0.00002
K1 = 0.50417
K2 = 0.07463
K3 = 0.01370
εW = Error in w
εT = εW TS1
|εT| 0.021°C
TRTD = TRTD1,
TRTD2,
INA #1
INA #2
The GUI collects 10 temperature estimates per second for each INA.
2012 Microchip Technology Inc.
DS52031A-page 33
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
4.5
RESISTANCE CALCULATIONS
For convenience of the user, the firmware gives two estimates of the RTD resistance.
We do not need these estimates, since we convert directly from voltage to temperature.
They are useful, however, to a user that wants to change the firmware or hardware.
The firmware gives two estimates of the same RTD resistance, based on the previously
calculated of TRTD1 and TRTD2; see Appendix C. “Conversion Polynomials”.
EQUATION
RRTD1 = RTD resistance from INA #1 (based on TRTD1)
RRTD2 = RTD resistance from INA #2 (based on TRTD2)
To accommodate fixed point arithmetic, TRTD and RRTD are scaled as follows:
EQUATION
p = TRTD / TS1
q = (RRTD – RS0)/ RS1
Where:
TS1 = +256°C
RS0 = 120 Ω
RS1 = 64 Ω
The RTD’s resistance is estimated with this polynomial approximation (with scaling):
EQUATION
q = A0 + p (A1 + p (A2)) ± εQ
RRTD = RSO + q RS1 ± εR
Where:
A0 = -0.31251
A1 = 1.56353
A2 = -0.05946
εQ = Error in q
εR = εQ RS1,
|εR| 0.0053 Ω
RRTD = RRTD1, INA #1
= RRTD2, INA #2
εT = εR / (dRRTD/dTRTD)
The GUI collects 10 resistance estimates per second for each INA.
4.6
CALIBRATING THE MCP6N11
When JP3 is set to position #1, the PIC MCU can send an mCal signal to the MCP6N11
device. This happens when the GUI sends a signal to the PIC MCU (via USB) to start
an mCal event. The effect of this signal is to internally re-calibrate the MCP6N11
devices’ input offset voltage; its output is high-impedance while this re-calibration is in
progress.
DS52031A-page 34
2012 Microchip Technology Inc.
Firmware
4.7
TEMPERATURE CALIBRATION
4.7.1
Single Point Calibration
One of the user options available in our Thermal Management GUI (see
Chapter 5. “Software GUI”) is a single-point (offset) calibration (usually at 50°C); it
helps to correct for both circuit and RTD errors.
The GUI calculates the errors and sends calibration coefficients to the USB PIC MCU,
which correct all subsequent temperature values. These corrections are simple
subtractions.
Note 1:
The demo board initially has the calibration coefficient set to zero.
2:
Only a single point calibration will be provided for our demo board.
4.7.2
Higher Order Calibrations
Higher order calibrations are potentially more accurate. With two temperature
measurements (e.g., at 0°C and +100°C), it is possible to do a linear correction (offset
and gain). With three temperature measurements (e.g., at 0°C, +50°C, and +100°C), it
is possible to do a quadratic correction (offset, gain, and bow). These corrections
require firmware modifications by the user.
4.7.3
Circuit Calibration
Separating the circuit calibration from the temperature calibration gives more insight
into error sources, but not more accuracy.
It is possible to do a separate circuit calibration by inserting a resistance in R2’s place.
For example, use the 0°C value of 100 (±0.01%).
4.8
PWM OUTPUT
The firmware sets the Vpwm output (pin 13) to produce a square wave with 5 VP-P, 25
kHz and 50% duty cycle. It runs continuously.
The manually set options on JP1 allow different coupling methods to the Wheatstone
bridge, including no connection.
4.9
USB COMMUNICATIONS
The USB PIC MCU device’s clock runs on a 20 MHz ceramic resonator, which supports
the microcontroller’s firmware overhead and USB communications (see
Section 3.5 “PIC Microcontroller”).
The GUI provides a means for sending the following configuration information to the
board via the USB:
• Temperature calibration coefficient
• Number of averages
• Strobe signal to trigger a VOS calibration event (mCal) in INA #1
The firmware will send the following data to the GUI (in this order): VAD1, VAD2, VAD3,
VAD4, TRTD1, TRTD2, RRTD1, and RRTD2.
2012 Microchip Technology Inc.
DS52031A-page 35
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
NOTES:
DS52031A-page 36
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Chapter 5. Software GUI
5.1
INTRODUCTION
This chapter discusses the PC’s GUI. Items discussed in this chapter include:
•
•
•
•
•
•
•
Platform Requirements
USB Communications
Display Data
Calibrate Temperature
Calibrate the MCP6N11
Configuration Options
Export Data to a File
The GUI is Microchip’s “Thermal Management Utility” software. It downloads data from
the USB PIC MCU, calculates one point calibration coefficients for TRTD1 and TRTD2,
sends them to the PIC MCU, sends an mCal event trigger signal, sends the number of
averages, displays the data on a strip chart and exports data in different file formats,
including text.
5.2
PLATFORM REQUIREMENTS
The “Thermal Management Utility” software used for many of our thermal demo boards
runs on Microsoft Windows (XP SP3, Vista, and 7 (64 and 32 bit)). It also needs the
.NET2 framework package; the installer package will install it, if it is not on your
machine.
5.3
USB COMMUNICATIONS
The GUI sends configuration information to the firmware, via the USB. This includes:
temperature calibration coefficient, number of averages and a strobe signal to trigger a
VOS calibration event (mCal) in INA #1.
The GUI collects the following (averaged) data from the firmware: VAD1, VAD2, VAD3,
VAD4, TRTD1, TRTD2, RRTD1 and RRTD2. This data is collected at adjustable time
intervals, with a default value of 200 ms (5 SPS per variable).
2012 Microchip Technology Inc.
DS52031A-page 37
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
5.4
DISPLAY DATA
The GUI uses a standard interface to display the temperature calibration coefficient and
number of averages.
Displays the four voltages (VAD1 through VAD4) on a strip chart. Also displays the RTD
temperature estimates (TRTD1 and TRTD2) on a strip chart, with a different y-axis. The
RTD resistance estimates (RRTD1 and RRTD2) are not displayed on the strip chart.
The user will select the data to display on the strip chart. The vectors in bold blue font
in Figure 5-1 can be displayed when their check boxes are checked (the ADC & F/W
block represents the ADC internal to the PIC MCU and the firmware stored in its memory). The GUI requests the latest data from the microcontroller, after each sampling
interval period has passed, and uses that data to perform the plotting and data calculations. Data between collection requests are not sent to the GUI.
RTD
&
Bridge
FIGURE 5-1:
5.5
INA
#1
VAD1
INA
#2
VAD3
VAD2
VAD4
TRTD1
ADC
&
F/W
RRTD1
TRTD2
RRTD2
Selecting Vectors to Display on Strip Chart.
CALIBRATE TEMPERATURE
Supports a single-point calibration of TRTD1 and TRTD2, at a temperature selected by
the user. The GUI sends these calibration coefficients to the USB PIC microcontroller,
which does the actual corrections.
5.6
CALIBRATE THE MCP6N11
The MCP6N11 MCU internally re-calibrates its input offset voltage when any one of
these three events occurs:
• Power up
• The user clicks the mCal push button in the GUI (which sends an mCal event
signal to the PIC microcontroller)
• The user presses the mCalSw switch (SW1)
5.7
CONFIGURATION OPTIONS
The GUI provides a means for sending the following configuration information to the
firmware, via the USB bus:
• Temperature calibration coefficient
• Number of averages (n)
• Strobe signal to trigger a VOS calibration event (mCal) in INA #1
DS52031A-page 38
2012 Microchip Technology Inc.
Software GUI
5.8
EXPORT DATA TO A FILE
Export all of the data (VAD1, VAD2, VAD3, VAD4, TRTD1, TRTD2, RRTD1 and RRTD2) to a
user-selected text file. The GUI either sends the displayed data (limited to 500 time
points) to the file, or sends “recorded” data over a selected time period (which has no
time point limit).
Export the strip chart to a user-selected file, in one of several formats.
5.9
SCREEN CAPTURES
All of the figures in this section have circled numbers that point to important features.
These numbers correspond to the associated numbered list. Cross references to these
numbers are displayed in the text as follows: (#1).
Some useful features, that may be hard to find, include:
• Resize Right Edge of Information Box (#4)
• Export Strip Chart
- See Section 5.9.4.8 “Exporting Data Acquisition Box”
- Data File
- Graphic File
• Maximize Strip Chart
- See Section 5.9.4.9 “Maximized Strip Chart Display”
- Maximizes Strip Chart only
• Chart Menu (more chart options)
- See Section 5.9.4.10 “Right Mouse Click (inside the Strip Chart Area)”
5.9.1
Data View Tab
Figure 5-2 shows the default screen. On the left, the Data View tab (#9) is selected,
and the Strip Chart is displayed.
2012 Microchip Technology Inc.
DS52031A-page 39
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
1
2
3
4
5
6
7
8
9
10
11
FIGURE 5-2:
12
13
14
15
16
17
18
Data View Tab, with Strip Chart.
The indicated features for Figure 5-2 are:
1. Data Collection buttons:
a) Data Acquisition Buttons (standard data collection).
Start Acquisition
Stop Acquisition
Reset Chart Buffer
b) Record Acquisition Buttons (data sent to a file, with user selected length).
Record Acquisition
Stop Record Acquisition
c) Zoom Buttons (resize Strip Chart).
Zoom Information (click and drag inside Strip Chart)
Zoom Out
d) Chart Customization Button.
Open chart customization dialog
2. Current measured values for INA #1:
a) RTD Resistance
b) RTD Temperature
c) VAD1 Reference Point (voltage which changes with temperature)
d) VAD2 Reference Point (voltage based on VREF)
DS52031A-page 40
2012 Microchip Technology Inc.
Software GUI
3. Current measured values for INA #2:
a) RTD Resistance
b) RTD Temperature
c) VAD3 Reference Point (voltage which changes with temperature)
d) VAD4 Reference Point (voltage based on VREF)
4. Right edge of Information Box. To resize this box, Left Click this edge with the
mouse and Drag.
5. Strip Chart area.
a) Double-click in this area to open the Strip Chart Dialog Box.
b) Right-click in this area to open another chart menu.
6. Strip Chart.
a) To zoom in on a smaller area, click on one corner and drag to the other
corner (the magnifier icon appears).
b) To zoom out to the full Strip Chart area, click on the Zoom Out button (#1).
7. Right edge of GUI window. To resize this window, click this edge and drag.
8. Help button
9. Data View tab is selected
10. Sampling Interval drop-down menu (interval between stored and displayed
points).
11. Data display boxes for ADC input voltages (VAD1 to VAD4).
12. Data display boxes for calculated temperatures (TRTD1 and TRTD2).
13. Y-axis for ADC input voltages (VAD1 to VAD4).
14. Y-axis for calculated temperatures (TRTD1 and TRTD2).
15. X-axis for collected data.
16. Strip Chart curves.
17. Bottom edge of GUI window. To resize this window, click this edge and drag.
18. Bottom Right corner of GUI window. To resize this window, click and drag.
2012 Microchip Technology Inc.
DS52031A-page 41
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
5.9.2
Additional Configuration and RTD Temp Tabs
Figure 5-3 shows that the Additional Configuration tab (#22) and RTD Temp sub-tab
(#23) are selected, and the Strip Chart is displayed (Figure 5-2 has more information).
19
20
21
22
23
24
25
26
27
FIGURE 5-3:
Additional Configuration and RTD Temp Tabs, with Strip Chart.
19. Calibration input boxes:
a) Calibration Temperature (press the Calibrate ! button (#20) when ready).
b) INA1 Calibration Offset (input temperature correction for INA1).
c) INA2 Calibration Offset (input temperature correction for INA2).
20. Calibrate ! button (enter Calibration Temperature (#19), then press when ready).
21. Reset button (resets Calibration inputs (#19) to 0).
22. Additional Configuration tab is selected.
23. RTD Temp sub-tab is selected.
24. Temperature Units selection radio buttons.
25. Number of Averages drop-down menu.
26. PWM inputs:
a) Enabled check box (turns on PWM output from PIC microcontroller and activates other features in the PWM box).
b) Duty input box (inputs duty cycle).
c) Set Duty button (sends updated value to the microcontroller).
d) Frequency Adjustment slider (changes the PWM frequency).
e) Approximate Frequency input box (adjusts PWM frequency; 25 kHz is
preferred).
27. Sampling Interval drop-down menu (shown at 100 ms per sample).
DS52031A-page 42
2012 Microchip Technology Inc.
Software GUI
5.9.3
Additional Configuration and MCP6N11 Tabs
Figure 5-4 shows that the Additional Configuration tab (#29) and MCP6N11 sub-tab
(#30) are selected, and the Strip Chart is displayed (Figure 5-2 has more information).
28
29
30
31
FIGURE 5-4:
Additional Configuration and MCP6N11 Tabs, with Strip Chart.
The indicated features for Figure 5-4 are:
28.
29.
30.
31.
Additional Configuration tab is selected.
MCP6N11 sub-tab is selected.
Trigger mCal button; sends a trigger to INA #1 to recalibrate its offset.
Sampling Interval pull-down menu (shown at 100 ms per sample)
2012 Microchip Technology Inc.
DS52031A-page 43
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
5.9.4
Strip Chart Customization
5.9.4.1
DATA ACQUISITION CUSTOMIZATION – GENERAL TAB
Figure 5-5 shows the dialog box, with the General tab selected. To open this box, double-click inside the Strip Chart area (#5). The overall look and feel of the Strip Chart
area can be changed here.
FIGURE 5-5:
5.9.4.2
Data Acquisition Customization – General Tab.
DATA ACQUISITION CUSTOMIZATION – PLOT TAB
Figure 5-6 shows the Data Acquisition dialog box, with the Plot tab selected. To open
this box, double-click inside the Strip Chart area (#5). The Strip Chart curve’s axes and
display types can be changed here.
FIGURE 5-6:
DS52031A-page 44
Data Acquisition Customization – Plot Tab.
2012 Microchip Technology Inc.
Software GUI
5.9.4.3
DATA ACQUISITION CUSTOMIZATION – SUBSETS TAB
Figure 5-7 shows that the Data Acquisition Dialog box, with the Subsets tab selected.
To open this box, double-click inside the Strip Chart area (#5). The Strip Chart curves
can be selected, or de-selected, here.
FIGURE 5-7:
5.9.4.4
Data Acquisition Customization – Subsets Tab.
DATA ACQUISITION CUSTOMIZATION – POINTS TAB
Figure 5-8 shows that the Data Acquisition dialog box, with the Points tab selected. To
open this box, double-click inside the Strip Chart area (#5). The number of points
displayed on the Strip Chart’s x-axis is changed here.
FIGURE 5-8:
2012 Microchip Technology Inc.
Data Acquisition Customization – Points Tab.
DS52031A-page 45
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
5.9.4.5
DATA ACQUISITION CUSTOMIZATION – FONT TAB
Figure 5-9 shows that the Data Acquisition dialog box, with the Font tab selected. To
open this box, double-click inside the Strip Chart area (#5). The Strip Chart’s font types
and sizes are changed here.
FIGURE 5-9:
5.9.4.6
Data Acquisition Customization – Font Tab.
DATA ACQUISITION CUSTOMIZATION – COLOR TAB
Figure 5-10 shows that the Data Acquisition dialog box, with the Color tab selected. To
open this box, double-click inside the Strip Chart area (#5). The font and object colors
are changed here.
FIGURE 5-10:
DS52031A-page 46
Data Acquisition Customization – Color Tab.
2012 Microchip Technology Inc.
Software GUI
5.9.4.7
DATA ACQUISITION CUSTOMIZATION – STYLE TAB
Figure 5-11 shows that the Data Acquisition dialog box, with the Style tab selected. To
open this box, double-click inside the Strip Chart area (#5). The Strip Chart’s curve colors are changed here.
FIGURE 5-11:
5.9.4.8
Data Acquisition Customization – Style Tab.
EXPORTING DATA ACQUISITION BOX
Figure 5-12 shows the Exporting Data Aquisition box. To open this dialog, double-click
inside the Strip Chart area (#5)., then press the Export button on the bottom right (e.g.,
see Figure 5-11). When the data is exported as a picture, its destination and size can
be selected. When the data is exported as text, only its destination can be selected.
FIGURE 5-12:
2012 Microchip Technology Inc.
Exporting Data Acquisition Box.
DS52031A-page 47
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
5.9.4.9
MAXIMIZED STRIP CHART DISPLAY
Figure 5-13 shows the maximized strip chart display. To open this window (on the PC’s
main display), double-click inside the Strip Chart area (#5), then press the Maximize …
button on the bottom right (e.g., see Figure 5-11). To exit this screen, click on the top
banner or press the “Esc” key on your keyboard.
FIGURE 5-13:
DS52031A-page 48
Maximized Strip Chart Display (under the Strip Chart Dialog Box).
2012 Microchip Technology Inc.
Software GUI
5.9.4.10
RIGHT MOUSE CLICK (INSIDE THE STRIP CHART AREA)
Figure 5-14 shows a chart menu with more chart options. This window is opened (on
the PC’s main display) by right-clicking inside the Strip Chart area (#5).
FIGURE 5-14:
Chart Menu With More Chart Options (Right Click inside the Strip Chart Area).
2012 Microchip Technology Inc.
DS52031A-page 49
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
5.9.4.11
ZOOMING IN AND OUT
Figure 5-15 shows the result of clicking and dragging inside the Chart Area; this will
cause the Strip Chart to display the selected area only. To zoom out to full view, click
the Zoom out button (second from right button under arrows #1 in Figure 5-2).
FIGURE 5-15:
DS52031A-page 50
Zoom In area in Strip Chart.
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Appendix A. Schematics and Layouts
A.1
INTRODUCTION
This appendix contains the following schematics and layouts for the MCP6N11 and
MCP6V2x Wheatstone Bridge Reference Design:
•
•
•
•
•
•
•
Board – Schematic
Board – Top Silk Screen and Pads
Board – Top Metal Layer
Board – Ground Plane (Second layer)
Board – Power Plane (Third layer)
Board – Bottom Metal Layer (top view)
Board – Bottom Silk and Pads (top view)
The Gerber files for this board are available on the Microchip website,
(www.microchip.com) and are contained in the zip file 00354R3_Gerbers.zip. This
is a four-layer PCB.
2012 Microchip Technology Inc.
DS52031A-page 51
vdd1
R9
3
2
20.0K 1%
R12
VR
SGND
SGND
R14
C14
10 5%
R21
5.0V
SGND
(0 or High-Z)
100K 5%
R15
10K 5%
1
10K 5%
R16
MCP6N11-SOIC8
Vref
Vout
Vdd
EN/CAL
INA #1
0603
4
5
L1
C15
10uH 20%
0805
Vdd1
10uF 16V 10%
1206
vdd1
R7
SGND
C9
10 5%
vdd1
100K 5%
Vdd3
499 0.1%
JP1
open
RC
C
Couple =
Vo1
20.0K 1%
R20
20.0K 1%
R19
Output Filter #1
Vdd1
N/C
100K 0.1%
SGND
SGND
R23
R18
R17
100nF 50V 10%
10uF 16V 10%
1206
C16
C8
10.0 1%
R13
100nF 50V 10%
220pF 50V 5%
478-1179-1-ND
C2
C1
1.0uF 25V 10%
1206
R22
Vdd2
Vref
30.1K 0.1%
Vref
5
6
7
8
Power Supply
Vss
Vip
Vim
Vfg
U2
C10
SGND
4
3
2
VIN-
VDD
MCP6001-SOT23-5
VIN+
VSS
VOUT
U1
100 Ohms 0.1%
VR
100 Ohms 0.1%
mCal #1
SGND 3
2
1
100nF 50V 10%
1.0uF 25V 10%
1206
E/C
SGND
C5
10nF 50V 10%
20.0K 1%
R11
C6
100nF 50V 10%
VL
C7
10nF 50V 10%
Input Filter #1
SGND
C3
R5
R3
VREF
100nF 50V 10%
SW-EVQ-P2R02M
1
4
SW1
C4
SGND
SGND
mCalSw
SGND
68.1K 1%
200 5%
R10
2.49K 0.1%
R6
SGND
100 0.1%
100 RTD 1%
(78.32 to 159.19)
External
2.49K 0.1%
R4
VL
R2
Vrtd
R1
vdd1
R8
121K 1%
Vdd1
Vdd1
SGND
1.0uF 25V 10%
1206
C13
Vad2
C12
1.0uF 25V 10%
1206
Vad1
1.0uF 25V 10%
1206
C11
SGND
JP2
Ext.
PIC
PWM=
Vpwmx
Vpwm
PWM Coupling
C17
R24
TP4
TP3
TP2
TP1
Rtd2
Rtd1
GND
+5.0V
20.0K 1%
R25
SGND
20 MHz
X1
SGND
Vpwm
E/C
N/C
N/C
N/C
Vad4
Vad3
Vad2
Vad1
SGND
1.0uF 25V 10%
1206
C27
VL
Vrtd
SGND
5.0V
10nF 50V 10%
C19
100nF 50V 10%
C18
10nF 50V 10%
20.0K 1%
RESONATOR-CSTCE
VL
SGND
SGND
VR
SGND
Vdd1
14
13
12
11
10
9
8
7
6
5
4
3
2
1
+B
TP8
TP7
TP6
TP5
VUSB
CCP1
RC1
RC0
OSC2
OSC1
VSS
RA5
RA4
AN3
AN2
AN1
AN0
VSS
SGND
100K 0.1%
GND
Vad2
Vad1
Vo1
VSS
OUTB
VDD
R31
R32
Vo3
Vad2
Vad1
PIC18F2553-SOIC28
GND
Vo4
Vo3
Vo2
VM
VP
RC6
RC7
VSS
VDD
RB0
RB1
RB2
RB3
RB4
RB5
PGC
PGD
TP12
TP11
TP10
TP9
Test Points
Vo1
15
16
17
18
19
20
21
22
23
24
25
26
27
28
MCP6V26
SOIC8
10.0K 0.1%
R29
499 0.1%
Vo2
10.0K 0.1%
R28
499 0.1%
R27
SGND
USB PIC
SGND
1.0K 1%
R37
7
U3B
R30
MCP6V27 100K 0.1%
-B
-A
MCLR
U5
5
6
2
3
C20
1.0uF 25V 10%
1206
U3A
MCP6V27
+A
SOIC8
VDD
1
OUTA
100nF 50V 10%
C21
10.0 1%
8
4
INA #2
VSS
VDD
R38
R39
N/C
N/C
N/C
N/C
N/C
N/C
N/C
N/C
SGND
SGND
1.0K 1%
R40
1.0K 1%
R34
6
SGND
Vdd2
Vdd3
N/C
N/C
1
2
3
4
5
6
5
4
3
2
1
J1
VBUS
D-
D+
SGND
USB
J2
Vpwmx
1206
1.0uF 25V 10%
Vad3
1206
1.0uF 25V 10%
Vad4
1.0uF 25V 10%
1206
SGND
Vad4
Vad3
ICSP
GND
Vad4
Vad3
SGND
C26
C25
C24
CONN_USB_MINI_B
TP15
TP14
Vdd2
SGND
R36
20.0K 1%
TP13
Vo4
SGND
C28
100nF 50V 10%
Vo4
Vo3
Vo2
10.0K 0.1%
SGND
-
+
C23
20K 1%
1.0uF 25V 10%
1206
10K 0.1%
C22
R35
Vref
SGND
Output Filter #2
R33
100nF 50V 10%
vdd1
1.0K 1%
2
U4
3
Vdd1
5
8
R26
1
7
4
vdd1
CON6
DS52031A-page 52
6
A.2
Shield
Input Filter 2
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
BOARD – SCHEMATIC
2012 Microchip Technology Inc.
Schematics and Layouts
A.3
BOARD – TOP SILK SCREEN AND PADS
2012 Microchip Technology Inc.
DS52031A-page 53
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
A.4
BOARD – TOP METAL LAYER
DS52031A-page 54
2012 Microchip Technology Inc.
Schematics and Layouts
A.5
BOARD – GROUND PLANE (SECOND LAYER)
2012 Microchip Technology Inc.
DS52031A-page 55
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
A.6
BOARD – POWER PLANE (THIRD LAYER)
DS52031A-page 56
2012 Microchip Technology Inc.
Schematics and Layouts
A.7
BOARD – BOTTOM METAL LAYER (TOP VIEW)
2012 Microchip Technology Inc.
DS52031A-page 57
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
A.8
BOARD – BOTTOM SILK AND PADS (TOP VIEW)
DS52031A-page 58
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Appendix B. Bill Of Materials (BOM)
TABLE B-1:
Qty
1
BILL OF MATERIALS – ASSEMBLED PCB
Reference
Designator
220 pF, 0603 SMD, Ceramic, NP0, 50V, 5%
Manufacturer
Part Number
AVX Corporation
06035A221JAT2A
10 C2-C4, C6,
C9, C10,
C18, C21,
C23, C28
100 nF, 0603 SMD, Ceramic, X7R, 50V, 10% AVX Corporation
06035C104KAT2A
4
10 nF, 0603 SMD, Ceramic, X7R, 50V, 10%
AVX Corporation
06035C103KAT2A
11 C8,
C11-C14,
C20, C22,
C24-C27
1.0 µF, 1206 SMD, Ceramic, X7R, 25V, 10% AVX Corporation
12063C105KAT2A
2
C15, C16
10 µF, 1206 SMD, Ceramic, X7R, 16V, 10%
AVX Corporation
1206YC106KAT2A
1
J2
Header, Mini USB
Tyco Electronics
1734035-2
1
C1
Description
C5, C7,
C17, C19
JP1
Header, 3×2, 100 mil
®
Molex
10-89-7062
10-89-7042
1
JP2
Header, 2×2, 100 mil
Molex®
1
L1
10 µH, 100 mA, 0805 SMD, 20%
Murata Electronics
LQM21FN100M70L
1
PCB
MCP6N11 and MCP6V2x Wheatstone
Bridge Reference Design,
4 layer PCB (2.60 in × 2.60 in)
—
—
2
R1, R6
2.49 kΩ, 0603 SMD, 0.1%, 1/10W
SUSUMU
RG1608P-2491-B-T5
1
R2
100Ω RTD, 0603 SMD, 1%
Vishay/Beyschlag
PTS060301B100RP100
3
R3-R5
100Ω, 0603 SMD, 0.1%, 1/10W
SUSUMU
RG1608P-101-B-T5
1
R7
30.1 kΩ, 0603 SMD, 0.1%, 1/10W
SUSUMU
RG1608P-3012-B-T5
1
R8
121 kΩ, 0603 SMD, 1%, 1/10W
Yageo
RC0603FR-07121KL
1
R9
68.1 kΩ, 0603 SMD, 1%, 1/10W
Yageo
RC0603FR-0768K1L
1
R10
200Ω, 0603 SMD, 5%, 1/10W
Yageo
RC0603JR-07200RL
8
R11, R12,
R19, R20,
R24, R25,
R35, R36
20.0 kΩ, 0603 SMD, 1%, 1/10W
Yageo
RC0603FR-0720KL
2
R13, R26
10.0Ω, 0603 SMD, 1%, 1/10W
Yageo
RC0603FR-0710RL
2
R14, R16
10 kΩ, 0603 SMD, 5%, 1/10W
Yageo
RC0603JR-0710KL
2
R15, R23
100 kΩ, 0603 SMD, 5%, 1/10W
Yageo
RC0603JR-07100KL
3
R17, R27,
R30
100 kΩ, 0603 SMD, 0.1%, 1/10W
SUSUMU
RG1608P-104-B-T5
3
R18, R28,
R29
499Ω, 0603 SMD, 0.1%, 1/10W
SUSUMU
RG1608P-4990-B-T5
2
R21, R22
10Ω, 0603 SMD, 5%, 1/10W
Yageo
RC0603JR-0710KL
Note 1:
The components listed in this Bill of Materials are representative of the PCB assembly. The released BOM
used in manufacturing uses all RoHS-compliant components.
2012 Microchip Technology Inc.
DS52031A-page 59
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
TABLE B-1:
Qty
BILL OF MATERIALS – ASSEMBLED PCB (CONTINUED)
Reference
Designator
Description
Manufacturer
Part Number
4
R31-R34
10.0 kΩ, 0603 SMD, 0.1%, 1/10W
SUSUMU
RG1608P-103-B-T5
4
R37-R40
1.00 kΩ, 0603 SMD, 1%, 1/10W
Yageo
RC0603FR-071KL
2
SH1, SH2
Shunt, 100 mil Thru-Hole, 2 Pos., 30 µm Au
Tyco Electronics Corp.
881545-2
1
SW1
SMD, Switch, Push Button, 1 Pos.,
SPST-NO
Panasonic-ECG
EVQ-P2R02M
1
U1
MCP6001, SOT-23-5, Single Op Amp
Microchip Technology Inc. MCP6001T-E/OT
1
U2
MCP6N11, SOIC-8, Single INA, GMIN = 100 Microchip Technology Inc. MCP6N11-100U/SN
1
U3
MCP6V27, SOIC-8, Dual Auto-Zeroed Op
Amp
Microchip Technology Inc. MCP6V27-E/SN
1
U4
MCP6V26, SOIC-8, Single Auto-Zeroed Op
Amp
Microchip Technology Inc. MCP6V26-E/SN
1
U5
PIC18F2553, SOIC-28 (300 mil), USB PIC
MCU
Microchip Technology Inc. PIC18F2553-I/SO
1
Y1
20 MHz Ceramic Resonator, SMT, E-temp
Murata Electronics
Note 1:
The components listed in this Bill of Materials are representative of the PCB assembly. The released BOM
used in manufacturing uses all RoHS-compliant components.
TABLE B-2:
BILL OF MATERIALS – OTHER PARTS IN KIT
Qty
Reference
Designator
1
—
Note 1:
Description
USB 2.0 Cable, A to Mini B, 1 m
Manufacturer
Part Number
Molex®
88732-8600
The components listed in this Bill of Materials are representative of the other parts in the kit. The released
BOM used in manufacturing uses all RoHS-compliant components.
TABLE B-3:
Qty
CSTCE20M0V53Z-R0
Reference
Designator
BILL OF MATERIALS – ALTERNATE COMPONENTS
Description
Manufacturer
Part Number
1
C27
470 nF, 1206 SMD, Ceramic, X7R, 25V, 10%
AVX Corporation
12063C474KAT2A
4
R31-R34
10.0 kΩ, 0603 SMD, 0.01%, 1/16W
Stackpole Electronics
Inc.
RNCF0603TKY10K0
1
U3
MCP6V07, SOIC-8, Dual Auto-Zeroed Op Amp
Microchip Technology MCP6V07-E/SN
Inc.
MCP6V02, SOIC-8, Dual Auto-Zeroed Op Amp
Microchip Technology MCP6V02-E/SN
Inc.
MCP6V06, SOIC-8, Single Auto-Zeroed Op Amp
Microchip Technology MCP6V06-E/SN
Inc.
MCP6V01, SOIC-8, Single Auto-Zeroed Op Amp
Microchip Technology MCP6V01-E/SN
Inc.
1
U4
Note 1:
The components listed in this Bill of Materials are representative of the alternate components used for
modifications. The released BOM used in manufacturing uses all RoHS-compliant components.
DS52031A-page 60
2012 Microchip Technology Inc.
Bill Of Materials (BOM)
TABLE B-4:
BILL OF MATERIALS – OPTIONAL PARTS
Reference
Designator
Qty
Description
Manufacturer
Molex®
Part Number
1
J1
Header, 1×6, 100 mil
16
TP1-TP16
SMD, Test Point
4
(for PCB mounting) Hemispherical Bumpon™ Standoff,
0.44 in × 0.20 in
3M™
SJ-5003 (BLACK)
4
(for PCB mounting) Stand-off, Hex, 0.500", 4 × 40 Thread,
Nylon,
0.285" max. O.D.
Keystone
Electronics
1902C
4
(for PCB mounting) Machine Screw, Phillips, 4 × 40 Thread,
1/4" long, Nylon
Building Fasteners
NY PMS 440 0025 PH
2
(for External RTD)
n/a
n/a
Note 1:
24 AWG wires, twisted strands, 2 m
22-28-4060
Keystone Electronics 5016
The components listed in this Bill of Materials are representative of the optional parts that the user may
wish to acquire and install. The released BOM used in manufacturing uses all RoHS-compliant components.
2012 Microchip Technology Inc.
DS52031A-page 61
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
NOTES:
DS52031A-page 62
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Appendix C. Conversion Polynomials
C.1
CIRCUIT RESPONSE
Table C-1 shows the nominal performance of critical circuit parameters used in this
appendix. VBRIDGE is the differential output voltage of the Wheatstone bridge. VINA_OUT
is the output voltage of the INAs. VREF shifts the INAs’ output above ground (about
1.80V). VINA is VINA_OUT – VREF, which corrects the measured voltages for VREF errors
(about ±0.03V).
TABLE C-1:
NOMINAL VALUES
TRTD
(°C)
RRTD
(Ω)
VBRIDGE
(V)
VINA_OUT
(V)
VINA
(V)
-55
78.32
-0.00565
0.665
-1.136
-50
80.31
-0.00510
0.774
-1.026
-45
82.29
-0.00456
0.883
-0.918
-40
84.27
-0.00403
0.989
-0.811
-35
86.25
-0.00351
1.095
-0.706
-30
88.22
-0.00299
1.199
-0.601
-25
90.19
-0.00248
1.303
-0.498
-20
92.16
-0.00197
1.404
-0.396
-15
94.12
-0.00147
1.505
-0.295
-10
96.04
-0.00097
1.605
-0.196
-5
98.04
-0.00048
1.703
-0.097
0
100.00
0.00000
1.801
0.000
5
101.95
0.00048
1.897
0.096
10
103.90
0.00095
1.992
0.191
15
105.85
0.00142
2.086
0.285
20
107.79
0.00188
2.179
0.378
25
109.74
0.00234
2.271
0.470
30
111.67
0.00279
2.362
0.561
35
113.61
0.00324
2.452
0.651
40
115.54
0.00368
2.541
0.740
45
117.47
0.00412
2.629
0.828
50
119.40
0.00455
2.716
0.915
55
121.32
0.00498
2.802
1.001
60
123.24
0.00540
2.887
1.086
65
125.16
0.00582
2.971
1.171
70
127.08
0.00623
3.055
1.254
75
128.99
0.00664
3.137
1.336
80
130.90
0.00705
3.219
1.418
85
132.80
0.00745
3.299
1.499
90
134.71
0.00785
3.379
1.579
95
136.61
0.00824
3.458
1.658
2012 Microchip Technology Inc.
DS52031A-page 63
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
TABLE C-1:
DS52031A-page 64
NOMINAL VALUES (CONTINUED)
TRTD
(°C)
RRTD
(Ω)
VBRIDGE
(V)
VINA_OUT
(V)
VINA
(V)
100
138.51
0.00863
3.537
1.736
105
140.40
0.00901
3.614
1.813
110
142.29
0.00939
3.691
1.890
115
144.18
0.00977
3.767
1.966
120
146.07
0.01014
3.842
2.041
125
147.95
0.01051
3.916
2.115
130
149.83
0.01088
3.990
2.189
135
151.71
0.01124
4.063
2.262
140
153.58
0.01160
4.135
2.334
145
155.46
0.01196
4.206
2.405
150
157.33
0.01231
4.277
2.476
155
159.19
0.01266
4.347
2.546
2012 Microchip Technology Inc.
Conversion Polynomials
C.2
RTD TEMPERATURE AND RESISTANCE
The RTD’s temperature error is Class F0.3:
EQUATION
TTol = ±(0.30°C + 0.005 |TRTD|),
Class F0.3
To accommodate fixed point arithmetic, RRTD and TRTD are scaled as follows:
EQUATION
p = TRTD / TS1
q = (RRTD – RS0)/ RS1
Where:
TS1 = +256°C
RS0 = 120 Ω
RS1 = 64 Ω
The RTD’s resistance is estimated with this polynomial approximation (with scaling):
EQUATION
q = A0 + p (A1 + p (A2)) ± εQ
RRTD = RS0 + q RS1 ± εR
Where:
A0 = -0.31251
A1 = 1.56353
A2 = -0.05946
εR = εQ RS1,
εT = εR / (dRRTD/dTRTD),
|εR| 0.0053 Ω
|εT| 0.013°C
The RTD’s temperature is estimated with this polynomial approximation (with scaling):
EQUATION
p = B0 + q (B1 + q (B2 + q (B3 + q (B4)))) ± εP
TRTD = p TS1 ± εT
Where:
B0 = 0.201424
B1 = 0.649547
B2 = 0.016232
B3 = 0.000768
B4 = 0.000163
2012 Microchip Technology Inc.
εT = εP TS1,
|εT 0.013°C
|
εR = εT (dRRTD/dTRTD)
|εR 0.0049 Ω
|
DS52031A-page 65
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
C.3
INA VOLTAGE TO TEMPERATURE
Preliminary work was done in an Excel® spreadsheet to fit polynomials to the TRTD
using VINA (see Table C-1). Different order approximations were produced for different
purposes: hand estimates, firmware algorithm and spreadsheet calculations.
To accommodate fixed point arithmetic, TRTD is scaled as before and VINA is scaled
based on its binary representation:
EQUATION
v = VINA / (VDD/2)
w = TRTD / TS1
Where:
VINA = VAD1 – VAD2
for INA #1
= VAD3 – VAD4,
for INA #2
VDD/2 = 2.5V
TS1 = +256°C
The reason VINA subtracts VREF (VAD2 or VAD4) is to correct the VREF’s inaccuracy
(about ±0.03V).
Since the PIC MCU’s ADC uses the supply as its reference voltage, v is easy to produce in firmware. Simply use the most significant bit (MSb) as the sign bit, and the other
bits to produce the magnitude.
The fitted polynomials are based on the average of the two nominal INA gains
(GINA = 201.2 V/V); the resulting nominal error is only ±0.1%, which is well within the
circuit’s error budget.
EQUATION
w = K0 + v (K1 + v (K2 + v (K3 + v (K4)))) ± εW
TRTD = w TS1
εT = εW TS1
Where several polynomials are of use:
Parameters
Polynomials
n=1
n=2
n=4
K0
0.01453
-0.00130
0.00002
0.00001
K1
0.55695
0.50628
0.50417
0.50465
K2
0
0.08652
0.07463
0.07452
0
0.01370
0.01116
0
0.00221
0.35
0.021
0.0013
K3
K4
εT (°C)
Note 1:
2:
DS52031A-page 66
n=3
6.1
n = 1 and 2 are useful for hand calculations, n = 3 for the firmware algorithm and n = 4 for spreadsheet design calculations.
Other RTDs will use different coefficients.
2012 Microchip Technology Inc.
MCP6N11 AND MCP6V2X
WHEATSTONE BRIDGE
REFERENCE DESIGN USER’S GUIDE
Appendix D. Board Validation Summary
This appendix summarizes the data collected during validation of two Rev. 1 boards.
D.1
THERMAL STEADY STATE RESPONSE
The boards’ bias voltages were within expected ranges. The difference between the
two INA outputs on each board was less than ±0.2°C (caused by resistor tolerances).
The difference between the two boards was less than ±1.4°C (caused by RTD tolerances).
The first board’s steady state temperature was 23.4°C (74.1°F).
D.2
THERMAL IMPULSE RESPONSE
A quick blast from a freeze spray can onto the RTD (on the first board) cooled it rapidly,
giving the thermal impulse response of the RTD and PCB. The following plots are from
the second INA on that board.
Figure D-1 shows VAD3, which is in range.
INA #2 Output Voltage (V)
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
FIGURE D-1:
2012 Microchip Technology Inc.
10 20 30 40 50 60 70 80 90 100 110 120
Time (s)
INA #2’s Output Voltage.
DS52031A-page 67
MCP6N11 and MCP6V2x Wheatstone Bridge Reference Design
Figure D-2 shows the RTD’s temperature as it changes over a two minute period of
time. It starts out of range for this RTD sensor (-55°C is the specified minimum).
30
RTD Temperature (°C)
20
10
0
-10
-20
-30
-40
-50
-60
-70
0
FIGURE D-2:
10 20 30 40 50 60 70 80 90 100 110 120
Time (s)
RTD Temperature.
Figure D-3 shows the difference between the steady state temperature (23.4°C) and
the temperature at each time point. Initially, the RTD’s thermal settling dominates, with
= 4.2 s (fP = 38 MHz). Later on, the PCB’s thermal settling (with still air, laying flat on
the bench) dominates, with = 94 s (fP = 1.7 MHz).
Magnitude of Change in
RTD Temperature (°C)
100
Time Constant = 4.2s
(RTD)
Time Constant = 94s
(PCB)
10
Steady State = 0°C, t = ∞
1
0 10 20 30 40 50 60 70 80 90 100 110 120
Time (s)
FIGURE D-3:
DS52031A-page 68
Exponential Settling of Temperature vs. Time.
2012 Microchip Technology Inc.
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DS52031A-page 70
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11/29/11
2012 Microchip Technology Inc.