Texas Instruments ADS1131REF User manual

User's Guide
SBAU175A–July 2010–Revised August 2011
ADS1131REF and ADS1231REF
The ADS1131REF and the ADS1231REF are reference designs for delta-sigma analog-to-digital
converters (ADCs). The ADS1131REF contains the ADS1131 18-bit device, while the ADS1231REF
contains the ADS1231 24-bit device. Both systems contain all the circuitry and user interface elements
needed for a weigh-scale digitizer, and are meant as examples of a good design for a basic weigh-scale
system. Each system is also suitable for general evaluation of the respectively installed ADC. Throughout
this document, the term ADS1x31REF is used to refer to the common features and functions for both
systems.
The ADS1x31REF hardware has the following features:
•ADS1131 ADC for the ADS1131REF and ADS1231 ADC for the ADS1231REF
•Connections for load cells or other voltage sources
•Low-side excitation switch on the load cell header connector
•Ample EMI/RFI suppression between the ADC and rest of design
•Eight-digit starburst LCD readout
•USB connection for firmware updates and remote control
•Designed for very low power consumption
•Battery (9V) or wall power
Version 1.0.0 of the firmware includes the following features:
•Weigh-scale mode with two-point calibration
•Complete configuration of the device
•Real-time peak-to-peak and RMS noise calculation
•Autoranging voltage display
•Noise displayed in volts, codes, and bits
•Voltage displayed in volts or codes
•Adjustable averaging mode
•Raw hexadecimal code display
•Simple and fast configuration
•Parameters saved to internal flash memory
•Computer link
Graphical PC software is also provided for histogram display, datalogging, and device control.
We welcome bug reports and suggestions for additional features; please contact the Texas Instruments
Precision Analog Applications Group.
Hyperterm is a trademark of Microsoft Corporation.
Windows is a registered trademark of Microsoft Corporation.
All other trademarks are the property of their respective owners.
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Contents
1 Getting Started .............................................................................................................. 3
2 Weigh Scale Mode .......................................................................................................... 7
3 Analysis Mode ............................................................................................................. 11
4 Using the PC Software ................................................................................................... 14
5 Serial Console ............................................................................................................. 18
6 Hardware ................................................................................................................... 20
Appendix A Schematic and Layout ........................................................................................... 23
List of Figures
1 ADS1x31REF Controls and Connectors................................................................................. 3
2 4-Wire Load Cell to Terminal Block ...................................................................................... 4
3 6-Wire Load Cell to Terminal Block ...................................................................................... 5
4 4-Wire Load Cell to Header................................................................................................ 5
5 6-Wire Load Cell to Header................................................................................................ 5
6 ADS1231REF PC Software Display .................................................................................... 15
7 ADS1x31REF Average Data............................................................................................. 16
8 ADS1x31REF Hardware Block Diagram ............................................................................... 20
9 ADS1x31REF PCB—Top Side .......................................................................................... 25
10 ADS1x31REF PCB—Layer 1............................................................................................ 25
11 ADS1x31REF PCB—Layer 2............................................................................................ 26
12 ADS1x31REF PCB—Bottom Side ...................................................................................... 26
13 ADS1x31REF Schematic—ADC ........................................................................................ 27
14 ADS1x31REF Schematic—MCU........................................................................................ 27
15 ADS1x31REF Schematic—USB......................................................................................... 28
List of Tables
1 Unit Conversion Factors and Display Formats.......................................................................... 8
2 Parameters in Configuration Mode ....................................................................................... 9
3 Modes and Example Displays ........................................................................................... 11
4 Voltage Display Ranges.................................................................................................. 12
5 Parameters in Analysis Mode............................................................................................ 13
6 Console Mode Commands............................................................................................... 19
7 Load Cell Header Pinout ................................................................................................. 21
8 Terminal Block Pinout..................................................................................................... 22
9 ADS1x31REF Bill of Materials .......................................................................................... 23
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1 Getting Started
A diagram of the ADS1x31REF is shown in Figure 1.
Figure 1. ADS1x31REF Controls and Connectors
1.1 Operating Modes
The ADS1x31REF operates in one of three modes:
•Scale mode: When the mode select switch is in Scale position, the ADS1x31REF acts as a basic
weigh scale. The scale has tare, range, and calibrate functions, and can display metric (SI) units. Other
parameters can be configured in configuration screens. Scale mode is described in detail in Section 2.
•Analysis mode: When the mode select switch is in Analysis position, codes are taken directly from the
installed ADS device and various measurements are made upon them. Several measurements are
available, including raw display, voltage, RMS noise, and peak-to-peak measurements. The ADS1131
or ADS1231 can also be configured directly from this mode. Analysis mode is described in detail in
Section 3.
•Configuration mode: Parameters governing the operation of the ADS1x31REF can be viewed and
altered in this mode. Scale and Analysis modes have different options in configuration mode:
configuration for Scale mode is described in Section 2.4, and configuration for Analysis mode is
described in Section 3.4.
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EXC-
SNS-
SNS+
SIG+
SIG-
EXC+
OUT+
EXC-
OUT-
EXC+
Getting Started www.ti.com
1.2 Controls
The main controls for the ADS1x31REF are the four buttons and the mode selection slide-switch (see
Figure 1).
The slide-switch selects between weigh-scale (Scale) and Analysis modes. The ADS1x31REF switches
modes only when it is displaying data. If the switch is changed in a configuration mode, nothing happens
until the configuration mode is exited. At that time the ADS1x31REF reads the switch and enters the
selected mode.
The four buttons have different functions, depending on the operating mode. In Scale mode, the switches
have the functions in the box labeled SCALE. In Analysis mode, the switches have the functions shown in
the box labeled ANALYSIS. In configuration mode, the switches have the functions shown in the box
labeled CONFIG.
The buttons also have different names in different modes. In this document, they are identifed by the
respective names they have in the mode under discussion.
1.2.1 Auxiliary Controls
The Reset switch resets the board, except for the USB interface.
The USB Reset switch resets the USB interface. If USB communication fails, pressing USB Reset may
solve the problem.
The Programming Mode switch is used to update the firmware. For normal operation, it should be set to
JTAG.
1.3 Power
To apply power to the ADS1x31REF, connect a 9V battery or plug in a 6V–9V ac wall adapter.
AC adapters must be tip positive/sleeve negative. When an ac adapter is plugged in, the board always
takes power from it, and not from the battery.
The ADS1x31REF is protected against polarity reversal. If a power source is connected in reverse by
mistake, the display remains blank. To prevent damage to the board, do not leave a reversed power
source connected for longer than a few seconds.
1.4 Connecting a Load Cell
The ADS1x31REF is specifically designed for connection to load cells. Two connectors are provided for
this application. The terminal block is used for load cells having stripped wire connections; the load cell
header is for load cells having a header connector. The terminal block provides connections to the
reference input (or power supply) and the header has switched excitation.
1.4.1 Connecting a 4-Wire Load Cell to the Terminal Block
Figure 2 shows the connection of a 4-wire load cell to the terminal block. In this configuration, the load cell
is excited by the +5V power supply, and the ADC reference is taken from the power supply.
For this configuration, the reference select switch must be in the +5VA position.
Figure 2. 4-Wire Load Cell to Terminal Block
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EXC-
SNS-
SNS+
SIG+
SIG-
EXC+
OUT+
EXC-
SENSE-
OUT-
SENSE+
EXC+
OUT+
EXC-
OUT-
EXC+
1
2
3
4
5
6
OUT-
EXC+
OUT+
EXC-
SENSE+
SENSE-
1
2
3
4
5
6
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1.4.2 Connecting a 6-Wire Load Cell to the Terminal Block
Figure 3 shows the connection of a 6-wire load cell to the terminal block. In this configuration, the load cell
is excited by the +5V power supply, and the ADC’s reference is taken from the sense wire returning from
the load cell. The sense wire connects to the excitation wire at the bridge sensor.
Figure 3. 6-Wire Load Cell to Terminal Block
1.4.3 Connecting a 4-Wire Load Cell to the Header
Figure 4 shows the connection of a 4-wire load cell to the header. In this configuration, the load cell is
excited by the +5V power supply, and the ADC reference is taken from the power supply.
Figure 4. 4-Wire Load Cell to Header
For this configuration, the reference select switch must be in the +5VA position; the EXT position does not
work.
1.4.4 Connecting a 6-Wire Load Cell to the Header
Figure 5 shows the connection of a 6-wire load cell to the header. In this configuration, the load cell is
excited by the +5V power supply, and the ADC reference is taken from the sense wire returning from the
load cell. The sense wire connects to the excitation wire at the bridge sensor.
Figure 5. 6-Wire Load Cell to Header
For this configuration, the reference select switch should be in the EXT position for best performance. The
+5V position also works, but the device may not perform as well.
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1.5 Connecting Other Signal Sources
In general, the ADS1x31REF can accurately measure any voltage in the input range of the installed ADS
device, as long as the following rules are observed:
•Never apply a negative voltage to the inputs of the ADS1x31REF. The installed ADS device cannot
accept negative voltages at its input. Applying negative voltages may damage both the device and the
ADS1x31REF. (The negative signs used in some signal names indicate inversion, not polarity.)
•For single-ended signals, ground the negative input or connect it to 2.5V. 2.5V is available from a
voltage divider by shorting J7; see Section 6.4.5 for details.
•The input range of the amplifier on the ADS1131 or ADS1231 does not extend to the supplies. See the
ADS1131 data sheet or the ADS1231 data sheet for details.
Note that Scale mode is designed only for use with load cells. Although it can be tested with a voltage
source or resistive divider, Scale mode does not, in general, display meaningful data unless a load cell is
connected and calibration is performed.
1.6 Connecting an External Clock
Note that only the ADS1231 can receive an external clock. To connect an external clock, connect a clock
oscillator to the EXTCLK test point. No settings need to be changed; the ADS1231 will automatically use
the attached clock.
The clock source must conform to 3.3V TTL or CMOS logic rules.
1.7 Common Tasks
1.7.1 Shorted-Input Noise Test
The noise measurements given in the product data sheet are taken with the inputs shorted to 2.5V. These
noise measurements can be replicated on the ADS1x31REF with no external hardware. To set up these
measurements on the ADS1x31REF, perform the following steps:
1. Move the mode switch to Analysis mode.
2. Short jumpers J8 and J7. (These jumpers are located very near the terminal block, and are marked
Input Shorting Jumpers in Figure 1.)
3. Set up the installed ADS device as desired, as described in the previous sections.
4. Hold down the DISP button. The display shows the current display mode. While holding down DISP,
press the MODE button until the word on the left side is RMS.
5. While still holding down DISP, press the UNIT button until the word on the right side is VOLT.
6. Release the DISP button. The display shows the word GOT followed by an increasing number. Once
the appropriate number of points is collected, the calculated noise voltage is displayed. This value is
the shorted-input RMS noise voltage, input-referred.
The first RMS noise measurement may be incorrect as a result of device settling. The second
measurement is generally correct.
For a detailed description of Analysis mode, see Section 3.
1.7.2 Measuring Mass
The following items are required to measure mass with the ADS1x31REF:
•A load cell, connected as described in Section 1.4
•An object of known mass within the load cell range
To avoid performing calibration on each power-up, you can save the calibration settings to flash memory.
See Section 3.4.1 for details.
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www.ti.com Weigh Scale Mode
Obtaining a calibration weight:
Before the ADS1x31REF can display the mass of an object, it must measure the output of the load cell for
a previously known mass. The known mass can be adjusted.
At power-up, the ADS1x31REF expects a 5kg mass. If this mass is not available, or if the load cell range
is not compatible with this mass, the calibration mass can be changed in the following manner:
1. Determine the mass of the calibration weight. The most accurate way to determine the mass is to
weigh the calibration object on an accurately calibrated scale. If the weight is precalibrated, its given
mass can be used, although this approach is not generally as accurate. (Note that the accuracy of the
ADS1x31REF as a scale directly depends on the accuracy to which the calibration weight can be
measured.)
2. Switch the mode switch to SCALE mode.
3. Press the PARM buttons simultaneously. The ADS1x31REF enters configuration mode.
4. Use the PARM buttons to select the screen which shows UNIT =. This screen allows you to select the
units used for the calibration mass. If the units shown are not correct, adjust them using the VALUE
buttons.
5. Use the PARM buttons to select the screen that shows CW= followed by the calibration mass. On this
screen, you can adjust each digit of the calibration mass separately. Select the digit using the PARM
buttons. The currently-selected digit flashes, and can be adjusted using the VALUE buttons.
6. Adjust the calibration mass to match the mass of the calibration weight.
7. Press the PARM buttons simultaneously to exit the configuration mode.
Preparing Scale mode:
Do the following steps to set up the scale mode:
1. Connect the load cell.
2. If the input shorting jumpers J8 and J7 are connected, disconnect them (see Figure 1).
3. Obtain a weight of known mass.
4. If the mass of the weight is not equal to the configured calibration mass, adjust the calibration mass as
described above.
5. Move the mode switch to SCALE position. If calibration has not been performed, the display reads
NO CAL.
6. Press the CAL button, and follow the calibration procedure given in Section 2.3.
If the calibration is performed properly, and the load cell is connected correctly, the ADS1x31REF will
measure the mass of an object placed on the load cell (provided that the mass of the object is within the
device range).
Weigh scale mode is described in detail in Section 2.
2 Weigh Scale Mode
In weigh scale mode, the ADS1x31REF displays mass.(1) Mass is displayed in either SI or avoirdupois
units based on the voltage received from a load cell.
The ADS1x31REF operates in Scale mode when the mode switch is set to the Scale position.
To accurately calculate mass, the ADS1x31REF must have calibration information for the load cell. When
scale mode is first entered, the ADS1x31REF displays NO CAL, because the ADS1x31REF has no
calibration data when it is powered on.
By default, mass is calculated from the average of four successive readings from the ADC. The number of
points for averaging can be adjusted, and averaging can be turned off.
(1) Load cells do not measure mass directly; they output a voltage proportional to the weight of an object. The mass of an object can be
accurately inferred from this voltage as long as a calibration is accurately performed, the downward force of gravity remains constant (as
it does if the load cell is not moved to a different altitude), and the tilt of the load cell does not change; if either of the latter conditions
change, a new calibration must be performed.
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m =
wfs
c-cfs zs
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2.1 Display
Weigh scale mode can display mass in these units: μg (displayed as ug), g, kg, pounds (lb), stone (st),
and ounces (oz).(2)
Internally, mass is measured in grams, and calibration factors are stored in grams. At display time, grams
are converted to the desired display unit using the conversion factors given in Table 1.
xxx
(2) The avoirdupois units used by the ADS1x31REF are equal in the imperial and U. S. customary systems.
Table 1. Unit Conversion Factors and Display Formats
Unit Multiplier Format
micrograms (μg) 106nnn.nnnug
milligrams (mg) 1000 nnn.nnnmg
grams (g) 1 nnn.nnng
kilograms (kg) 10–3nnn.nnnkg
ounces (oz) 0.035274 nnn.nnnoz
pounds (lb) 0.0022046 nnn.nnnlb
stone (st) 157.473 ×10−6nnstnn.nn
The display format for stone differs from the format used for the other units. One stone is equal to fourteen
pounds; weight in stone is commonly expressed as a number of stone followed by a number of pounds.
On the ADS1x31REF, two digits are shown for stone, followed by st, followed by pounds displayed to two
decimal places.
2.1.1 Calculation of Mass
Mass is calculated from ADC code using the formula:
w=m●c+wzs –wt
where:
•w= mass
•c= the ADC code
•wt= tare weight
•m, wzs, wt= values determined in the calibration process
mis a calibration constant, and is calculated using Equation 1:
(1)
where wfs is the user-specified calibration mass, cfs is the ADC code taken with the calibration mass
applied, and czs is the ADC measurement taken with no load.
wzs , the zero-scale mass, is calculated from m and czs using Equation 2:
wzs =–m●czs (2)
2.2 Button Functions
2.2.1 TARE
The tare function allows the mass of a substance to be measured separately from the mass of its
container. When the TARE button is pressed, the scale measures the mass and records it. This reading is
subtracted from each subsequent measurement.
The recorded tare value can be reset to zero by holding the TARE button down for at least one second.
When the tare value is to be reset, the display reads TARE OFF.
Tare is also reset following a calibration.
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2.2.2 RANGE
The RANGE button changes the units or range of the display. RANGE cycles through all available units;
see Section 2.1 for details. When the RANGE button is depressed, it displays the selected unit.
2.2.3 CAL
Pressing CAL initiates the two-point calibration sequence. See Section 2.3 for details.
2.3 Calibration
Two-point calibration is performed by pressing the CAL button. When this button is pressed, the board
executes the following command sequence:
1. The board scrolls the message, REMOVE WEIGHT.
2. The user removes all weight from the load cell and presses any button.
3. The board measures the load cell voltage and records it as the zero point.
4. The board scrolls the message PLACE CAL WEIGHT.
5. The user places a weight on the load cell and presses any button. The weight should have the mass
that was selected in the configuration mode.
6. The board measures the load cell voltage and records it as the calibration weight.
New calibration data is lost when power is removed. To prevent this data loss, calibration data can be
saved, with other board settings, to flash memory. See Section 3.4.1 for details.
2.4 Configuration
The parameters for Scale mode can be adjusted in the configuration mode.
To enter configuration mode, press the PARM buttons simultaneously. The four buttons then assume the
functions shown in the CONFIG box. To exit Configuration mode, press the PARM buttons simultaneously
again. This function does not cause parameters to be adjusted, because only button releases are detected
in Configuration mode.
Configuration mode contains a number of adjustable parameters. To scroll through the available
parameters, use the PARM buttons. To change the parameter values, use the VALUE buttons.
Some items in configuration mode are not parameters, but commands or gateways to a submenu. These
items are labelled as words with a question mark. To enter these or to execute the command, press SEL
or ENT.
All parameters in the analysis and scale configuration menus are independent, including the parameters
found in both modes.
Table 2 summarizes the available parameters.
Table 2. Parameters in Configuration Mode
Parameter Display Value Range Description
Display units UNIT= μg, mg, g, kg, lb, st, oz Display units
Averages AVGS= 2–128 Number of points to
average
ADC speed SPD= FAST, SLOW Acquisition rate
Calibration mass CW= 0–99.9 in various units Calibration mass and unit
Save parameters SAVE? —Save parameters; see text
Version number V1.0.0 —Firmware version number
display
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2.4.1 Parameters
Display units: The units to use when displaying mass. Available units are μg (displayed as ug), mg, g, kg,
ounces (oz), pounds, (lb), and stone (st). See Section 2.1 for details.
Averages: Number of points to use when reading weight. The choices available are 2, 4, 8, 10, 16, 32,
50, 64, 100, and 128. The default is 50.
ADC speed: This parameter selects between the two data rates on the installed ADS device, which are
called high-speed and low-speed. When high-speed mode is selected, FAST is shown; for low-speed
mode, SLOW is shown.(1)
The actual data rate of the ADS1131 or ADS1231 depends on the frequency of the master clock, fCLK. In
fast mode, the data rate is fCLK/61440; in slow mode, the data rate is fCLK/491520. See the ADS1131 data
sheet or the ADS1231 data sheet for further information.
The default setting is low-speed mode.
Calibration weight: This parameter gives the expected mass of the calibration weight used in the
calibration procedure (Section 2.3). The calibration mass can be given in any of the available units, in
three significant figures. The unit is independent of the display unit.
Each digit in the mass is adjusted separately. The currently-selected digit flashes, and can be adjusted
with the VALUE buttons. The PARM buttons are used to select the digit. The unit is adjusted in the same
manner, and flashes when selected.
When the unit is changed, the value changes to the equivalent mass in the new unit.
Save parameters: This screen allows the settings of the ADS1x31REF to be stored in flash memory. It
functions the same as it does in Analysis mode. When ENT or SEL is pressed on this screen, the
ADS1x31REF saves its settings to flash memory. These settings are loaded from flash memory when the
board is reset or powered on. All operating parameters are saved, including scale calibration settings,
voltage reference, display mode, and units.
Version number: This screen displays the version number of the ADS1x31REF firmware.
(1) The ADS1x31REF cannot detect the frequency of the ADS1231 master clock, so it cannot display the actual data rate of the ADS1231
device.
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