Texas Instruments ADS1131REF Manuel utilisateur

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.

SBAU175A–July 2010–Revised August 2011 ADS1131REF and ADS1231REF

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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

2ADS1131REF and ADS1231REF SBAU175A–July 2010–Revised August 2011

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Getting Started

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.

SBAU175A–July 2010–Revised August 2011 ADS1131REF and ADS1231REF

EXC-

SNS-

SNS+

SIG+

SIG-

EXC+

OUT+

EXC-

OUT-

EXC+

Getting Started

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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

4ADS1131REF and ADS1231REF SBAU175A–July 2010–Revised August 2011

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EXC-

SNS-

SNS+

SIG+

SIG-

EXC+

OUT+

EXC-

SENSE-

OUT-

SENSE+

EXC+

OUT+

EXC-

OUT-

EXC+

OUT-

EXC+

OUT+

EXC-

SENSE+

SENSE-

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Getting Started

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.

SBAU175A–July 2010–Revised August 2011 ADS1131REF and ADS1231REF

Getting Started

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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.

6ADS1131REF and ADS1231REF SBAU175A–July 2010–Revised August 2011

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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.

SBAU175A–July 2010–Revised August 2011 ADS1131REF and ADS1231REF

m =

wfs

c-cfs zs

Weigh Scale Mode

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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.

8ADS1131REF and ADS1231REF SBAU175A–July 2010–Revised August 2011

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Weigh Scale Mode

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

SBAU175A–July 2010–Revised August 2011 ADS1131REF and ADS1231REF

Weigh Scale Mode

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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.

10 ADS1131REF and ADS1231REF SBAU175A–July 2010–Revised August 2011

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