Texas Instruments ADS1230REF Manuale utente

User's GuideSBAU125A – August 2007 – Revised September 2007

ADS1230REF User's Guide

The ADS1230REF is a reference design for the ADS1230 20-bit, delta-sigma analog-to-digital converter(ADC). It contains all the circuitry and user interface elements needed for a weigh-scale digitizer, and ismeant as an example of good design for a basic weigh-scale system. The ADS1230REF is also suitablefor general evaluation of the ADS1230 device.

The ADS1230REF hardware has the following features:•ADS1230 ADC•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 1.1.0 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 InstrumentsPrecision 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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Contents1 Getting Started ............................................................................................................... 32 Weigh Scale Mode........................................................................................................... 73 Analysis Mode .............................................................................................................. 114 Using the PC Software .................................................................................................... 145 Serial Console .............................................................................................................. 186 Hardware .................................................................................................................... 20Appendix A Schematic and Layout ........................................................................................... 24

List of Figures

1 ADS1230REF Controls and Connectors .................................................................................. 32 4-Wire Load Cell to Terminal Block ....................................................................................... 43 6-Wire Load Cell to Terminal Block ....................................................................................... 54 4-Wire Load Cell to Header ................................................................................................ 55 6-Wire Load Cell to Header ................................................................................................ 56 ADS1230REF PC Software Display ..................................................................................... 157 ADS1230REF Average Data ............................................................................................. 168 ADS1230REF Hardware Block Diagram ................................................................................ 20A-1 ADS1230REF PCB—Top Side ........................................................................................... 26A-2 ADS1230REF PCB—Bottom Side ....................................................................................... 26A-3 ADS1230REF Schematic—ADC ......................................................................................... 27A-4 ADS1230REF Schematic—MCU ......................................................................................... 28A-5 ADS1230REF Schematic—USB ......................................................................................... 29

List of Tables

1 Unit Conversion Factors and Display Formats ........................................................................... 82 Parameters in Configuration Mode ........................................................................................ 93 Modes and Example Displays ............................................................................................ 114 Voltage Display Ranges ................................................................................................... 125 Parameters in Analysis Mode............................................................................................. 136 Console Mode Commands ................................................................................................ 197 Load Cell Header Pinout .................................................................................................. 218 Terminal Block Pinout ..................................................................................................... 22A-1 ADS1230REF Bill of Materials ............................................................................................ 24

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1 Getting Started

1.1 Operating Modes

Getting Started

A diagram of the ADS1230REF is shown in Figure 1 .

Figure 1. ADS1230REF Controls and Connectors

The ADS1230REF operates in one of three modes:•Scale mode: When the mode select switch is in Scale position, the ADS1230REF acts as a basicweigh scale. The scale has tare, range, and calibrate functions, and can display metric (SI) units. Otherparameters 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 theADS1230 device and various measurements are made upon them. Several measurements areavailable, including raw display, voltage, RMS noise, and peak-to-peak measurements. The ADS1230can also be configured directly from this mode. Analysis mode is described in detail in Section 3 .•Configuration mode: Parameters governing the operation of the ADS1230REF can be viewed andaltered 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 isdescribed in Section 3.4 .

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1.2 Controls

1.2.1 Auxiliary Controls

1.3 Power

1.4 Connecting a Load Cell

1.4.1 Connecting a 4-Wire Load Cell to the Terminal Block

EXC-

SNS-

SNS+

SIG+

SIG-

EXC+

OUT+

EXC-

OUT-

EXC+

Getting Started

The main controls for the ADS1230REF are the four buttons and the mode selection slide-switch (seeFigure 1 ).

The slide-switch selects between weigh-scale (Scale) and Analysis modes. The ADS1230REF switchesmodes only when it is displaying data. If the switch is changed in a configuration mode, nothing happensuntil the configuration mode is exited. At that time the ADS1230REF reads the switch and enters theselected mode.

The four buttons have different functions, depending on the operating mode. In Scale mode, the switcheshave the functions in the box labeled SCALE. In Analysis mode, the switches have the functions shown inthe box labeled ANALYSIS. In configuration mode, the switches have the functions shown in the boxlabeled CONFIG.

The buttons also have different names in different modes. In this document, they are identifed by therespective names they have in the mode under discussion.

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 maysolve the problem.

The Programming Mode switch is used to update the firmware. For normal operation, it should be set toJTAG.

To apply power to the ADS1230REF, 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 alwaystakes power from it, and not from the battery.

The ADS1230REF is protected against polarity reversal. If a power source is connected in reverse bymistake, the display remains blank. To prevent damage to the board, do not leave a reversed powersource connected for longer than a few seconds.

The ADS1230REF is specifically designed for connection to load cells. Two connectors are provided forthis application. The terminal block is used for load cells having stripped wire connections; the load cellheader is for load cells having a header connector. The terminal block provides connections to thereference input (or power supply) and the header has switched excitation.

Figure 2 shows the connection of a 4-wire load cell to the terminal block. In this configuration, the load cellis 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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1.4.2 Connecting a 6-Wire Load Cell to the Terminal Block

EXC-

SNS-

SNS+

SIG+

SIG-

EXC+

OUT+

EXC-

SENSE-

OUT-

SENSE+

EXC+

1.4.3 Connecting a 4-Wire Load Cell to the Header

OUT+

EXC-

OUT-

EXC+

1.4.4 Connecting a 6-Wire Load Cell to the Header

OUT-

EXC+

OUT+

EXC-

SENSE+

SENSE-

Getting Started

Figure 3 shows the connection of a 6-wire load cell to the terminal block. In this configuration, the load cellis excited by the +5V power supply, and the ADC’s reference is taken from the sense wire returning fromthe load cell. The sense wire connects to the excitation wire at the bridge sensor.

Figure 3. 6-Wire Load Cell to Terminal Block

Figure 4 shows the connection of a 4-wire load cell to the header. In this configuration, the load cell isexcited 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 notwork.

Figure 5 shows the connection of a 6-wire load cell to the header. In this configuration, the load cell isexcited by the +5V power supply, and the ADC reference is taken from the sense wire returning from theload 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

1.6 Connecting an External Clock

1.7 Common Tasks

1.7.1 Shorted-Input Noise Test

1.7.2 Measuring Mass

Getting Started

In general, the ADS1230REF can accurately measure any voltage in the input range of the ADS1230ADC, as long as the following rules are observed:•Never apply a negative voltage to the inputs of the ADS1230REF. The ADS1230 cannot acceptnegative voltages at its input. Applying negative voltages may damage both the device and theADS1230REF. (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 avoltage divider by shorting J7; see Section 6.4.5 for details.•The input range of the amplifier on the ADS1230 does not extend to the supplies. See the ADS1230data sheet for details.

Note that Scale mode is designed only for use with load cells. Although it can be tested with a voltagesource or resistive divider, Scale mode does not, in general, display meaningful data unless a load cell isconnected and calibration is performed.

To connect an external clock, connect a clock oscillator to the EXTCLK test point. No settings need to bechanged; the ADS1230 will automatically use the attached clock.

The clock source must conform to 3.3V TTL or CMOS logic rules.

The noise measurements given in the product data sheet are taken with the inputs shorted to 2.5V. Thesenoise measurements can be replicated on the ADS1230REF with no external hardware. To set up thesemeasurements on the ADS1230REF, 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 markedInput Shorting Jumpers in Figure 1 .)3. Set up the ADS1230 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. Oncethe appropriate number of points is collected, the calculated noise voltage is displayed. This value isthe shorted-input RMS noise voltage, input-referred.

The first RMS noise measurement may be incorrect as a result of device settling. The secondmeasurement is generally correct.

For a detailed description of Analysis mode, see Section 3 .

The following items are required to measure mass with the ADS1230REF:•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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2 Weigh Scale Mode

Weigh Scale Mode

Obtaining a calibration weight:

Before the ADS1230REF can display the mass of an object, it must measure the output of the load cell fora previously known mass. The known mass can be adjusted.

At power-up, the ADS1230REF expects a 5kg mass. If this mass is not available, or if the load cell rangeis 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 toweigh the calibration object on an accurately calibrated scale. If the weight is precalibrated, its givenmass can be used, although this approach is not generally as accurate. (Note that the accuracy of theADS1230REF as a scale directly depends on the accuracy to which the calibration weight can bemeasured.)

2. Switch the mode switch to SCALE mode.3. Press the ENT and SEL buttons simultaneously. The ADS1230REF enters configuration mode.4. Use the PARM buttons to select the screen which shows UNIT =. This screen allows you to select theunits used for the calibration mass. If the units shown are not correct, adjust them using the VALUEbuttons.

5. Use the PARM buttons to select the screen that shows CW= followed by the calibration mass. On thisscreen, you can adjust each digit of the calibration mass separately. Select the digit using the PARMbuttons. 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 VALUE 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 asdescribed above.5. Move the mode switch to SCALE position. If calibration has not been performed, the display readsNO 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 ADS1230REF willmeasure the mass of an object placed on the load cell (provided that the mass of the object is within thedevice range).

Weigh scale mode is described in detail in Section 2 .

In weigh scale mode, the ADS1230REF displays mass.

(1)

Mass is displayed in either SI or avoirdupoisunits based on the voltage received from a load cell.

The ADS1230REF operates in Scale mode when the mode switch is set to the Scale position.

To accurately calculate mass, the ADS1230REF must have calibration information for the load cell. Whenscale mode is first entered, the ADS1230REF displays NO CAL, because the ADS1230REF has nocalibration data when it is powered on.

By default, mass is calculated from the average of four successive readings from the ADC. The number ofpoints 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 beaccurately inferred from this voltage as long as a calibration is accurately performed, the downward force of gravity remains constant (asit 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 conditionschange, a new calibration must be performed.

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2.1 Display

2.1.1 Calculation of Mass

m =

wfs

c-cfs zs

(1)

2.2 Button Functions

2.2.1 TARE

Weigh Scale Mode

Weigh scale mode can display mass in these uints: 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, gramsare converted to the desired display unit using the conversion factors given in Table 1 .

xxx(2)

The avoirdupois units used by the ADS1230REF are equal in the imperial and U. S. customary systems.

Table 1. Unit Conversion Factors and Display Formats

Unit Multiplier Format

micrograms ( μg) 10

nnn.nnnugmilligrams (mg) 1000 nnn.nnnmggrams (g) 1 nnn.nnngkilograms (kg) 10

–3

nnn.nnnkgounces (oz) 0.035274 nnn.nnnozpounds (lb) 0.0022046 nnn.nnnlbstone (st) 157.473 ×10

–6

nnstnn.nn

The display format for stone differs from the format used for the other units. One stone is equal to fourteenpounds; weight in stone is commonly expressed as a number of stone followed by a number of pounds.On the ADS1230REF, two digits are shown for stone, followed by st, followed by pounds displayed to twodecimal places.

Mass is calculated from ADC code using the formula:w = mc + w

– w

twhere:

•w= mass•c= the ADC code•w

= t are weight•m, w

, wt = values determined in the calibration process

mis a calibration constant, and is calculated using Equation 1 :

where w

is the user-specified calibration mass, c

is the ADC code taken with the calibration massapplied, and c

is the ADC measurement taken with no load.

, the zero-scale mass, is calculated from m and c

using Equation 2 :w

= –mc

(2)

The tare function allows the mass of a substance to be measured separately from the mass of itscontainer. When the TARE button is pressed, the scale measures the mass and records it. This reading issubtracted 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

2.2.3 CAL

2.3 Calibration

2.4 Configuration

Weigh Scale Mode

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.

Pressing CAL initiates the two-point calibration sequence. See Section 2.3 for details.

Two-point calibration is performed by pressing the CAL button. When this button is pressed, the boardexecutes 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 CALWEIGHT.

5. The user places a weight on the load cell and presses any button. The weight should have the massthat 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 besaved, with other board settings, to flash memory. See Section 3.4.1 for details.

The parameters for Scale mode can be adjusted in the configuration mode.

To enter configuration mode, press the VALUE buttons simultaneously. The four buttons then assume thefunctions shown in the CONFIG box. To exit Configuration mode, press the VALUE buttonssimultaneously again. This function does not cause parameters to be adjusted, because only buttonreleases are detected in Configuration mode.

Configuration mode contains a number of adjustable parameters. To scroll through the availableparameters, 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. Theseitems are labelled as words with a question mark. To enter these or to execute the command, press SELor ENT.

All parameters in the analysis and scale configuration menus are independent, including the parametersfound 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 unitsAverages AVGS= 2–128 Number of points toaverageADC speed SPD= FAST, SLOW Acquisition rateCalibration mass CW= 0–99.9 in various units Calibration mass and unitOffset calibration OCAL? — Offset calibration; see textSave parameters SAVE? — Save parameters; see textVersion number V1.1.0 — Firmware version numberdisplay

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2.4.1 Parameters

Weigh Scale Mode

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 ADS1230, which are calledhigh-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 ADS1230 depends on the frequency of the master clock, f

CLK

. In fast mode, thedata rate is f

CLK

/61440; in slow mode, the data rate is f

CLK

/491520. See the ADS1230 data sheet forfurther information.

The default setting is low-speed mode.

Calibration weight: This parameter gives the expected mass of the calibration weight used in thecalibration procedure (Section 2.3 ). The calibration mass can be given in any of the available units, inthree 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 adjustedwith the VALUE buttons. The PARM buttons are used to select the digit. The unit is adjusted in the samemanner, and flashes when selected.

When the unit is changed, the value changes to the equivalent mass in the new unit.

Offset calibration: This screen has the same function as it does in Analysis mode. See Calibrationsection 2.3 for information.

Save parameters: This screen allows the settings of the ADS1230REF to be stored in flash memory. Itfunctions the same as it does in Analysis mode. When ENT or SEL is pressed on this screen, theADS1230REF saves its settings to flash memory. These settings are loaded from flash memory when theboard 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 ADS1230REF firmware.(1)

The ADS1230REF cannot detect the frequency of the ADS1230 master clock, so it cannot display the actual data rate of the ADS1230device.

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