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.
www.ti.com Contents 1 Getting Started .............................................................................................................. 3 2 Weigh Scale Mode .......................................................................................................... 7 3 Analysis Mode ............................................................................................................. 11 4 Using the PC Software ...............................................................................
Getting Started www.ti.com 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.
Getting Started 1.2 www.ti.com 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.
Getting Started www.ti.com 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. OUTSENSE+ EXC+ EXCSNSSNS+ SIG+ SIGEXC+ OUT+ EXCSENSE- Figure 3. 6-Wire Load Cell to Terminal Block 1.4.
Getting Started 1.5 www.ti.com 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.
Weigh Scale Mode www.ti.com 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.
Weigh Scale Mode 2.1 www.ti.com 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.
Weigh Scale Mode www.ti.com 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.
Weigh Scale Mode 2.4.1 www.ti.com 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.
Analysis Mode www.ti.com 3 Analysis Mode In Analysis mode, the ADS1x31REF analyzes code output from the installed ADS device and displays it in different ways. Table 3 summarizes the numerous display modes available, together with example displays. NOTE: The values shown in Table 3 are consistent for a 24-bit device, such as the ADS1231. However, the ADS1131 is an 18-bit device, so the actual number of codes will be fewer than the examples shown here. Table 3.
Analysis Mode 3.2 www.ti.com Measurement Modes Raw: In this measurement, codes are read from the installed ADS device and displayed. No processing or analysis is done on the sample stream. Data can be displayed as hexadecimal codes, decimal codes, or volts.
Analysis Mode www.ti.com The number of codes used in the calculation is selectable in Configuration mode. Averaged: In this mode, a number of codes are collected, and the average is calculated. The result can be displayed in decimal codes or volts. (Hexadecimal is not available because the result may be fractional.) Volts are calculated as in raw mode. The number of codes used in the calculation is selectable in Configuration mode. 3.
Using the PC Software www.ti.com Power-down mode: When ENT or SEL is pressed on this screen, the ADS1x31REF causes the installed ADS device to enter power-down mode. This action occurs by pulling the PDWN line low. While the PDWN line is low, the display reads POWER DN. When a button is pressed from this display, the ADS1x31REF powers the converter on and returns to the analysis display, exiting configuration mode.
Using the PC Software www.ti.com Status display: This display shows messages indicating the current state of the program. Acquire button: This button starts and stops the running acquisition of data. 4.3 Starting the Program When the program launches, a screen similar to that shown in Figure 6 appears. Immediately after launch, the program searches all available serial ports for the board.
Using the PC Software 4.4 www.ti.com Analysis or Record Mode The program will operate in analysis mode or record mode. Both modes acquire the data and update the displays, but record mode also writes the result to the selected file. 4.5 Adjusting Device Parameters To adjust device parameters, use the controls in the Device control box. Each control corresponds to a setting that can also be made from the board itself. 4.
Using the PC Software www.ti.com 4.6.2 Block Analysis Length The histogram, RMS, and peak-to-peak calculations require a set of samples. These calculations are updated with every group of samples received, and are performed on a block of the most recently acquired (or averaged) samples. The number of samples used is set using the Analysis points control. By default, this number is 100, but can be changed at any time. This value also controls the RMS and peak-to-peak noise analysis lengths.
Serial Console www.ti.com Recording time is not measured, but calculated from the number of samples collected. The speed is used to determine the amount of time for each sample. If the data rate is changed at the board, the recording time will be incorrect. This change does not affect the data file, except that samples continue to be collected with the different settings.
Serial Console www.ti.com The available commands are listed in Table 6. In this table, values in brackets indicate a range or list of possible characters. A dash (–) indicates a range, and commas (,) indicate a list. Table 6. Console Mode Commands COMMAND FORMAT OPERATION P — Prints the current gain setting R R [F,S] Set data rate V V Show firmware version S S Start streaming D D Read data once Q Q Query parameters Console mode does not interrupt standalone operation.
Hardware 5.2.4 www.ti.com S—Start Streaming When S is issued, the ADS1x31REF begins printing raw output codes from the installed ADS device in hexadecimal format, separated by new lines. The device iterates continuously until a character is received from the serial port. S is used primarily by the EVM software for data collection. 5.2.5 D—Collect One Sample Issuing the D command causes the ADS1x31REF to report the latest collected sample from the installed ADS device.
Hardware www.ti.com 6.1 Microcontroller The ADS1x31REF uses the MSP430F449 microcontroller. This device provides an LCD controller, hardware SPI and UART interfaces, and a multiplier. The latter is important because the firmware must perform many multiplications. 6.2 Power Supply The ADS1x31REF operates from +5VDC and +3.3VDC. These supplies are generated by linear regulators U1 and U2. Input power comes from either wall-adapter connector J2, or battery connector BT1.
Hardware 6.4.3 www.ti.com Input Terminal Block The input terminal block consists of J4 and J5. It provides connections to all of the analog inputs on the installed ADS device, and connections to the ADS1x31REF voltage reference network. The terminal block pinout is given in Table 8. Table 8. Terminal Block Pinout TERMINAL NAME 6.4.
Appendix A www.ti.com To allow firmware to be downloaded through USB, U4 is connected to the microcontroller bootstrap loader pins. The DTR and RTS pins are connected to the RST and TCK inputs on the microcontroller to allow the serial bootstrap loader to operate. These lines are not isolated; instead, they are switched through SW9, which also connects USB ground and power to the microcontroller ground and power.
Bill of Materials www.ti.com Table 9. ADS1x31REF Bill of Materials (continued) Item No ADS1131 ADS1231 23 1 1 Ref Des Description J5 3.
PCB Layout www.ti.com A.2 PCB Layout Figure 9. ADS1x31REF PCB—Top Side Figure 10.
PCB Layout www.ti.com Figure 11. ADS1x31REF PCB—Layer 2 Figure 12.
Schematics www.ti.com A.3 Schematics C30 TP13 100pF TP11 EXTCLK C27 2.2uF +5VA 3 2 13 10 9 7 8 5 6 12 11 C28 2.2uF C45 C31 C42 TP15 GND +5VA C38 TP3 +5V C39 R24 MUR_NFM21PC105F1C3D 0 D1 CUI-STACK PJ-102BH 5 VIN VOUT ENABLE GND NR/ADJUST C8 10uF 4 R26 20K +5VA 1 2 3 4 5 6 +5VA HEADER-6 +5V U1 1 3 2 R23 0 100pF 20K EXC+ EXCSNS+ SIG+ SIGEXCSNSEXC- BT1 J2 C7 0.
Schematics www.ti.com +3.3VD SW2 SW3 RANGE SW4 TARE SW5 CAL CONFIG C13 C14 C15 C5 0.01uF R19 0.01uF R20 0.01uF R21 0.01uF 221K 221K 221K R8 10K R9 10K +3.3VD SIMO0 R22 200K PDWN C23 2.2uF COM(3) COM(2) COM(1) COM(0) S(39) R7 10K DOUT SCLK SPEED TP18 TP8 MCURX MCUTX 10K R10 TP7 DNP 75 74 73 72 71 70 69 68 67 66 65 64 63 62 61 60 59 58 57 56 55 54 53 52 51 MCURX MCUTX J3 2 4 6 8 10 12 14 TDO/TDI TDI/CLK TMS TCK 1 3 5 7 9 11 13 C25 2.2uF RST +3.
Revision History www.ti.com Revision History Changes from Original (July, 2010) to A Revision .......................................................................................................... Page • • • • • • Deleted references to Offset Calibration in Table 2 .................................................................................. 9 Removed information on Offset Calibration from Section 2.4.1 ..................................................................
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