System Board 5985

MAXREFDES75#: 24-Bit Weigh Scale with a 16-Bit DAC, 0 to 10V Output



MAXREFDES75# System Board Enlarge+

Introduction

Weigh scales perform the critical function of mass measurement for a wide variety of industrial applications. From ultra-small reactant amounts to large cargo containers, weighing provides key information in an array of processes from chemical reactions to commodity management. The critical component of any modern weigh scale is the analog-to-digital converter. The MAXREFDES75# brings a full 24-bit ADC into a high-performance system performing small-signal (down to ±25mV full scale) 24-bit measurements, while generating the high-voltage sensor supply, and producing a 0 to 10V output proportional to the input signal.

The MAXREFDES75# reference design features:

  • Ratiometric measurement or measurement compared to a precision voltage reference
  • User-programmable offset and gain calibration for sensor (load cell)
  • User-programmable offset and gain calibration for DAC
  • User-programmable voltage output range mapping

MAX11270

  • 24-bit resolution, 64ksps
  • 6nV/√Hz Integrated programmable-gain amplifier (PGA) with gains up to 128

MAX542

  • 16-bit resolution with no missing codes
  • High accuracy: INL ±2 LSB (max)

Introduction

Weigh scales perform the critical function of mass measurement for a wide variety of industrial applications. From ultra-small reactant amounts to large cargo containers, weighing provides key information in an array of processes from chemical reactions to commodity management. The critical component of any modern weigh scale is the analog-to-digital converter. The MAXREFDES75# brings a full 24-bit ADC into a high-performance system performing small-signal (down to ±25mV full scale) 24-bit measurements, while generating the high-voltage sensor supply, and producing a 0 to 10V output proportional to the input signal.

The MAXREFDES75# reference design features:

  • Ratiometric measurement or measurement compared to a precision voltage reference
  • User-programmable offset and gain calibration for sensor (load cell)
  • User-programmable offset and gain calibration for DAC
  • User-programmable voltage output range mapping

MAXREFDES75# System Board Enlarge+

MAX11270

  • 24-bit resolution, 64ksps
  • 6nV/√Hz Integrated programmable-gain amplifier (PGA) with gains up to 128

MAX542

  • 16-bit resolution with no missing codes
  • High accuracy: INL ±2 LSB (max)

Detailed Description

Figure 1 shows the block diagram of the MAXREFDES75#.


Figure 1. The MAXREFDES75# reference design block diagram.

The signal chain begins with the sensor signal feeding directly into the MAX11270. No external op amp is necessary because the PGA is integrated in the MAX11270. Following isolation via the MAX14850, the MAXQ622 reads the input signal, applies zero-offset and weight-factor, and presents the resulting weight. The MAXQ622 calculates a DAC value, based on DAC calibration and output range mapping and writes this to the DAC. A variety of sensor power options are available, outlined in Table 1.

Table 1. Sensor Supply Configuration

ADC Input Configuration Connectors Shunt Positions
10V generated on MAXREFDES75# Output: S+ Bridge
Output: S- Bridge
Ratiometric setup
Ratiometric setup
JP5: horizontal
JP6: horizontal
JP2: 2-3
JP3: 2-3
3.3V generated on MAXREFDES75# Output: S+ Bridge
Output: S- Bridge
Ratiometric setup
Ratiometric setup
JP5: vertical
JP6: vertical
JP2: 2-3
JP3: 2-3
Sensor supplied externally ratiometric measurement Input: S+ Bridge
Input: S- Bridge
Ratiometric setup
Ratiometric setup
JP5: open
JP6: open
JP2: 2-3
JP3: 2-3
Sensor supplied externally use voltage reference Input: S+ Bridge
Input: S- Bridge
Ratiometric setup
Ratiometric setup
JP5: open
JP6: open
JP2: 1-2
JP3: 1-2

Power Supply

The MAXREFDES75# is equipped with multiple sensor-supply options. Either 10V or 3.3V may be used to excite the load cell. Excitation at 10V will provide the highest performance, however, it will require additional components, and hence increase cost, in any final design. The 3.3V rail is required for ADC operation so it will be available in all configurations. Furthermore, the ADC may be configured to operate in a ratiometric mode, with the reference-derived resistor-divider from the 10V rail, or from the MAX6126 voltage reference. The ratiometric method is generally preferred, however, the voltage reference provides the user flexibility to evaluate the system without the changes in gain contributed by the system.

The MAXREFDES75# board receives power from a single DC source of 9V to 40V, 200mA through the X2 terminal block. The MAX17501 generates an +8V rail from the variable input supply. The MAX16910 LDO generates 3.3V for the MAXQ622 from the 8V rail. The MAX13256 generates isolated positive and isolated negative rails for the ADC, DAC, voltage reference, load cell, and analog output. The MAX16910 LDO generates VDD for the MAX542. The MAX8510 LDOs generate AVDD and DVDD for the MAX11270. The MAX9633 generates a low-noise 10V supply for the load cell.

Quick Start

Required equipment:

  • MAXREFDES75#
  • PC with Windows 7 or later operating system
  • Micro USB cable
  • +12V DC power-supply (9V to 40V acceptable voltage range)
  • Precision voltmeter
  • Load-cell setup to measure weight
  • Tiny screwdriver (2mm) for slotted screws

Procedure

The MAXREFDES75# board is fully assembled and tested. Follow the steps below to verify board operation:

  1. Connect the load cell to MAXREFDES75#.
  2. Positive supply for load cell S+, negative supply for load cell S-.
    Load cell output connects to IN+ and IN-.

  3. Apply power to the X2 connector on the left side of the board. (Note: Voltage should be between 9V and 40V). The input is reverse-polarity protected. Lower pin is (+), upper pin is (-).
  4. Upon power-up, the board will take 10 measurements and display the ADC LSB value during this process. These 10 measurements will be averaged and the average will be used as the zero-position.
  5. Note: Remove all weights from the load cell prior to power-up, avoid vibrations while the first 10 measurements are taken.

  6. The board has not been calibrated for the load cell, but the display should show ~0 grams after power-up and the numbers should change if a weight is placed at the load-cell platform. A Maxim Integrated logo should move from left to right after the first 10 measurements are taken.
  7. To calibrate the board, download the MAXREFDES75# calibration GUI
  8. Install the MAXREFDES75# calibration GUI and start it.
  9. Connect the micro USB cable between the PC and the MAXREFDES75#.
  10. The MAXREFDES75# calibration GUI displays weight history and the results on the left. On the right are two tabs for “Automatic Calibration” and “Manual Calibration.”
  11. The recommended process is to use the “Automatic Calibration” tab, which leads the user through the entire process. The calibration factors established by the automatic calibration are populated in the appropriate text boxes in the “Manual Calibration” tab and can be reviewed after calibration.

  12. Click the “Calibrate Weight” button, a message box opens and reminds the user to remove all weights from the platform. Vibration and any air movement should be avoided during the measurement as much as possible. Click the“OK” button.
  13. The MAXREFDES75# will take 10 measurements while there is no weight on the platform and shows the progress.
  14. After this, a pop-up window opens and asks the user to place a known weight on the platform and the weight in grams should be entered in the text box. After entering the weight, click the “OK” button.
  15. The MAXREFDES75# will take 10 measurements while the known weight is placed on the platform and shows the progress.
  16. After the second measurement, the MAXREFDES75# calibration GUI will calculate the offset and weight factor and send the results to the MAXREFDES75# board. The results are also populated in the appropriate text boxes in the “Manual Calibration” tab.
  17. DAC output also needs to be calibrated for the offset and gain errors.
  18. Connect a precision multimeter between AOUT and AGND on the right side (if oriented such that the display can be read) of the board. Click the “Calibrate DAC” button.

  19. The MAXREFDES75# will generate DC 0.625V. Measure the exact result with the multimeter and enter the measured result into the text box and click the “OK” button.
  20. The MAXREFDES75# will generate DC 9.375V. Measure the exact result with the multimeter and enter the measured result into the text box and click the “OK” button.
  21. After the second measurement, the MAXREFDES75# calibration GUI will calculate offset and gain for the analog output and will send the results to the MAXREFDES75# board. The results are also populated in the appropriate text boxes in the “Manual Calibration” tab.
  22. The output mapping between weight and output voltage can be calibrated as well.
  23. Click “Calibrate Output”.
    A pop-up window asks the user to enter a weight in grams that should produce 0V output.
    Note: Weights below that will also produce 0V.
    And enter the weight that should produce 10V output.
    Note: Weights above that will also produce 10V.
    Click the “OK” button.

  24. Verify that the weight scale now measures weight accurately and that the analog output changes appropriately.
  25. To store the calibration values to flash, click the “Save All Calibrations to Flash” button. This saves the calibration data into the MAXQ622 mounted on the MAXREFDES75#. The PC is not required for stand-alone system use after this step.
  26. Disconnect the USB.
  27. Power cycle the MAXREFDES75# and verify that the weight is measured correctly and that the analog output changes are calibrated.

Stand-alone operation (after the first calibration was done)

  • The weight on display shows weight in grams.
  • Analog output voltage is produced based on the calibration done with the MAXREFDES75# calibration GUI.
  • To reset the weight back to null, push the “S1” button (far right button if board is oriented such that the display can be read), this sets the weight back to zero.
  • To reset the system, push the “S3” button (far left button if board is oriented such that the display can be read).

Windows is a registered trademark and registered service mark of Microsoft Corporation.

Quick Start

Required equipment:

  • MAXREFDES75#
  • PC with Windows 7 or later operating system
  • Micro USB cable
  • +12V DC power-supply (9V to 40V acceptable voltage range)
  • Precision voltmeter
  • Load-cell setup to measure weight
  • Tiny screwdriver (2mm) for slotted screws
Procedure
Stand-alone operation (after the first calibration was done)
 
Status:
Package:
Temperature:

MAX11270
24-Bit, 10mW, 130dB SNR, 64ksps Delta-Sigma ADC with Integrated PGA

  • High Resolution for Instrumentation Applications that Require a Wide Dynamic Range
  • Longer Battery Life for Portable Applications
  • High Accuracy for DC Measurements

MAX17501
60V, 500mA, Ultra-Small, High-Efficiency, Synchronous Step-Down DC-DC Converter

  • Eliminates External Components and Reduce Total Cost
  • Reduces Number of DC-DC Regulators to Stock
  • Reduces Power Dissipation

MAX14850
Six-Channel Digital Isolator

  • Protection from High-Voltage Environments
  • Complete Digital Isolation Solution
  • Compatible with Many Interface Standards

MAX13256
36V H-Bridge Transformer Driver for Isolated Supplies

  • Simple, Flexible Design
  • Integrated System Protection
  • Saves Space on Board

MAX9633
Dual 36V Op Amp for 18-Bit SAR ADC Front-End

  • High-Resolution ADC Driver
  • Support a Wide Range of Industrial Applications
  • Improved Reliability

MAXQ622
16-Bit Microcontrollers with Infrared Module and Optional USB

  • High-Performance, Low-Power, 16-Bit RISC Core
  • DC to 12MHz Operation Across Entire Operating Range
  • 1.70V to 3.6V Operating Voltage

MAX16910
200mA, Automotive, Ultra-Low Quiescent Current, Linear Regulator

  • Enables System Designers to Meet Stringent Module Requirements for 100μA Quiescent Current
  • Tiny Output Capacitors Reduce Board Space and BOM Cost
  • Accurate Active-Low RESET Output with Adjustable Delay Eliminates Need for Separate Reset IC

MAX8719
High-Voltage, Low-Power Linear Regulators for Notebook Computers

  • 4V to 28V Input Range
  • 18µA Quiescent Supply Current
  • <3µA Shutdown Supply Current

  • MAX8510
    Ultra-Low-Noise, High PSRR, Low-Dropout, 120mA Linear Regulators

    • Space-Saving SC70 and TDFN (2mm x 2mm) Packages
    • 11µVRMS Output Noise at 100Hz to 100kHz Bandwidth (MAX8510)
    • 78dB PSRR at 1kHz (MAX8510)

    MAX6126
    Ultra-High-Precision, Ultra-Low-Noise, Series Voltage Reference

    • Ultra-Low 1.3µVP-P Noise (0.1Hz to 10Hz, 2.048V Output)
    • Ultra-Low 3ppm/°C (max) Temperature Coefficient
    • ±0.02% (max) Initial Accuracy

    MAX542
    +5V, Serial-Input, Voltage-Output 16-Bit DACs

    • Full 16-Bit Performance Without Adjustments
    • +5V Single-Supply Operation
    • Low Power: 1.5mW
    Title Edition Updated