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Auto calibration Algoritm

Amir Shaddel
Prodigy 25 points

Other Parts Discussed in Thread: PGA309, PGA309EVM-USB, MSP430F2274

Hi

1-I want use pga309 in my design with strain gauge , please explain for auto calibration process .

2- how much maximum current for Vexc for non linear bridge sensor?

Thank you very much.

  • 0
    TI__Mastermind 27715 points

    Hello Amir,

    Thank you for your interest in the PGA309.

    1. The attached .zip file contains a document which details the measurements and calculations required for a full calibration. If this is your first time working with the device, I highly recommend using the PGA309EVM-USB to more easily familiarize yourself. It can be ordered at: http://www.ti.com/tool/pga309evm-usb. We also offer a system which can calibrate up to 64 PGA309 modules at a time, potentially eliminating the need for you to develop any hardware or software to do this in production. Let me know if this would interest you. 
    2. The Vexc pin should source no more than 5mA in order to maintain an accurate voltage. However, for low-impedance sensors it is possible to connect a pull-up resistor to Vcc so that additional current can be sourced from the main power supply. The attachment contains a guide on how to perform this as well as a TINA circuit for simulated testing.

    Best regards,

    Ian Williams
    Linear Applications Engineer
    Precision Analog and Sensing Products 

    PGA309.zip
  • 0 in reply to Ian Williams
    Mastermind 7675 points

    Hi Ian It is possible to use an catalytic sensor with the pga309? I'm asking because is a bridge sensor and I'm wondering if this could work.

    Regards

    Gastón

  • 0 in reply to Gaston_Melo_Arg
    TI__Mastermind 27715 points

    Hello Gastón,

    If it is a bridge sensor, it should work with the PGA309. If you provide a link to a data sheet for the sensor I can take a more thorough look.

    Best regards,

    Ian Williams


  • 0 in reply to Ian Williams
    Mastermind 7675 points

    Ian,

             here is the link to the datasheet of the sensor : http://www.citytech.com/PDF-Datasheets/4p75ct4.pdf

    Only needs a few external resistor to complete the bridge. 

    There's an application note (SBOA10) that explain that connecting the TEST pin to Vcc, the I2C will be released (In my case I won't need the EEPROM Memory). I will be using the pga309 with the msp430f2274. I have a question regarding the Test Pin in the pga309:

    In the application note pag3 says that :

    "it may be optimal for the microcontroller to control the Test pin in order to keep VOUT at high impedance until the microcontroller has configured the PGA309 for the desired settings"

    But in pag1 says that if the Test pin is pulled high: a microcontroller can modify the register and that Vout will be enable.

    Is this a contradiction? What I understand of this is that I need to pull high the Test Pin to configure the registers but in that case Vout will be enable too.

    How can I modify the registers of the pga309 while keeping Vout at high impedance?

    This configuration only need to be done once, at the beginning of the program or requires multiple configs?

    Regards

    Gastón

  • 0 in reply to Gaston_Melo_Arg
    TI__Mastermind 27715 points

    Hi Gastón,

    The only issue I can see is the high current consumption of the sensor (75 ±7 mA). This exceeds the PGA309's maximum VEXC current of 50mA. Therefore you would need a separate power supply for the sensor. 

    If I understand correctly, you desire the following operation from your system (PGA309 + MSP430):

    1. Power on system, PGA309 VOUT at high impedance
    2. Use MSP430 to configure PGA309 registers
    3. Enable PGA309 VOUT

    It is entirely possible to achieve this. Simply keep the TEST pin low ('0') at power-up so that the PGA309 begins its usual power-up sequence. VOUT will be disabled and the PGA309 will attempt to read its register settings from an external EEPROM. Since the EEPROM will not be present, VOUT will not enable. You can then write the initial register settings to the PGA309 using your MSP430 (with the TEST pin still low), since the power-on state machine cycle will reset once you address the PGA309 over the communication bus. Once the settings have been written, you can bring the TEST pin high ('1'), VOUT will enable, and at that point you have full control over the registers.

    The above information comes from section 3.1, "Power-On Sequence and Normal Stand-Alone Operation," of the PGA309 User's Guide.

    Best regards,

    Ian Williams 

  • 0 in reply to Ian Williams
    TI__Mastermind 27715 points

    Hi Gastón,

    Please disregard my previous comment about the current requirements of the catalytic sensor. As I stated in an earlier post on this thread, you can use a pull-up resistor to VCC to provide the additional current. The attachment contains more information and a simulation to support this.

    Best regards,

    Ian Williams

  • 0 in reply to Ian Williams
    Mastermind 7675 points

    thanks Ian

    Regards

    Gastón