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Original question:

PGA309: Maximum excitation voltage, VEX

PGA309: Internal Register Configuration - Static or Dynamic

Torin Vanderpool
Prodigy 60 points

Part Number: PGA309

Hi there,

I've developed a program to generate the PGA309 lookup table based on data we collect from our bridge sensors (3 temp, 2 pressure at hi/low and 1 midpoint at room temp). These sensors have no electronics attached when we collect this data.

I'm trying to determine how to programmatically develop the internal register configuration, and whether these are dynamic or statically calculated based on the LUT. I had difficulty understanding this. I read through the register descriptions, do the internal registers hold the "calculated" value of the LUT variables (Gain/Zero) based on the current temperature ADC reading? If so, how would the Linearization DAC setting[7:0] be determined for the internal registers? 

Is this the Klin number that is calculated (this is all based on the retired spreadsheet PGA309DK Calibration Spreadsheet, Version 1.2.4, (C) 2004 Texas Instruments) what would be written to this register?

Thank you,

TV

  • 0
    TI__Guru 55466 points

    Hi Torin,

    The PGA309 provides calibration over temperature for both span (gain) and offset. The device corrects for the non-linear gain drift and non-linear offset drift errors vs temperature using the correction coefficients stored on a lookup table during the calibration routine.  The lookup table is stored on an external EEPROM (non-volatile memory).

    The KLIN factor and Linearity Register DAC setting is a separate coefficient from the gain/offset coefficient factors above, where the linearity DAC corrects for the Bridge sensor non-linearity over pressure change by adjusting the excitation voltage. Note that the device only stores a single Linearization DAC coefficient (more below).

    PGA309 offset and Gain Correction Over Temperature.

    The PGA309 performs offset and gain calibration correction and corrects for the non-linear gain and offset drift errors over temperature by performing linear interpolation of the different calibration points on a lookup table.   Essentially by storing the gain and offset correction coefficients during an initial calibration routine at different temperatures, the different gain and offset factors are stored on an external EEPROM (coefficients stored on non-volatile memory). The PGA309 uses the internal temperature sensor ADC to choose the appropriate coefficients on the lookup table depending on temperature. The plots below from the User Guide show how the device corrects for the non-linear temperature errors using linear interpolation. The device can store on the EEPROM a look-up table with up to 17 temperature index values/coefficients. Please review section 3.2 p.62-65 of the PGA309 User Guide for a detailed example calculation of the correction factors of the lookup table.

    PGA309 Linearization Function vs applied Pressure.

    The PGA309 contains a dedicated circuit for sensor voltage excitation and linearization, as shown in Figure 2-10 of the PGA309 User Guide. The Linearization Circuit scales the selected VREF and sums it together with a portion of the output voltage (VOUT) through the feedback pin (VFB) to compensate for the bow-shaped nonlinearity of the bridge sensor output versus pressure. By adjusting the excitation voltage, the bridge non-linearity vs pressure is corrected.

     

    Please review section 2.6 of the PGA309 User Guide for a detailed example on how to calculate the Linearity DAC register settings. The non-linearity of the bridge must be known. It is possible to measure or estimate the non-linearity of the sensor by measuring the bridge output at minimum pressure, maximum pressure and mid-scale pressure to estimate the max non-linearity of the bridge vs pressure. The PGA309 linearization circuitry can only compensate for the parabolic-shaped portions of a sensor’s nonlinearity with applied pressure. This nonlinearity is assumed to be constant over temperature variation. Only one linearization DAC register coefficient is stored on the EEPROM.

     

    Thank you and Regards,

    Luis