SBOU024C august   2004  – july 2023 PGA309

 

  1.   1
  2.   Read This First
    1.     About This Manual
    2.     Related Documentation from Texas Instruments
    3.     If You Need Assistance
    4.     Information About Cautions and Warnings
    5.     FCC Warning
    6.     Trademarks
  3. 1Introduction
    1. 1.1  PGA309 Functional Description
    2. 1.2  Sensor Error Adjustment Range
    3. 1.3  Gain Scaling
    4. 1.4  Offset Adjustment
    5. 1.5  Voltage Reference
    6. 1.6  Sensor Excitation and Linearization
    7. 1.7  ADC for Temperature Sensing
    8. 1.8  External EEPROM and Temperature Coefficients
    9. 1.9  Fault Monitor
    10. 1.10 Over-Scale and Under-Scale Limits
    11. 1.11 Power-Up and Normal Operation
    12. 1.12 Digital Interface
    13. 1.13 Pin Configuration
  4. 2Detailed Description
    1. 2.1  Gain Scaling
      1. 2.1.1 PGA309 Transfer Function
      2. 2.1.2 Solving For Gain Settings
    2. 2.2  Offset Scaling
    3. 2.3  Zero DAC and Gain DAC Architecture
    4. 2.4  Output Amplifier
    5. 2.5  Reference Voltage
    6. 2.6  Linearization Function
      1. 2.6.1 System Definitions
      2. 2.6.2 Key Linearization Design Equations
        1. 2.6.2.1 Lin DAC Counts Conversion
      3. 2.6.3 Key Ideal Design Equations
        1. 2.6.3.1 Linearization Design
        2.       37
    7. 2.7  Temperature Measurement
      1. 2.7.1 Temp ADC Start-Convert Control
      2. 2.7.2 External Temperature Sensing with an Excitation Series Resistor
    8. 2.8  Fault Monitor
    9. 2.9  Over-Scale and Under-Scale
      1. 2.9.1 Over-Scale and Under-Scale Calculation
      2.      44
    10. 2.10 Noise and Coarse Offset Adjust
    11. 2.11 General AC Considerations
  5. 3Operating Modes
    1. 3.1 Power-On Sequence and Normal Stand-Alone Operation
    2. 3.2 EEPROM Content and Temperature Lookup Table Calculation
      1. 3.2.1 Temperature Lookup Table Calculation
        1. 3.2.1.1 Temperature Lookup Table Calculation
        2.       52
        3.       53
    3. 3.3 Checksum Error Event
    4. 3.4 Test Pin
    5. 3.5 Power-On Initial Register States
      1. 3.5.1 PGA309 Power-Up State
  6. 4Digital Interface
    1. 4.1  Description
    2. 4.2  Two-Wire Interface
      1. 4.2.1 Device Addressing
      2. 4.2.2 Two-Wire Access to PGA309
    3. 4.3  One-Wire Interface
    4. 4.4  One-Wire Interface Timeout
    5. 4.5  One-Wire Interface Timing Considerations
    6. 4.6  Two-Wire Access to External EEPROM
    7. 4.7  One-Wire Interface Initiated Two-Wire EEPROM Transactions
    8. 4.8  PGA309 Stand-Alone Mode and Two-Wire Transactions
    9. 4.9  PGA309 Two-Wire Bus Master Operation and Bus Sharing Considerations
    10. 4.10 One-Wire Operation with PRG Connected to VOUT
    11. 4.11 Four-Wire Modules and One-Wire Interface (PRG)
  7. 5Application Background
    1. 5.1 Bridge Sensors
    2. 5.2 System Scaling Options for Bridge Sensors
      1. 5.2.1 Absolute Scale
      2. 5.2.2 Ratiometric Scale
    3. 5.3 Trimming Real World Bridge Sensors for Linearity
    4. 5.4 PGA309 Calibration Procedure
  8. 6Register Descriptions
    1. 6.1 Internal Register Overview
    2. 6.2 Internal Register Map
      1. 6.2.1 Register 0: Temp ADC Output Register (Read Only, Address Pointer = 00000)
      2. 6.2.2 Register 1: Fine Offset Adjust (Zero DAC) Register (Read/Write, Address Pointer = 00001)
      3. 6.2.3 Register 2: Fine Gain Adjust (Gain DAC) Register (Read/Write, Address Pointer = 00010)
      4. 6.2.4 Register 3: Reference Control and Linearization Register (Read/Write, Address Pointer = 00011)
      5. 6.2.5 Register 4: PGA Coarse Offset Adjust and Gain Select/Output Amplifier Gain Select Register (Read/Write, Address Pointer = 00100)
      6. 6.2.6 Register 5: PGA Configuration and Over/Under-Scale Limit Register (Read/Write, Address Pointer = 00101)
      7. 6.2.7 Register 6: Temp ADC Control Register (Read/Write, Address Pointer = 00110)
      8. 6.2.8 Register 7: Output Enable Counter Control Register (Read/Write, Address Pointer = 00111)
      9. 6.2.9 Register 8: Alarm Status Register (Read Only, Address Pointer = 01000)
  9.   A External EEPROM Example
    1.     A.1 PGA309 External EEPROM Example
      1.      A.1.1 Gain and Offset Scaling for External EEPROM
      2.      94
  10.   B Detailed Block Diagram
    1.     B.1 Detailed Block Diagram
  11.   C Glossary
  12.   Revision History

5.4 PGA309 Calibration Procedure

The PGA309 calibration can be done by applying zero, middle, and full-scale signals to the sensor module and measuring the output response of the PGA309. This calibration is done over temperature and different Gain DAC, and Zero DAC values will be determined for each temperature applied. A lookup table is derived using the Gain DAC and Zero DAC values over temperature. The actual calibration algorithm used can be modified to accommodate different requirements. Here is one possible calibration algorithm.

The PGA309 transfer function is shown below. This equation represents all the gain and offset control blocks in the PGA309. The equation will be rearranged to solve for various gain and offset settings throughout the calibration procedure.

Equation 17. V O U T = m u x _ s i g n ∙ V I N + V C O A R S E _ O F F S E T ∙ G I + V Z E R O _ D A C ∙ G D ∙ G O
Table 5-1 Summary of Terms in Equations
Term Definition
mux_sign This term changes the polarity of the input signal. See Table 6-9
VIN The input signal
VCOARSE_OFFSET Coarse offset DAC output voltage. See Table 6-10
GI Input stage gain. See Table 6-8
VZERO_DAC Zero DAC output voltage. See Table 6-4
GD Gain DAC. See Table 6-5
GO Output Stage Gain. See Table 6-7
Equation 18. V Z E R O _ D A C = V O U T - G D ∙ G I ∙ G O ∙ V C O A R S E _ O F F S E T + V I N ∙ m u x _ s i g n G D ∙ G O
Equation 19. V C O A R S E _ O F F S E T = V O U T - G D ∙ G O ∙ V Z E R O _ D A C + G I ∙ V I N ∙ m u x _ s i g n G D ∙ G I ∙ G O
Equation 20. T o t a l _ G a i n = G D ∙ G I ∙ G O
Equation 21. T o t a l _ G a i n = V O U T _ M A X - V O U T _ M I N V I N _ M A X - V I N _ M I N

Calibration Algorithm

  1. Apply minimum stimulus (for example, pressure). Adjust PGA309 gain to lowest possible level and set Zero DAC to drive the output to midscale (1/2Vs).
    Equation 22. G I = 4 ,     G D = 1 + 0.3333 / 2 = 0.667 ,     G O = 2 ,     V C O A R S E _ O F F S E T = 0 V
    Equation 23. V Z E R O _ D A C = ( 0.5 ∙ V S ) - G D ∙ G I ∙ G O ∙ V C O A R S E _ O F F S E T + V I N ∙ m u x _ s i g n G D ∙ G O

    Back calculate Vin, based on Vout measured.

    Equation 24. V I N = V O U T - G D ∙ G O ∙ ( V Z E R O _ D A C + G I ∙ V C O A R S E _ O F F S E T ) G D ∙ G I ∙ G O ∙ m u x _ s i g n
  2. Adjust the gain according to the if-then relationship and re-do step 1. This will provide a more accurate value for Vin.

    If (Vin > 0.131)

    GI=4;

    Else if ((Vin>0.035) && (Vin<=0.131))

    GI=8;

    Else if ((Vin>0.023) && (Vin<=0.035))

    GI=16;

    Else if ((Vin>0.015) && (Vin<=0.023))

    GI=32;

    Else

    GI=64;

  3. Apply maximum stimulus and do the same procedure as in step 1 and 2. This gives you Vin_max. Now the values Vin_min and Vin_max have been calculated from Vout_max and Vout_min. Use this information to calculate total gain.
    Equation 25. T o t a l _ G a i n = V O U T _ M A X - V O U T _ M I N V I N _ M A X - V I N _ M I N
  4. Search through all combinations of GI × GO × 0.667 to find the values that are closest to the total gain. This allows for maximum adjustment range of the Gain DAC.
  5. Solve for the value of GD to get the exact Total_Gain.
    Equation 26. G D = T o t a l _ G a i n G I ∙ G O
  6. Set the Zero DAC to ½ of its full-scale value (0.5×Vref). This allows for maximum adjustment range for coarse offset.
    Equation 27. V Z E R O _ D A C = 0.5 ∙ V R E F
  7. Find the coarse offset that sets the output to the target output at full-scale stimulus. For example, if the output is supposed to be 4.5 V with minimum pressure applied, then select the coarse offset to make the output as close as possible to the target of 4.5 V. Note that the coarse offset adjustment resolution steps are large, so the output will not hit the target exactly. The Zero DAC must be adjusted to improve the accuracy of the minimum output.
    Equation 28. V C O A R S E _ O F F S E T = V O U T _ M A X - G D ∙ G O ∙ V Z E R O _ D A C + G I ∙ V I N _ M A X ∙ m u x _ s i g n G D ∙ G I ∙ G O
  8. After adjusting the coarse offset, measure the output. Use the new output voltage to adjust the Zero DAC to get an accurate zero and full-scale output.
    Equation 29. V Z E R O _ D A C = V O U T _ M A X - G D ∙ G I ∙ G O ∙ V C O A R S E _ O F F S E T + V I N _ M A X ∙ m u x _ s i g n G D ∙ G O
  9. Now the gain and offset corrections give approximately Vout_max and Vout_min for maximum and minimum stimulus. At this point the stimulus is still at maximum, so the output is close to the full-scale target. However, this is not the best accuracy. To further improve the accuracy, do a linear correction to the Zero DAC and Gain DAC. Remeasure the output and calculate the input. Then calculator a new value for the Zero DAC.
    Equation 30. V I N _ M A X = V O U T _ M A X - G D ∙ G O ∙ ( V Z E R O _ D A C + G I ∙ V C O A R S E _ O F F S E T ) G D ∙ G I ∙ G O ∙ m u x _ s i g n
  10. Apply minimum stimulus, and measure the output. Use this to calculate Vin_min. Use the values of Vin_min, and Vin_max from steps 9 and 10 to calculate a new Gain DAC and Zero DAC.
    Equation 31. V I N _ M I N = V O U T _ M I N - G D ∙ G O ∙ ( V Z E R O _ D A C + G I ∙ V C O A R S E _ O F F S E T ) G D ∙ G I ∙ G O ∙ m u x _ s i g n
    Equation 32. T o t a l _ G a i n = V O U T _ M A X - V O U T _ M I N V I N _ M A X - V I N _ M I N
    Equation 33. G D = T o t a l _ G a i n G I ∙ G O
    Equation 34. V Z E R O _ D A C = V O U T _ M I N - G D ∙ G I ∙ G O ∙ V C O A R S E _ O F F S E T + V I N _ M I N ∙ m u x _ s i g n G D ∙ G O
  11. Apply minimum stimulus, and measure the output signal. Confirm that the adjustments in step 10 meet your accuracy requirement. Technically, the adjustment was complete in step 10 so this step is only to confirm that the device and programing are working as expected.
  12. Steps 7 to 10 of the procedure are repeated for all calibration temperatures. Thus, over all temperature the only variables that change are the Gain DAC and Zero DAC. All other gain and offset blocks remain constant. Use the Gain DAC and Offset DAC across temperature to generate the lookup table. The maximum length of the lookup table is 17 points. In many cases the temperature calibration is done at only three temperatures. For a thee temperature calibration, you can use a polynomial interpolation to estimate the Gain DAC and Zero DAC at other temperatures in the lookup table. The PGA309 applies linear gain and offset adjustment versus temperature between the points in the lookup table, so using interpolation to fill all 17 points in the lookup table generally improves accuracy.