SBOA506 December   2020 OPA2991-Q1 , TLV197-Q1 , TLV2197-Q1 , TLV4197-Q1 , TMP61 , TMP61-Q1 , TMP63 , TMP63-Q1 , TMP64-Q1

 

  1.   Design Goals
  2.   Design Description
  3.   Design Notes
  4.   Design Information
  5.   Design Steps
  6.   Design Simulations
  7.   DC Simulation Results
  8.   Sensor Circuit Accuracy Using a Linear Approximation
  9.   Improving Accuracy of the Sensor Circuit
  10.   Design References
  11.   Design Featured Op Amp
  12.   Design Alternate Op Amp

Improving Accuracy of the Sensor Circuit

  1. The temperature accuracy of the circuit can be improved while continuing to use a linear approximation to solve for the resistor network. In this example the temperature range will be optimized from 0°C to 90°C. This is accomplished by decreasing the offset term of the VOUT(T) equation.
    Equation 1. VoutT=VPTCT×1+R4×R3+R5R3×R5-Vbias×R4R5
  2. Recall, VOUT(T) is treated as a linear equation to solve for the resistor network:
    Equation 1. YX = X×M +B
  3. Therefore, the y-intercept, B, of the equation can be used to offset the curve along the Y-Axis. In this case the y-intercept of the equation is:
    Equation 1. B = Vbias×R4R5
  4. To shift the center of the VOUT(T) curve the negative inverting gain will be decreased by approximately 1.86%. The 1.86% decrease was selected by calculating the change needed to shift the midpoint of the simulated VOUT(T) curve towards the midpoint of the theoretical VOUT(T) curve. To calculate shift the VOUT(T) curve can be determined as follows:
    Equation 1. Vout_simmidpoint = 2.68
    Equation 1. Vout_newmidpoint = 2.63
    Equation 1. 1 - 2.632.68 = 0.0186
  5. The new VOUT(T) can be obtained by multiplying the ratio of R4 to R5 by 98.1% as follows:
    Equation 1. Vbias×R4R5 = Vbias×0.981×R4R5 
    Equation 1. 5V×10kΩ6.251kΩ = 5V×0.981×10kΩR5 
    Equation 1. R5 = 6.123kΩ
  6. The final resistor, R3, can be solved for using the new R5 value. Note, this approach will slightly affect the inverting gain of the circuit however, the non-inverting gain of the circuit will remain unchanged which will not impact the gain of the voltage divider output voltage as a function of temperature (VPTC(T)).
    Equation 1. 4.07 = 1+10kΩ×R3+6.123kΩR3×6.123kΩ ; R3 =6.959kΩ
  7. Using the linear approximation the resistor network for the desired temperature range is as follows:
    1. R1 = 10kΩ

    2. R2 = TMP61

    3. R3 = 6.96kΩ

    4. R4 = 10kΩ

    5. R5 = 6.12kΩ

    In the following image, the comparison of VOUT(T) to the linear approximation is used to calculate the resistor values. VOUT(T) represents the output voltage including the non-linear portion of the TMP61 temperature range. Linear approximation represents the calculated temperature assuming the TMP61 resistance response is linear across full temperature range.
    GUID-20201209-CA0I-GD1R-VLSD-BGH2396LNWQB-low.png
  8. Solving the resistor network for the optimized temperature range improved the accuracy across the overall temperature range of the sensor. The temperature error across the optimized temperature range is approximately 1.65°C which occurs at 49.1°C. The temperature reading from the ADC is approximately 50.75°C while the actual temperature is 49.1°C. However, the error at the end points increases.