TIDUF45 May   2024

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Design Considerations
    3. 2.3 Highlighted Products
      1. 2.3.1 ADS127L21
      2. 2.3.2 PGA855
      3. 2.3.3 REF70
  9. 3System Design Theory
  10. 4Hardware, Software, Testing Requirements, and Test Results
    1. 4.1 Hardware Description
      1. 4.1.1 Board Interface
      2. 4.1.2 Power Supplies
      3. 4.1.3 Clocking Tree
    2. 4.2 Software Requirements
    3. 4.3 Test Setup
    4. 4.4 Test Results
      1. 4.4.1 DC Accuracy Tests
      2. 4.4.2 Gain and Offset Temperature Drift
      3. 4.4.3 Nonlinearity
      4. 4.4.4 SNR and Noise Performance
  11. 5Design and Documentation Support
    1. 5.1 Design Files
      1. 5.1.1 Schematics
      2. 5.1.2 BOM
    2. 5.2 Software
    3. 5.3 Documentation Support
    4. 5.4 Support Resources
    5. 5.5 Trademarks
  12. 6About the Author

4.4.2 Gain and Offset Temperature Drift

Offset drift is the change in the offset voltage across a temperature range. The signal chain offset drift can be calculated from the offset data collected using the box method. The box method takes the maximum and minimum offset over the range and divides the offset by the temperature range as shown in Equation 2:

Equation 2. O f f s e t   D r i f t   μ V ° C = 10 6 × V O F S M A X - V O F S M I N T M A X - T M I N

Gain error is the difference between the actual and the ideal slopes of the signal chain. Gain error is measured by applying dc test voltages at –95% and 95% of the full-scale range. The error is calculated by subtracting the difference of the dc test voltages (ideal slope), scaled by the gain (G), from the difference in the output voltages (actual slope), as shown in Equation 3:

Equation 3. G a i n   E r r o r   ( p p m   o f   F S R )   =   10 6 × ∆ V O U T - G ∆ V I N G ∆ V I N

The gain drift can be found by using the box method again, using the minimum and maximum gain error across gain stages:

Equation 4. G a i n   D r i f t   p p m ° C = 10 6 × G E M A X - G E M I N T M A X - T M I N

The typical gain and offset drift errors an be estimated by using adding the typical values listed in the PGA855 and ADS127L21 data sheets. The same can be done for the maximum errors:

Table 4-3 Estimated Offset and Gain Drift
PGA855

(TYP)

 ADS127L21 (TYP) ESTIMATED VALUE (TYP) FOR PGA855 + ADS127L21 PGA855 (MAX) ADS127L21 (MAX) ESTIMATED VALUE (MAX) FOR PGA855 + ADS127L21
Offset Drift (μV/°C) 0.3 0.05 0.35 1 0.2 1.2
Gain Drift (ppm/°C) 1 0.5 1.5 2 1 3

For the range 20°C–30°C, the expected error from drift is around ± 3.5μV of offset and ± 15ppm of gain error. Below, the offset and gain error across temperature measured from one unit is displayed:

TIDA-010945 Offset Voltage (RTI) Versus Temperature Figure 4-5 Offset Voltage (RTI) Versus Temperature
TIDA-010945 Gain Error Versus Temperature Figure 4-6 Gain Error Versus Temperature

From Figure 4-5 and Figure 4-6, we can calculate the temperature drift for the unit, using Equation 2 and Equation 4:

GAIN (V/V) 0.25 1 4 16
Offset Drift (μV/°C) 0.81 0.11 0.04 0.02
Gain Drift (ppm/°C) 2.06 0.89 0.64 1.28