SBOA597 November   2024 OPA928

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Architecture of Small Current Measurement
    1. 2.1 Coulombmeter
    2. 2.2 Using the Coulombmeter to Determine IB
    3. 2.3 Leakage of Integration Capacitor
  6. 3Benchmarking
    1. 3.1 Point to Point Wiring
    2. 3.2 Shielding
    3. 3.3 PCB Cleaning
    4. 3.4 Temperature Stability
  7. 4Calibration Using a Coulombmeter for Application Circuits
    1. 4.1 Calibration of Common Application Circuits
    2. 4.2 Calibration of Inverting Input
    3. 4.3 Calibration of Non-Inverting Input
    4. 4.4 Determine Resistance of the Capacitor Using Zero-Cross Method
    5. 4.5 Dielectric Absorption and Relaxation
    6. 4.6 Calibration at 85°C
    7. 4.7 Calibration at 25C
  8. 5Summary
  9. 6References

4.6 Calibration at 85°C

IB typically can have a logarithmic relationship relative to temperature. This means that as the temperature increases, the IB can be significantly higher. For this reason, highly accurate systems can need to calibrate IB across temperatures. Following is a summary of the steps necessary to achieve ultra-low current calibration. Each step is then described in detail.

  • Transform the circuit into calibration mode.
  • Monitor the output voltage over time.
  • Find a point where the output voltage crosses zero volts.
  • Calculate current using the derivative of the output over time multiplied by the capacitance.
  • Plot IB versus output voltage to determine at what voltage the dielectric relaxation has settled. Make sure the zero-cross point is away from dielectric relaxation.
  • Apply a temperature coefficient of the capacitor for calculation.

In the example, the output voltage (buffer with a gain of ten) moved from +1.0V to -3.4V.

Calibration at 85C With Zero-Cross Method (Buffer Out Over Time) Figure 4-20 Calibration at 85C With Zero-Cross Method (Buffer Out Over Time)

The curve crosses zero volts, and a derivative at the point shows -33.7µV/sec. Assuming the number is the smallest leakage current condition across the capacitor, IB is calculated as -33.7µV/sec / 10.1 gain x 108.6pF x (1-200 x 10-6 x (85-20)) = -357.6aA. The calculation includes the temperature coefficient of the capacitor of -200ppm/°C.

Calibration at 85C with zero-cross method (buffer out over time) zoom Figure 4-21 Calibration at 85C with zero-cross method (buffer out over time) zoom

Next, make sure the zero-cross point is away from dielectric relaxation. Calculate the derivative of output voltage over time. The plot of current over output voltage settles at around output voltage closes zero volts. This indicates dielectric relaxation settles at around the output of zero volts. A fitting curve shows -50.5aA/V. The resistance of the integration capacitor is calculated as 1 / (-50.5aA/V) / 10.1 (gain) = 1.96PΩ. The intercept of the fitting curve indicates IB is -354.8aA.

Calibration at 85C with zero-cross method (current over buffer out) Figure 4-22 Calibration at 85C with zero-cross method (current over buffer out)

Comparing the number between the intercept of the fitting curve (-354.8aA) and the zero-cross method (-357.6aA) gives a delta of 2.8aA. With that, IB is most likely between -354.8aA to -357.6aA at 85°C.