DESIGN
WOMENT&INMT: ETCIPHS
Figure 7. Shortening the coaxial cable.
The output voltage measured by the DMM increased with a constant slope and reached around 4.16V in all measurements. The corresponding current shows a value of about 50fA. Furthermore, the red line in Figure 9 shows the waveform of the remeasurement with a shorter coaxial cable connected to the noninverting input terminal (C p = 26.5pF). The slope of the voltage rise is as large as the theoretical calculation. From these measurement results, it was found that the conductance component on the input side has a significant adverse effect on the measurement accuracy. Conclusion Although the fA level measurement can be performed in a general lab environment, the path of the leakage current on the input side of the operational amplifier needs to be carefully considered. In order to improve the accuracy of the measurement, it is recommended to use a Teflon terminal block on the input side or a triaxial cable together with the evaluation board. Acknowledgements The author would like to thank Scott Hunt, Iku Nagai, and Jun Kakinuma for their technical advice. Figure 9. Measurement results after removing the test box. The blue, orange, and green lines are measurement results at Cp = 35.2pF. The red line is the measurement result when C p = 26.5pF.
• A shield box was installed on the evaluation board inside the
thermostatic chamber (see Figure 6)
• The coaxial cable connected to the non-inverting input terminal was shortened to reduce the C p (see Figure 7) For one, we expect to reduce the effect of external noise, and for two, we expect to reduce the small leakage current in the cable (the re-calculated C p is 35.2pF). However, although these measures were taken and remeasured, no reproducibility was observed, similar to the results obtained in 'Capacitive Integration Measurement'. The waveforms differed significantly from the expected ones. Remove the test box The test box used was removed and the SW was changed by directly shorting and opening the ground (see Figure 8). In other words, the conductance component called the test box was removed and the measurement was performed. As a result, we were able to obtain the waveform as shown in Figure 9.
Figure 8. Measurement with test box removed. Short and open operation by hand instead of the SW.
28 ELECTRONICSPECIFIER.COM
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