High Precision Measurement of Sub-Nano Ampere Current in ATE Environment

Abstract

Background: In IoT system devices, currents become smaller and they have to operate for ten years with a coin cell battery. Then their accurate and fast measurement is required at the mass production shipping stage. However, the conventional method needs a large resistor (Rm, MΩ-order in Fig. 1) which makes the testing slow, and the ATE environment is noisy.Research Target: Our target is the development of a testing technique to measure the current in the order of nano or sub-nano ampere with high linearity in the noisy ATE environment and in short time as well as with only additional low cost built-out self-test (BOST) circuits.Approach: Fig. 2 shows the proposed current measurement circuits. The current under test is converted to the voltage through an op-amp and a resistor Rm of 10kΩ, and it is then converted to the AC voltage; these conversions are done with small BOST circuits. The AC voltage is amplified and converted to the digital signal through an AC amp, a sample/hold circuit and an ADC. FFT is performed and its power spectrum is calculated; the input current value is obtained. The resistor (Rm) of 10k generates spike noises which are spurious components in the power spectrum. However, usage of the sample/hold circuit reduces their effects. Thanks to the DC-AC conversion, the measurement accuracy is not degraded by the system noise in the low frequency region. The nano-ampere current and the resistor of 10kΩ produces several tens μV level voltage and our previous research in [1, 2] shows that the DC-AC conversion method can measure this level of the voltage accurately and in short time. Also, its multi-channel measurement is possible.Experiment Verification: Preliminary experiment with the prototype system in Fig. 2 was performed and its measured result is shown in Fig. 3. The measurement circuit gain in Fig. 2 was calibrated with 1.0nA input current (Iin), which corresponds to Vin of 10.0μV. Also an offset of 0.2μV due to electromotive force (EMF) was calibrated. We see in Fig. 3 that the proposed method can measure the current as low as 50pA. So far EMF limits the lowest measurable current. Fig. 4 shows 100 times of measurements for 1.0nA without averaging. Each measured current value is obtained by 1K-point FFT with 25.6 ksps, 16-bit ADC (myDAQ) usage and the measurement time of 40ms. The measured data is within 0.94nA to 1.07nA; the variation range is 0.13nAp−p.Conclusion: A method of fast and accurate current measurement as low as 50pA using IV conversion and DC-AC conversion in ATE environment has been developed.