Low Frequency Impedance Measurement Using Source Measure Units
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Solution Overview
Problem
Measuring very high impedances at low frequencies is challenging due to the need for large voltages or small currents, which can lead to device breakdown or arcing, and existing technologies struggle to accurately quantify capacitances below 1 pF at low frequencies.
Innovation Solution
A method involving a source measure unit (SMU) that applies an AC signal with a non-zero frequency below 1 kHz to a device under test, synchronously digitizes voltage and current signals, and calculates impedance using techniques like discrete Fourier transforms, while optionally using two SMUs to account for stray impedances and enable multipin capacitance measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a DC voltage signal is applied to measure high impedance, then the measurement can be performed with standard SMUs, but the voltage becomes exceedingly large causing device breakdown or arcing
Solution Approach 1:
The patent applies periodic AC voltage signals at low frequencies (e.g., 1 Hz, 10 Hz, 100 Hz) instead of DC signals to measure high impedances. This periodic action allows the use of relatively low voltage amplitudes (e.g., 1 Vpp) while still obtaining measurable current responses, thereby avoiding device breakdown or arcing that would occur with high DC voltages.
Solution Approach 2:
The patent changes the measurement parameter from DC voltage to low-frequency AC voltage. By transforming the signal type and frequency, the measurement can be performed with much lower voltage levels while maintaining the ability to measure high impedances through impedance calculation from the AC response.
2Measurement precision
If the measurement frequency is reduced to measure capacitance, then the impedance increases making measurement easier, but the required voltage becomes excessively large
Solution Approach 1:
The patent uses periodic AC signals at specific low frequencies (1 Hz, 10 Hz, 100 Hz) to measure capacitance. The periodic nature allows for synchronized detection of the current response, enabling precise capacitance calculation from the measured current amplitude and phase without requiring excessively high voltages.
Solution Approach 2:
The patent replaces direct DC voltage measurement with AC signal analysis using lock-in detection techniques. By using synchronous detection of the AC current response, the system can extract precise capacitance information from very small current signals without needing large driving voltages.
3Device complexity
If standard SMUs are used for low frequency AC measurement, then the existing equipment can be utilized, but stray impedances and cable effects significantly impact measurement accuracy
Solution Approach 1:
The patent extracts and separately measures the stray impedance components (cable impedance, SMU output impedance) by performing open and short calibration procedures. These extracted stray impedance values are then subtracted from the DUT measurements to obtain accurate results, effectively removing the harmful influence of parasitic elements.
Solution Approach 2:
The patent implements a feedback-based calibration procedure where the measured stray impedances are used to correct subsequent DUT measurements. The system automatically applies correction factors based on the calibrated stray impedance values, continuously improving measurement accuracy through feedback from the calibration data.
Data Source
AI summary
A method for measuring the impedance of a DUT having a capacitance of less than 1 pF includes applying a voltage or current signal to the DUT, the voltage or current signal including an AC component having a non-zero frequency of less than 1 kHz; monitoring a current or voltage signal, respectively, through the DUT in response to the voltage or current signal; digitizing the voltage signal and the current signal synchronously; and calculating the impedance from the digitized voltage and current signals.