LCR Meter Sub-Balancing for Fast, Accurate Impedance Measurement

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Solution Overview

Problem

LCR meters face inaccuracies due to leakage currents at high frequencies, where existing methods like guard and auto-balancing bridges have low gain and stability issues, requiring multiple measurements and long settling times, especially at low and medium frequencies.

Innovation Solution

A sub-balancing method using a trans-impedance amplifier (TIA) with an additional digital-to-analog converter (DAC) to apply inverted voltage, reducing residual unbalanced voltage and enabling instant balancing at low and medium frequencies with a single voltage measurement, and leakage compensation to correct for leakage current errors.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If a trans-impedance amplifier (TIA) is used for auto-balancing, then instant balancing is achieved at low and medium frequencies, but the gain is low at high frequencies resulting in significant residual unbalanced voltage

Engineering Contradiction:

Improvebalancing speedVSAvoidbalancing accuracy at high frequencies

Core Design Contradiction:

SpeedVSMeasurement precision

Solution Approach 1:

An additional DAC is introduced as an intermediary component to generate a sub-balancing voltage that compensates for the residual unbalanced voltage at high frequencies. This mediator allows the TIA to maintain its fast response while achieving accurate balancing across the full frequency range.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The system dynamically changes the balancing approach based on frequency: using TIA for instant balancing at low and medium frequencies, and switching to DAC-based sub-balancing at high frequencies where TIA gain is insufficient. This parameter-based selection resolves the contradiction between speed and precision.

Inventive Principle:

Principle #35Parameter changes

2Measurement precision

If modem-type auto-balancing bridges with low-speed integrators are used, then good balancing accuracy at high frequencies is achieved, but measurement time increases at low and medium frequencies

Engineering Contradiction:

Improvebalancing accuracy at high frequenciesVSAvoidmeasurement time at low and medium frequencies

Core Design Contradiction:

Measurement precisionVSLoss of time

Solution Approach 1:

The system dynamically selects the balancing method based on operating frequency. At low and medium frequencies, the fast TIA-based method is used for instant balancing. At high frequencies, the system transitions to the DAC-based sub-balancing method. This dynamic adaptation eliminates the need for slow integrators across all frequencies.

Inventive Principle:

Principle #15Dynamics

3Object-generated harmful factors

If guard method or TIA auto-balancing is used, then leakage current is decreased, but common voltage suppression is low at high frequencies

Engineering Contradiction:

Improveleakage currentVSAvoidcommon voltage suppression at high frequencies

Core Design Contradiction:

Object-generated harmful factorsVSMeasurement precision

Solution Approach 1:

The additional DAC acts as a mediator that generates a sub-balancing voltage to suppress common-mode voltage at high frequencies. This intermediary component enables the system to maintain both low leakage current and high common voltage suppression across the full frequency range.

Inventive Principle:

Principle #24Intermediary (Mediator)

4Measurement precision

If multiple voltage measurements are performed for balancing, then measurement accuracy is improved, but measurement speed decreases

Engineering Contradiction:

Improvebalancing accuracyVSAvoidmeasurement speed

Core Design Contradiction:

Measurement precisionVSProductivity

Solution Approach 1:

The system performs only one voltage measurement at high frequencies using the DAC-based sub-balancing method, rather than requiring multiple measurements. This partial action approach achieves sufficient accuracy while maintaining fast measurement speed, avoiding the need for iterative multi-measurement processes.

Inventive Principle:

Principle #16Partial or excessive action

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach provides fast and accurate impedance measurements at low and medium frequencies with instant balancing, while minimizing errors at high frequencies by reducing leakage current, thus improving measurement speed and accuracy with reduced costs.

Implementation Method 1

A trans-impedance amplifier (TIA) provides an analog method for fast balancing at a low cost. The TIA has instant balancing at low and medium frequencies.

Methodology Applied

Scientific EffectTrans-impedance amplification:

Implementation Method 2

The sub-balancing method comprises the measurement of the unbalanced voltage and applying some voltage with inverted polarity from an additional digital-to-analog converter (DAC) to the non-inverting input of the TIA.

Methodology Applied

Scientific EffectVoltage inversion:

Implementation Method 3

The impedance of the DUT is calculated by knowing the current through and voltage drop of the DUT. In this case, the leading cause of an error is a leakage current through a leakage impedance between a middle point of the DUT and the range resistor to the ground.

Methodology Applied

Scientific EffectLeakage current compensation:

Data Source

PatentUS11592858B1Fast LCR meter with sub-balancing

Publication Date: 2023.02.28 KOKORIN OLEKSANDR

AI summary

An LCR meter to increase accuracy of balancing uses sub-balancing method, additional to analog balancing by trans-impedance amplifier (TIA). For this, the LCR meter, based on TIA, to correct analog auto-balancing, applies the inverted voltage equal to unbalanced voltage to noninverting input of the TIA. And only one measurement of voltages is needed for.