Application and Principle Analysis of LCR Digital Bridges in Circuit Parameter Measurement

In the field of electronic testing, digital bridges play a vital role. Traditionally, digital bridges were primarily used for measuring parameters such as inductance, capacitance, resistance, and impedance, a practice considered conventional. While early impedance measurements indeed utilized traditional bridge methods, the advent of modern analog and digital technologies gradually phased out this approach. Nevertheless, the nomenclature of digital bridges, such as LCR bridge, persists to this day. Notably, when a digital bridge incorporates microprocessor technology, it is referred to as an LCR digital bridge. Users commonly refer to these devices as LCR meters, LCR bridges, or LCR testers.

Digital bridges are extensively employed in measuring the parameters of impedance components, including AC resistance (R), inductance (L) along with its quality factor (Q), and capacitance (C) along with its dissipation factor (D). Consequently, they are often referred to as digital LCR meters. Their measurement frequency range spans from power frequency to approximately 100 kHz, with a basic measurement error of 0.02%, typically around 0.1%.

As electronic circuit signals continue to evolve towards higher speeds and frequencies, understanding the practical model of LRC components becomes essential, moving beyond simplistic ideal models. Precise measurement of the voltage and current applied to the Device Under Test (DUT) facilitates accurate determination of the DUT’s impedance. The measurement process involves utilizing phase-sensitive detectors (PSDs) within the LCR digital bridge to measure the in-phase and quadrature components of the voltages Vx and Vr corresponding to a reference phase vector. Subsequently, these values are digitized through analog-to-digital converters (ADCs) and subjected to complex computations by a computer to obtain the resistive and reactive components constituting the impedance Zx of the DUT.

Despite inheriting the nomenclature of traditional bridges, digital bridges have transitioned away from the composition of classical AC bridges. Instead, they have evolved to operate at a higher level, reverting to the principles and circuits of current and voltage meters based on Ohm’s law for impedance measurement.

Various methods exist for measuring the phase difference between voltage and current signals:

1. Zero-Crossing Comparison + Time Measurement Technique:

A common method involving triggering a level from a zero-crossing point and measuring the time difference between two zero-crossing points.

2. Dual-Channel Synchronous Sampling Technique:

Involves performing calculations and analysis after sampling using two ADC channels.

3. Analog Multiplier Method: By multiplying two signals, the phase difference can be determined, enabling the calculation of impedance.

Digital bridges find widespread application in metrology departments for calibrating and transferring impedance standards, as well as routine measurements of impedance components in general departments. Many digital bridges come equipped with standard interfaces for automatic classification of measured components based on accuracy. They can also be directly connected to automated test systems for product line testing, thereby facilitating quality control in the manufacturing process.

By the mid-1980s, there were dozens of digital bridges with a general error rate below 0.1%. Digital bridges are advancing towards higher accuracy, increased functionality, higher speeds, greater integration, and enhanced intelligence.

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