Precision Impedance Measurement Using Auto-Balancing Bridge Methodology

Can impedance be accurately measured across a wide range (< 1 Ω to 100 MΩ) and broad frequency spectrum (20 Hz to 100 kHz) whilst maintaining high resolution (>20-bits) and minimizing measurement errors introduced by the test circuit itself?

Traditional impedance measurement techniques struggle to maintain accuracy across such an expansive impedance range without requiring manual tuning or introducing significant loading effects that distort the measurement.

Signal-Chain Solution

The solution employs an auto-balancing bridge architecture with a precision transimpedance amplifier (TIA) configuration that maintains a virtual ground at the measurement node.

Figure 1. Impedance Analyzer Signal Chain solution for Data Acquisition

This approach directly addresses the stated problem through three key mechanisms:

1. Virtual Ground Maintenance: The TIA uses a high-impedance JFET or CMOS input operational amplifier (such as the ADA4510) configured to actively maintain the measurement node at virtual ground potential. This eliminates loading effects on the device under test (DUT), ensuring the impedance measurement reflects the true component characteristics rather than being influenced by the test circuit. The auto-balancing technique inherently solves the wide impedance range challenge because the virtual ground condition is maintained regardless of DUT impedance—the TIA feedback automatically adjusts to accommodate impedances from sub-ohm to tens of megohms without manual rebalancing.
2. Architecture: The measurement system comprises three primary stages:
   1. Signal source: A precision excitation source (10 mV to 1 Vrms, 20 Hz to 100 kHz).
   2. Measurement Signal Paths: This includes a voltage measurement path and a current measurement path. 
      • The voltage across the DUT is measured via an instrumentation amplifier (InAmp) such as the AD8421, with low DC offset (+25 µV max.), low noise (3 nV/ÖHz typ.), and low current noise (200 fA/ÖHz typ.).
      • The current is measured by the Auto-Balancing Stage using the ADA4510-2 as a transimpedance amplifier (TIA). It also has extremely low DC offset (+10 µV typ.), low input bias current (+2.5 pA typ.), and low noise (5 nV/√Hz typ.).
   3. Digitizing Stage: High-resolution ADCs capture both voltage and current measurements—the AD4630-24 (24-bit, 2 MSPS) is a dual channel SAR ADC that enables both measurements to be made simultaneously. It achieves ±1 ppm INL (max.) with no missing codes, 105.7 dB SNR (typical), and -127 dB THD (typical) at 2 MSPS throughput.
3. Precision Reference and Drive Capability: The LTC6655-5 voltage reference provides exceptional stability (2 ppm/°C max drift) and low noise (0.25ppm_p-p), whilst the ADA4945-1 fully differential ADC driver ensures accurate signal conditioning with minimal offset drift (±0.1 µV/°C).

Use Cases and Applications

This signal-chain solution is optimally suited for:

LCR Meters and Impedance Analyzers: The architecture is commonly deployed in benchtop and handheld LCR meters that characterize passive components (inductors, capacitors, resistors) across their operating frequency ranges. The 0.1% accuracy specification makes this design ideal for component sorting, quality control, and incoming inspection in electronics manufacturing.

Battery Impedance Spectroscopy: Electrochemical impedance spectroscopy (EIS) applications, such as lithium-ion battery state-of-health monitoring, require measurement of milliohm-range impedances over wide frequency sweeps. The auto-balancing approach with current excitation capability enables accurate characterization of battery internal resistance and electrochemical processes without requiring high voltage excitation.

Analytical Instrumentation: Precision impedance measurement finds application in analytical instruments including bioimpedance sensors, material characterization systems, and chemical impedance spectroscopy platforms where both DC and AC impedance characteristics provide diagnostic information.

Signal Chain Designer Circuits

See examples of the measurement path signal chains using ADI’s Signal Chain Designer tool. These examples illustrate fixed gain solutions but can easily be converted to variable gain input stages.

Signal Chain Designer | Precision Studio | Analog Devices-Impedance Analyzer Voltage Measurement

Signal Chain Designer | Precision Studio | Analog Devices – Impedance Analyzer Current Measurement