LCR meters and impedance analyzers are used to measure unknown values of passive components like resistors, capacitors, inductors, or a combination of these elements. These lab equipment are similar, except that an impedance analyzer allows measurements at different test frequencies. The auto-balancing (ABB) method, compared to the other architectures, offers good measurement accuracy over a wide range of values of impedance, and is discussed in this technical report.
Figure 1-1 shows a representative schematic of an analog front-end using the ABB method. ZDUT is the unknown impedance (device under test or DUT) and RF is a known feedback resistance in this circuit. A known voltage VIN is forced at input to the signal chain. For a voltage VDUT across ZDUT and a current IDUT flowing through it,
Amplifier A1 is used as an inverting amplifier, whose output voltage is given as,
From (1) and (3), the unknown impedance ZDUT is given by,
A few things need careful consideration when designing an LCR meter analog front-end circuit using the ABB method:
Amplifier A3 can be eliminated for a simplified analog front-end design; however, to maintain measurement accuracy, amplifier A1 should have a large enough open-loop gain (AOL), and hence a gain-bandwidth product at the highest measurement frequency of interest. With a large AOL at the test frequency, a virtual ground is maintained at A1’s inverting input.
Eliminating A3 allows for single-point ground-referenced measurements with need for smaller number of amplifier channels and single-ended ADCs. A general rule-of-thumb is to ensure that A1 has >60-dB AOL at the highest frequency of interest for high accuracy measurements. For higher test frequencies, two-point measurements for VDUT and IDUT are needed to calculate ZDUT with high-accuracy, which requires more amplifiers (A3 in Figure 1-2) and differential input ADCs.
The TIDA-060029 reference design describes this LCR meter analog front-end and the associated challenges in detail. An analog front-end with impedance measurements accurate to 0.1% is implemented in this reference design. Impedance values in the range 1 Ω to 10 MΩ can be measured at frequencies from 100 Hz to 100 kHz. Table 1-1 lists Texas Instruments amplifiers suitable for use in an LCR meter design:
Device | Architecture | GBW | Quiescent Current | Noise | Function |
---|---|---|---|---|---|
OPA810 | FET-input, voltage-feedback | 70 MHz | 3.7 mA | 6.3 nV/rtHz | Unity-gain buffer for VDUT and IDUT measurements |
OPA656 | FET-input, voltage-feedback | 230 MHz | 14 mA | 7 nV/rtHz | High-frequency VDUT and IDUT measurements |
THS4551 | Low-power fully differential amplifier | 135 MHz | 1.37 mA | 3.3 nV/rtHz | ADC input driver for differential VDUT and IDUT measurements |
BUF634A | High IOUT buffer | 210 MHz | 8.5 mA | 3.4 nV/rtHz | High IOUT buffer for driving small-value DUT with VIN |
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