SBOA496 January 2021 BUF634 , BUF634A , OPA2810 , OPA656 , OPA810 , THS4551

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. Z_{DUT} is the unknown impedance (device under test or DUT) and
R_{F} is a known feedback resistance in this circuit. A known voltage
V_{IN} is forced at input to the signal chain. For a voltage V_{DUT}
across Z_{DUT} and a current I_{DUT} flowing through it,

Equation 1.

Amplifier A1 is used as an inverting amplifier, whose output voltage is given as,

Equation 2.

Equation 3.

From (1) and (3), the unknown impedance
Z_{DUT} is given by,

Equation 4.

A few things need careful consideration when designing an LCR meter analog front-end circuit using the ABB method:

- A single value of R
_{F}will not suffice for measuring a wide range of values of Z_{DUT}. To increase the measurement range and sensitivity of the LCR meter multiple feedback resistors (R_{F1,2,3}) are switched into the circuit through series switches (SW_{F1,2,3}), shown in Figure 1-2. - A large value of R
_{F}forms a zero in the noise-gain transfer function causing 40 dB/decade rate-of-closure and potential instability. Use of a capacitor C_{F}in parallel with the large R_{F}, shown in Figure 1-1, introduces a pole to cancel this zero and restores phase margin, but it is difficult to find a single value of C_{F}for stability with all values of capacitive Z_{DUT}. This problem is solved using series resistors R_{G1,2,3}with Z_{DUT}, as Figure 1-2 shows. Use of R_{G}introduces a pole in noise-gain, cancelling the zero and restoring phase for a stable circuit. Multiple values of R_{G}(equal to corresponding R_{F1,2,3}) with corresponding series switches (SW_{G1,2,3}) need to be used. The same R_{F}and R_{G}pairs (marked with the same color in Figure 1-2) are switched in every time for the required measurement range to ensure stable operation. - For high accuracy measurements with large
value DUTs, V
_{DUT }and I_{DUT}should be buffered with high-Z input amplifiers (A2 and A3 here, CMOS or FET-input amplifiers with ≈pA range bias currents).

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 V_{DUT} and I_{DUT} are
needed to calculate Z_{DUT} 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 V_{DUT} and I_{DUT} measurements |

OPA656 | FET-input, voltage-feedback | 230 MHz | 14 mA | 7 nV/rtHz | High-frequency V_{DUT} and I_{DUT} measurements |

THS4551 | Low-power fully differential amplifier | 135 MHz | 1.37 mA | 3.3 nV/rtHz | ADC input driver for differential V_{DUT} and I_{DUT}
measurements |

BUF634A | High I_{OUT} buffer | 210 MHz | 8.5 mA | 3.4 nV/rtHz | High I_{OUT} buffer for driving small-value DUT with
V_{IN} |