TIDUEU6B September 2020 – December 2021 OPA810
The goal of any test and measurement system is to measure a device under test (DUT) as simply as possible, while only introducing errors significantly smaller than those present in the measured device. For impedance measurements, there are several existing techniques that provide various tradeoffs between measurement accuracy, complexity, and frequency range. For this design, the auto balancing circuit method was chosen because it provides good accuracy over a wide impedance measurement range without any tuning requirements. Table 1-1 lists the advantages and disadvantages of several common impedance measurement techniques along with their frequency ranges and typical applications.
|METHOD||ADVANTAGES||DISADVANTAGES||APPLICABLE FREQUENCY RANGE||COMMON APPLICATION|
||DC to 300 MHz||Standard Lab|
||10 kHz to 70 MHz||High Q device measurement|
|Network analysis method||
||5 Hz to above||RF component measurement|
|Auto balancing method (Method used in this design)||
||20 Hz to 120 MHz||Generic component measurement|
The auto balancing technique is very useful for a wide range of impedance measurements at a frequency range of 20 Hz to 120 MHz. The auto balancing technique uses an op-amp as shown in Figure 1-1.
The fundamental idea in this technique is to convert the current ( IX ) through unknown impedance (ZX) into voltage ( VO ). The unknown impedance value is determined using the value of current flowing through it. The non-ideal properties of the amplifier and circuit play a very crucial role in the design of an LCR meter. For example, the parasitic capacitance at the inverting input of the amplifier will cause instability for a high value of RF. The circuit's stability is also sensitive to both the type of component and value used for ZX. The circuit is particularly prone to instability when capacitive impedances are measured. In this design, these stability problems are addressed using a multi-path capacitive compensation technique. This design illustrates the analog signal chain of an LCR meter which is tested up to 100 kHz.