TIDUEU6B September   2020  – December 2021 OPA810

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 OPA2810
      2. 2.2.2 BUF634A
    3. 2.3 Design Considerations
      1. 2.3.1 Existing architecture
        1. 2.3.1.1 Circuit Stability Issue
        2. 2.3.1.2 Solution in Existing Architecture (Compensation Cap)
      2. 2.3.2 Proposed Design
        1. 2.3.2.1 Stability Analysis of the Proposed Design
          1. 2.3.2.1.1 Without Measurement of Voltage at Inverting Node of A2
          2. 2.3.2.1.2 With Measuring Voltage at Inverting Node of A2
        2. 2.3.2.2 RG = RF Settings and Respective Impedance Ranges
        3. 2.3.2.3 Impedance Measurement Procedure
          1. 2.3.2.3.1 Short Cal
          2. 2.3.2.3.2 Impedance Cal
          3. 2.3.2.3.3 100k Setting Calibration
          4. 2.3.2.3.4 Open Cal
          5. 2.3.2.3.5 Calculations
          6. 2.3.2.3.6 Correction in ZX
          7. 2.3.2.3.7 Data Acquisition and Processing
          8. 2.3.2.3.8 Mathematical Explanation
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Required Hardware and Software
      1. 3.1.1 Hardware
    2. 3.2 Testing and Results
      1. 3.2.1 Test Setup
      2. 3.2.2 Test Results
  9. 4Design Files
    1. 4.1 Schematics
    2. 4.2 Bill of Materials
    3. 4.3 PCB Layout Recommendations
      1. 4.3.1 Layout Prints
    4. 4.4 Altium Project
    5. 4.5 Gerber Files
    6. 4.6 Assembly Drawings
  10. 5Software Files
  11. 6Related Documentation
    1. 6.1 Trademarks
    2. 6.2 Third-Party Products Disclaimer
  12. 7Revision History

1 System Description

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.

Table 1-1 Impedance Measurement Methods
METHOD ADVANTAGES DISADVANTAGES APPLICABLE FREQUENCY RANGE COMMON APPLICATION
Bridge method
  • High Accuracy
  • Wide frequency range with different types of bridges
  • Manual balancing needed
  • Narrow frequency coverage with single bridge
 DC to 300 MHz Standard Lab
Resonant method
  • Good Q measurement accuracy up to high Q
  • Tuning required
  • Low impedance measurement accuracy
 10 kHz to 70 MHz High Q device measurement
Network analysis method
  • Wide frequency coverage
  • Good accuracy
  • Narrow impedance measurement range
 5 Hz to above RF component measurement
Auto balancing method (Method used in this design)
  • Good accuracy over wide range of impedances
  • Grounded device measurement
  • High frequency ranges are not available
 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.

GUID-0009E04A-2EBA-4A63-9C11-2F21D90E90EB-low.gif Figure 1-1 Auto Balancing Circuit Amplifier Configuration

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.