Skip to content

Hardware Design

Core Concept

The M181 LCR Meter operates by applying a fixed sine wave voltage to the Device Under Test (DUT) through a series resistor. It measures the voltage drop (V), current (I), and the phase difference between V and I to calculate the complex impedance. Using this impedance, the following parameters of the DUT can be derived:

  • Inductance (L)
  • Capacitance (C)
  • Resistance (R)

Additionally, the Dissipation Factor (D) and Equivalent Series Resistance (ESR) can also be calculated.

Image title
Core Concept
Image title
Implemented Method

Capability

The manufacturer (JYETech) specifies the following capabilities:

Image title

Image source: JYETech M181 LCR Meter

Design Files

Block-wise Explanation

Power Tree

  1. USB input voltage: 5V
  2. Charge pump (TPS60403 IC) generates dual voltage: ±5V
  3. LDO regulates voltage to 3.3V

Signal Generator (DAC)

To generate a sine wave with a fixed frequency, a DAC is required. Since the MCU lacks an inbuilt DAC, an external R-2R ladder DAC is used.

Detailed explanation: Digital-to-Analog Conversion (R-2R DAC)

  • R-2R ladder DAC is implemented using digital pins (8-bit resolution).
  • The DAC output is connected to a voltage follower (U4C).
  • Two RC low-pass filters (\(F_c = 723Hz\)) are connected to smooth the signal.
  • The output is tapped for Automatic Factor Correction (AFC) and connected to the MCU's ADC.
  • Further, an RC high-pass filter (\(F_c = 312Hz\)) blocks DC, followed by another voltage follower (U4D) to provide the final output.

Trans-impedance Amplifier

This section converts the current (\(I_X\)) through the DUT into a voltage (\(V_O\)) using the formula:
$$ V_O = I_X \times R_F $$

Kelvin (4-wire) Measurement

  • Two dedicated wires measure the voltage across the DUT.
  • A switch toggles between voltage and current measurement modes.
Image title

Image source: Kelvin Resistance Measurement

Note: Calibration is required to compensate for probe errors.

Amplifier with Gain Controller Section

Image title
  • Instrument amplifiers (U2A, U2C, U2D) with a fixed gain of 5.25.
  • The gain is selectable via the GS pin, allowing unity gain or attenuation by a factor of \(\frac{1}{101}\).
  • Next it is connected to high-pass filter which DC block signal.
  • A non-inverting amplifier with a gain of 100 adds an offset voltage (-1.77V) and connects to the MCU's ADC.
Image title
\[ V_{out}=V_{in}.(1+\frac{R_1}{R_2}+\frac{R_1}{R_3}) - V_{offset} .(\frac{R_1}{R_3}) \]

Here \(R_1 = R_3 = 5.1\)kohm, \(R_2 = 51\)ohm, \(V_{offset}=-1.77\)V

\[ V_{out}=102\times V_{in} + 1.77 V \]

Miscellaneous Section

  1. I2C 128x64 resolution SSD1306 display.
  2. Three push buttons for user input/control.
  3. SWD signals terminated at the J3 header.
  4. UART signals connected to the CH340N USB serial interface.