AN-1053: AD5933 Evaluation Board Example Measurement

Introduction

This application note details how to set up the AD5933 evaluation board, and how to make a measurement of the impedance of the on-chip 15 pF capacitor. The AD5933 data sheet provides additional information and should be consulted when using the evaluation board.

Functional Block Diagram

Figure 1. Functional Block Diagram.

Evaluation Board Software

The four steps for measuring an impedance with the evaluation board include the following:

System setup
Calibration routine
Measure unknown impedance
Download data

System Setup

To set up the evaluation board system, do the following:

Load the AD5933 evaluation board software, which is supplied with the evaluation board or is alternatively available on the AD5933 product page.
When the software is loaded on the PC, connect the evaluation board using a USB cable.
Place the following jumpers: LK3, LK5.
Place a 200 kΩ through-hole resistor in the Z position on the evaluation board; this is the calibration impedance.
Place a 200 kΩ through-hole resistor in the RFB position; this is the feedback resistor.

Calibration Routine

Set up the system as shown in Figure 2. Fill out the columns from left to right as shown in Figure 2.

Figure 2. Calibration Routine.

Measure Unknown Impedance

Remove the 200 kΩ resistor from Z and insert a 15 pF capacitor to the board. Click Start Sweep to see the plot shown in Figure 3. Note that the impedance of the capacitor reduces with frequency according to the following equation:

where f is the Start Frequency + (Delta Frequency × Number of Increments).

Click the Impedance Phase 0 tab to check that the phase is approximately −90°.

Figure 3. Measuring the Impedance of Capacitor.

Download Data

Click Download Impedance Data to save the measured impedance data in an excel file.

The following data is downloaded to the excel file:

Frequency column, excitation frequency (Column A).
Real data register contents, R (Column D).
Imaginary data register contents, I (Column E).
where Gain Factor is calculated in Step 1

It does not give the phase in this column because the phase in this column is actually equal to (X in degrees) − (calibration phase [or system phase] in degrees). The system phase is calculated when the gain factor is calculated using, for example, the midpoint calibration. The calibration sets up the system phase, and then to determine the phase of the sensor, the system phase is deducted.

Figure 4. Downloaded Data.