Principle and Application of AD5933 Impedance Measurement Chip

1.1 Main Performance
AD5933 is a high-precision impedance measurement chip, which integrates a frequency generator with a 12-bit AD converter with a sampling rate of up to 1MSPS. This frequency generator can generate a specific frequency to excite the external resistor. The response signal obtained on the resistor is sampled by the ADC and discrete Fourier transform is performed through the on-chip DSP. After the Fourier transform, the real value R and the imaginary value I obtained at this output frequency are returned. In this way, the modulus of the Fourier transform and the phase angle of the resistor at each scanning frequency can be easily calculated. Where the modulus = , the phase angle = .
AD5933 has the following features:
 Programmable frequency generator, the highest frequency can reach 100KHz
 As a device through the port and host communication, to achieve frequency scanning control
 Frequency resolution is 27 bits (<0.1Hz)
 Impedance measurement range is 100Ω to 10MΩ
 Internal temperature sensor, measurement error range is ±2℃
 With internal clock
 Can achieve phase measurement
 System accuracy is 0.5%
 Available power supply range is 2.7V to 5V
 Normal operating temperature range is -40℃ to +125℃
 16-pin SSOP package
1.2 Pin definition of AD5933
Figure 1 shows the package diagram of AD5933, and Table 1 shows the pin definition of AD5933. It is recommended to connect all power pins 9, 10, and 11 together and connect them to the power supply. Similarly, all ground pins 12, 13, and 14 are also connected together and connected to the system ground.

Figure 1 AD5933 pin arrangement

Table 1 AD5933 pin definition

1.3 Main applications
AD5933 can be widely used in many fields such as electrochemical analysis, bioelectrode impedance measurement, impedance spectrum analysis, complex impedance measurement, corrosion monitoring and instrument protection, biomedical and automatic control sensors, non-invasive detection, raw material performance analysis, and fuel and battery status monitoring. It provides great convenience for impedance measurement. The monolithic integration technology greatly reduces the size of the instrument, making the instrument more convenient to use. The simple I2C communication method is convenient for user operation and reduces the difficulty of user programming. Since it directly gives the real and imaginary data of the transformed impedance, it greatly simplifies the user programming process and saves development time.

2 AD5933 working principle
2.1 AD5933 parameter setting
AD5933 has a 27-bit DDS on chip to provide output specific frequency excitation signal. The data input to the DDS status register is provided by the start frequency registers at addresses 82h, 83h, and 84h on the AD5933 chip. Although the status register provides 27-bit accuracy, the upper three bits of the start frequency register are actually internally reset to zero, so the user can only control the lower 24 bits of the start frequency. AD5933 can achieve a frequency resolution of 0.1Hz, which is controlled by the on-chip 24-bit frequency increment register. The addresses of the frequency increment register are 85h, 86h, and 87h. The code of the start frequency and frequency component register is calculated by dividing the required start frequency value or frequency increment value by one-quarter of the system clock and then multiplying it by 2 to the 27th power. The system clock can be set by the control register to select whether to select an external clock or an internal clock. The internal clock of the AD5933 is 16MHz. The number of frequency points can also be set in registers 88h and 89h. For example, if the user wants to measure 150 frequency points, the user stores 00H and 96H in 88h and 89h respectively. After these three parameters are set, the scan can be initialized by writing the start frequency scan command to the control register. After the scan of each frequency point is completed, the second bit of the status register will be automatically set. You can query this bit to determine whether the measurement is completed. The user can control it to jump to the next frequency point. The real part of the measurement result is saved in 94h and 95h, and the imaginary part is saved in 96h and 97h. This result should be read before jumping to the next frequency point. If you want to measure the value of the same frequency point multiple times to make the measurement result more accurate, just write the repeat current frequency command word in the control register after a measurement is completed. When all frequency points are scanned, the third bit of the status register will be automatically set. Once this bit is set, further frequency scanning will not be possible.

The specific process of frequency scanning includes three parts:
(1) Entering the standard mode. Before writing the start frequency scanning control word to the control register,
the standard mode control word must first be written to the control register. In this mode, the VOUT and VIN pins are internally connected to ground, so there is no DC bias between the external resistor or the resistor and ground.
(2) Entering the initialization mode. After writing the start frequency control word to the control register, the initialization mode will be entered.
In this mode, the resistor has been stimulated by the starting frequency signal, but no measurement has been performed. The user can set the time before writing the frequency scanning command to the control register to start entering the frequency scanning mode through the program.
(3) Entering the frequency scanning mode. The user writes the frequency scanning control word. In this mode, the ADC
starts measuring after the set time period has passed. The user can control the number of cycles of the output frequency signal by setting the values ​​of registers 8Ah and 8Bh before measuring each frequency point.
The signal output by the on-chip DDS passes through a programmable gain amplifier. By controlling the gain, four different ranges
of peak-to-peak output can be achieved. The control of this output range is implemented in the 9th and 10th bits of the control register.
During the receiving process, the signal obtained from the resistor first enters the current-to-voltage conversion amplifier, followed by a programmable gain amplifier. The gain of this amplifier has two values, 5 and 1, which can be selected by setting the 8th bit of the control register. The signal after the programmable gain amplifier is sampled by the ADC, and the sampled data is sent to the DSP for Fourier transform. Each frequency samples 1024 points for Fourier transform, and the result of the transform is stored in two 16-bit registers to represent the real and imaginary parts after the transform. Each 16-bit register is composed of two 8-bit registers, which will be specifically given in the following register introduction.
The system clock of AD5933 can be given in two ways. Users can connect a high-precision and stable clock to the external clock pin MCLK. Alternatively, the internal 16.776MHz clock of AD5933 can be used. The specific selection of which clock can be achieved by setting the 3rd bit of the control register. The system is powered on by default to select the internal clock.
2.2 Definition and setting of internal registers
There are 9 registers on the AD5933 chip. These registers implement different parameter setting functions respectively. Table 2 gives their names, addresses and read-write characteristics.
Table 2 Internal Registers

Most of the registers have been mentioned above. First, let's introduce the control register. The control register mainly implements the setting of various parameters of AD5933 and the setting of working status. Table 3 gives the functional definition of each bit of the control register. The control register consists of two 8-bit registers, with addresses 80h and 81h respectively. When in use, the user can only change the value of one register, while the value of the other register remains unchanged. When writing a reset command to the control register, the already programmed settings related to the frequency scan will not be reset. The values ​​related to the frequency scan are the starting frequency, frequency increment and frequency points. After the reset command, the start frequency command must be written to the control register to restart the frequency scan process. The default value of the control register after power-on is A000H.
Table 3 AD5933 Internal Register Bit Definition

In addition to the control register, it is also necessary to pay attention to the status register. The address of the status register is 8Fh. The status register marks the end of the measurement. Each bit from D7 to D0 represents the different functional states of AD5933, among which D4 to D7 have no practical meaning and do not represent any measurement status. (D0 indicates that the temperature measurement is completed, and this bit is set to 1). D1 indicates the impedance measurement of a frequency point. When the impedance measurement of the current frequency point is completed, this bit is set to 1, and it also indicates that the measurement results have been stored in the real and imaginary data registers. This bit is automatically reset when receiving the start, jump to the next frequency point, repeat the current frequency or reset command, and this bit is also reset when power is turned on. D2 indicates that the frequency scan is completed. This bit is set to 1 when all frequency points are measured. This bit is reset when a reset command is received or power is turned on.
2.3 Data transmission between AD5933 and the control system
The communication between AD5933 and the control system is implemented as a device and complies with the timing of communication. It has a seven-bit device address of 0001101. There is no special description when the control system writes to AD5933. When reading data from AD5933, first write B0h to AD5933, then write the register address to read the data, and read the register value.
2.4 Temperature measurement implementation
The temperature sensor on the AD5933 chip is a 13-bit digital temperature sensor, and the 14th bit is a flag bit. The temperature sensor can accurately measure the temperature of the surrounding devices. The measurement range of the temperature sensor is -40℃ to +125℃. When the temperature reaches +150℃, the structural integrity will be damaged when working at the maximum specifications of voltage and temperature. The accuracy of the measured temperature is ±2℃.
The conversion clock of the temperature measurement process is generated internally, and an external clock is required only when reading or writing data from the serial port. In general mode, the internal clock automatically completes the conversion process. By default, the temperature sensor is in a power-off state. To start the temperature measurement, the temperature measurement control word needs to be written in the control register. After the measurement is completed, the temperature sensor automatically shuts down until the next command is received and restarted. The user can check whether the temperature measurement is completed by reading the value of the status register. The temperature measurement results are saved in 92h and 93h. Among them, 14 bits are useful data, and the highest two bits are meaningless. DB13 is a flag bit. Table 4 shows the correspondence between some measured data and actual temperature. The specific temperature measurement data can be obtained through the formula. If the measured temperature is positive, it is equal to the decimal value of the obtained data divided by 32. If the measured temperature value is negative, and the value of DB13 is also calculated, it is equal to the decimal value of the measured data minus 16384 and then divided by 32. If the value of DB13 is not calculated, it is equal to the decimal value of the measured data minus 8192 and then divided by 32.