# Design of Three-Dimensional Electrical Impedance Tomography System for Rock Samples
## 4 Measurement System  
The measurement system primarily consists of the CPLD acquisition circuit board, the signal source circuit board based on AD9956, and two relay circuit boards. The acquisition circuit board, as shown in Figure 4, is responsible for data acquisition, including modules such as the front-end circuit, ADC circuit, FPGA, and power supply circuit. The acquisition circuit board comprises six channels, consisting of five voltage measurement channels and one current measurement channel. When the signal source supplies an AC signal of a specific frequency to the object under test, the acquisition circuit board can measure the electrical signals at different positions of the object, converting the analog signals into stable digital signals and transmitting them to the embedded system circuit board [25]. It concurrently undertakes the responsibility of receiving commands from the control system through the SPI bus, subsequently parsing them to configure other modules of the measurement system. This configuration facilitates calibration and data acquisition functionalities. Figure 4: Design structure diagram for the circuit board of acquisition. Figure 3: (a) EIT system for rock samples with high-density microelectrode. (b) Interface of host panel. To meet the high precision and low power consumption design requirements of the system, the control core of the acquisition board adopts Lattice's MachXO2-2000 ZE CPLD. This series of CPLDs utilizes a 65 nm low-power process with a core supply voltage of 1.2 V and standby power consumption of only 22 uW. Simultaneously, the onboard resources of the CPLD fully meet the requirements of our design. During the acquisition process, the analog signal is converted into a 32-bit digital signal through the high-precision Analog-to-Digital Converter ADS1282 on the CPLD. The ADS1282 is a low-power, low-noise, and high-precision industrial-grade Analog-to-Digital Converter (ADC) manufactured by Texas Instruments (TI). The ADC incorporates a dual-channel input multiplexer, a low-noise programmable gain amplifier (PGA), and a programmable digital filter. It exhibits outstanding noise immunity and linearity, providing a data sampling rate ranging from 250 SPS to 4000 SPS. The on-chip gain and offset calibration registers support system calibration, making it widely used in geophysical instruments, especially seismic exploration instrument acquisition systems [26]. Figure 5 illustrate the short-circuit input noise data curves of the six channels of the acquisition circuit board at a sampling rate of 1000 SPS and a gain value of 1. It is evident from the illustrated figures that the overall short-circuit noise of the acquisition channels is relatively low, with only minor variations among individual channels, thereby meeting the design specifications. The signal source circuit board represents a compact, low-power, and broad-amplitude-range signal generator autonomously developed by our research team [27]. This signal generator not only enhances the instrument's portability but also proficiently delivers undistorted excitation to rock or ore specimens with extremely low or high impedance, thereby significantly elevating the precision of the system measurements. Based on the principle of Direct Digital Synthesis (DDS), the signal source converts the digitally generated continuous-time varying signal with a certain pattern (typically a sinusoidal function) into the corresponding analog waveform signal through Digital-to-Analog Conversion. During operation, the configuration of frequency output is accomplished by transmitting frequency control words through the SPI bus, ensuring highly efficient and straightforward frequency control. The signal generator features two operational modes: constant voltage and constant current. In the constant voltage mode, it offers five levels: 10 V, 1 V, 100 mV, 10 mV, and 1 mV, while in the constant current mode, it provides five levels: 10 mA, 1 mA, 100 uA, 10 uA, and 1 uA.