Design of Small Capacitance Measurement Circuit Using Capacitance Detection Chip PS021


Capacitive sensors are devices that convert measured changes into capacitance changes, and are widely used in many fields. It has the advantages of simple structure, good temperature stability, high resolution, and good dynamic response, and can work under harsh conditions such as high temperature, radiation, and strong vibration.

Since the capacitance signal output by the capacitive sensor is very small (1 fF to 10 pF), and there is the influence of the stray capacitance and parasitic capacitance of the sensor and its connecting line, the measurement circuit must meet the requirements of large dynamics. Range, high and low measurement sensitivity Requirements for noise and spurious immunity.

At present, it is very difficult to measure the capacitance below 10 pF at home and abroad. The measurement circuit mostly adopts the charge transfer method or the AC method to convert the capacitance into voltage or current. Circuits are often affected by the charge injection effect of electronic switches. The contradiction between improving the measurement speed and improving the resolution is also difficult to solve.

This paper plans to use the general capacitance detection chip PS021 chip of ACAM company in Germany to design a small capacitance measurement circuit. The chip converts capacitance measurements into precise time measurements. The internal algorithm can well suppress the influence of parasitic capacitance on the measurement results. The temperature compensation module integrated in the chip can also ensure good stability, which can be achieved at a refresh frequency of 10Hz. The effective accuracy is 6 aF, and the maximum refresh rate can reach 50 kHz. The high precision and high refresh rate can alleviate the contradiction between measurement speed and resolution.

1 Microcapacitance measurement module

The overall design principle block diagram is shown in Figure 1, which is mainly composed of a pressure-bearing shell, a power management circuit, a PS021 chip, and a single-chip microcomputer.

The PS021 chip converts the capacitance signal generated by the change of the pressure shell into the corresponding 16-bit digital quantity; the MSP430 single-chip microcomputer controls the PS021 through the SPI interface, and stores the data in the MSP430 flash memory; after the data collection is completed, it is transmitted to the computer through the infrared module. The VisualBasie6.O soft panel displays the measurement result curve; the power management section can provide time-sharing controllable power supply for MSP430 and PS021.

1.1 Main features of PS021

Based on TDC technology, PS021 chip is a fully integrated ultra-low power consumption, ultra-high precision measurement chip. This digital measurement principle offers very high measurement flexibility, a wide measurement range and an effective accuracy of 22 bits. The chip can communicate with a microcontroller or DSP through an SPI-compatible serial port. It also has an independent temperature measurement port and parasitic capacitance compensation circuit. It is a high-end chip that can be used for pressure sensors, accelerometers, and gap measurement.

1.2 Measurement principle

The sense and reference capacitors are connected with resistors to form a low-pass filter. PS021 controls the on-off of the analog switch, the on-time of the two is equal, and the two capacitors are charged and discharged in turn within the on-time. The time to discharge to the same voltage will be measured by a high precision TDC.

The actual measured charge and discharge of the reference capacitor τ1=RCref, the actual measured charge and discharge of the sensor capacitor τ2=RCsensor, according to the internal algorithm of the chip, τ2/τ1=Csensor/Cref, where Cref is the known capacitance, and finally the effect data is obtained in 16 bits. Realize the measurement of sensor capacitance. PS021 controls the analog switch to charge and discharge the two capacitors repeatedly, and then calculates the ratio of the measured values of the capacitors. As shown in Figure 2, the graph is obtained by shifting the on-time of the charge-discharge curve of one of the two capacitors on the time axis. The ns-level interval in the figure corresponds to the difference between the two capacitors. When the sensor is in the initial state, the capacitance of the reference terminal is basically equal to the initial capacitance of the sensor, and the charge and discharge curves of the two can be basically overlapped by translation; when the measured capacitance changes, the ns-level interval Δt in the figure corresponds to the two capacitances difference value. The value of ΔC, or the change in capacitance ΔC, will cause a delay in the discharge time Δt.

2 Measurement system circuit design

2.1 System State Design

In order to achieve low power consumption, the system enters an ultra-low power consumption state after power-on, and requires an external level signal to wake up. The system state design is shown in Figure 3. In order to avoid system malfunction, when the capacitance signal needs to be measured, the trigger signal is set to high level. If the trigger signal is always at a high level within 15 s, the system enters the cyclic acquisition and storage state. In order to obtain the complete capacitance signal curve including before and after the trigger, once the capacitance signal reaches the preset trigger value, the system enters the trigger state and stores the capacitance signal in the flash memory. When the flash memory is full, the FIFO data in the RAM is imported into the flash memory reserved address.

2.2 Control Module

The measurement circuit needs a control chip to control the reading and writing of data. Because the peripheral interface of PS021 is SPI, the control part adopts the ultra-low power consumption microcontroller MSP-430FG4618 of American TI Company, with 8 kB RAM and 113 kB flash memory. When working, after the SPI communication is correct, the microcontroller is responsible for sending read and write commands to set up the PS021 and control the start and stop of its measurement, and to receive and store digital signals to achieve digital internal triggering. The RAM cyclically stores the sampled data before the trigger and stores the data in the flash memory after the trigger. After acquisition is complete, data storage achieves a negative latency of 2 kB.

2.3 Power Management Module

In order to realize the design of low power consumption, in each working link of the system, the single-chip microcomputer controls the power switch states of different modules in time to supply or cut off the power supply, so as to save power. As shown in Figure 4, the circuit power supply uses LDO chip LP5966 to output two independent 3.3V voltages: VDD=3.3V to provide the power supply voltage of the microcontroller, its power supply is normally open, LVDD=3.3V to supply power to PS021, and ONA controls its switch Status: Select the charge pump chip MAX1595 to output HVDD=5V to PS021, ONB controls its switching state; the power supply of the two chips is directly provided by the battery.

4 Measurement

When the circuit is used for the capacitive manometer of the internal ballistic pressure test system, the 22 cm3 test system is placed in the explosion pressure field.

5 Conclusion

This circuit is used in an internal ballistic stress test system with good results. The circuit completely records the change curve of the capacitance signal before and after the explosion with a refresh rate of 10 kHz. At the same time, the circuit board adopts a six-layer design, the area is less than 2.5cm2, and the low power consumption current is 0.04μA, which reflects the advantages of low power consumption and small size. The measurement solution is very flexible and can be modularized. The circuit design can be transplanted into the design of many capacitive sensors, which reduces the difficulty of product development and is of great significance for accelerating product development and reducing production costs.