The design of a small capacitance measurement circuit using capacitance detection chip PS021
公開時間: 2020-08-26 11:17:31
The capacitive sensor is a device that converts the measured change into the capacitance change, and has been widely used in many fields. It has the advantages of simple structure, good temperature stability, high resolution, 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～10 pF), and there is the influence of the stray capacitance and parasitic capacitance of the sensor and its connecting wires, the measurement circuit must meet the requirements of large dynamic range, high and low measurement sensitivity Requirements for noise and anti-spurious.
At present, there are great difficulties in measuring the capacitance below 10 pF at home and abroad. The measurement circuit mostly adopts the charge transfer method or the AC method, which converts the capacitance into voltage or current. The circuit is often affected by the charge injection effect of the electronic switch. And it is difficult to resolve the contradiction between improving the measurement speed and improving the resolution.
This article intends to use the general capacitance detection chip PS021 chip of German ACAM company to design the small capacitance measurement circuit. The chip converts capacitance measurement into accurate time measurement. The internal algorithm can well suppress the influence of parasitic capacitance on the measurement result. The integrated temperature compensation module of the chip can also ensure good stability, which can be reached at a refresh frequency of 10 Hz. Effective accuracy of 6 aF, the highest refresh rate can reach 50 kHz, high precision and high refresh rate can ease the contradiction between measurement speed and resolution.
1 Micro capacitance measurement module
The block diagram of the overall design principle is shown in Figure 1. It is mainly composed of pressure-bearing shell, power management circuit, PS021 chip, and single-chip microcomputer.
The PS021 chip converts the capacitance signal generated by the pressure-bearing shell change 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 is collected, it is transmitted to the computer through the infrared module In, use the VisualBasie6.O soft panel to display the measurement result curve; the power management department can provide time-sharing and controllable power supply to MSP430 and PS021.
1.1 Main features of PS021
The PS021 chip is based on TDC technology, making it a fully integrated ultra-low power, ultra-high precision measurement chip. This digital measurement principle provides very high measurement flexibility, has a wide measurement range, and the effective accuracy can reach up to 22 bits. The chip can communicate with the microcontroller or DSP through the 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, acceleration sensors, and gap measurements.
1.2 Measuring principle
The sensing capacitor and the reference capacitor are connected with the resistor to form a low-pass filter. PS021 controls the analog switch to turn on and off, the on-time of the two is equal, and the two capacitors are charged and discharged in turn during the on-time in turn. The time to discharge to the same voltage will be measured by a high-precision TDC.
Reference capacitance charge and discharge measured τ1=RCref, sensor capacitance charge and discharge measured τ2=RCsensor, according to the internal algorithm of the chip, τ2/τ1=Csensor/Cref, where Cref is a known capacitance, and finally a 16-bit effect data is obtained. Realize the measurement of sensor capacitance. PS021 controls the analog switch so that charging and discharging are repeated on the two capacitors, and then the ratio of the capacitance measurement values is calculated. 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 basically overlap by translation; when the measured capacitance changes, the ns-level interval △t in the figure corresponds to the difference between the two capacitances. The value △C, or the change of the capacitance △C, causes the delay of the discharge time △t.
2 Measuring 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 state of the system is designed as 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. If the trigger signal is always high within 15 s, the system enters the state of cyclic acquisition and storage. In order to obtain a complete capacitance signal curve including before and after triggering, once the capacitance signal reaches the preset trigger value, the system enters the trigger state and stores the capacitance signal in the flash memory. After 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 microcontroller MSP-430FG4618 of American TI Company, which has 8 kB RAM and 113 kB of flash memory. At work, after the SPI communication is correct, the microcontroller is responsible for sending read and write commands to set PS021 and control the start and stop of its measurement, and receive and store digital signals to achieve digital internal triggering. RAM cyclically stores the sampled data before the trigger, and stores the data in the flash memory after the trigger. When the acquisition is completed, the data storage realizes a negative delay of 2 kB.
2.3 Power Management Module
In order to achieve low power consumption design, in each working link of the system, the single-chip microcomputer controls the power supply switch status of different modules in time, provides power for them or cuts off power, so as to achieve the purpose of power saving. As shown in Figure 4, the circuit power supply selects the LDO chip LP5966 to output two independent 3.3 V voltages: VDD=3.3 V supplies the power supply voltage of the single-chip microcomputer, its power supply enable is always on, LVDD=3.3 V supplies PS021, and ONA controls its switch Status: The charge pump chip MAX1595 is selected to output HVDD=5 V to PS021, and the ONB controls its switching state; the power supply of the two chips is directly provided by the battery.
2.4 Data reading module
The data transmission adopts the GP2W0116YPS infrared module produced by SHARP, which has the characteristics of low power consumption, strong anti-interference ability and high input sensitivity. It can realize wireless data communication with computer, it supports infrared IrDA1.2 standard, and the data transmission rate is 2.4 kb/s～115.2 kb/s.
3 System software design
In the system design, the control function is mainly realized through software, and the data collection and transmission are controlled. The single-chip microcomputer is programmed in C language, which makes the program readable and easy to transplant. The main program flow chart is shown as in Fig. 5.
The main program structure is very simple, the system power is powered on, the watchdog is turned off, the I/O port is initialized, and the low power consumption state LPM4 is entered, waiting for power-on interruption, computer reading request interruption, and returning to the low power consumption state LPM4 after the interrupt response is completed .
When the circuit is used in the capacitive manometer of the internal ballistic pressure test system, the 22 cm3 test system is placed in the explosion pressure field.
The circuit is used in the internal ballistic pressure test system and has achieved good results. The circuit completely records the change curve of the capacitance signal before and after the explosion at a refresh frequency of 10 kHz. At the same time, the circuit board adopts a six-layer design with an area less than 2.5 cm2 and a low power consumption current of 0.04μA, which embodies the advantages of low power consumption and small size. The measurement scheme is very flexible and can be modularized. The circuit design can be transplanted to 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.