For the measurement of capacitance sensor, the traditional circuit mode has its insurmountable limitations. The design of analog circuit is complex, and the measurement range of capacitance is difficult to expand, which will bring great resistance to development.
PICOCAP patent of acam company in Germany? The measurement principle provides a revolutionary breakthrough for capacitance measurement. 20 1 1 introduced the latest single chip capacitance measurement scheme PCAP0 1 with built-in DSP single chip microcomputer, which improved the capacitance measurement to an unprecedented level.
2. Overview
PCap0 1 is a single-chip capacitance digital conversion scheme, which uses a single-chip processing unit to measure capacitance. The measurement range of this chip is from several fF to hundreds of nF, and it can be configured very simply to meet the needs of various applications. PCap0 1 is not only suitable for ultra-low power measurement as low as a few uA, but also suitable for high-precision and high-performance measurement with 2 1 significant bits, and can also carry out rapid measurement up to 500,000 times per second. The chip perfectly combines high-precision measurement, low-power measurement and fast measurement applications. Sensor data can be calibrated in the chip and then transmitted through SPI or IIC data serial interface. In addition, the chip can also send PWM/PDM output voltage signal through IO port. The remaining IO ports can be used as interrupt pins, horizontal alarm signal pins or ordinary IO ports.
PCap0 1 has a very small package size of QFN, and only few external components (at least two external double-pass capacitors) are needed, which makes the design of the whole system very compact, reduces the cost and is suitable for large-scale capacitance measurement.
3. Introduction of picocard measurement principle
Principle of capacitance measurement scheme pcap01src = "/uploadfile/dygl//2012/20121408 551925.
The principle of fur cap measurement provides a new revolutionary method for capacitance measurement. In this principle, the capacitor of the sensor and the reference capacitor are connected to the same discharge resistor to form a low-pass low-pass filter.
The capacitor is first charged to the supply voltage and then discharged through the resistor. The high-precision time-to-digital converter TDC inside the chip discharges the recording to a controllable voltage level.
The measurement process will be repeated and staggered on the sensor and reference capacitor, and the same resistance will be applied. The calculated result is the measured ratio result, which is related to the temperature correlation between the resistor and the comparator. The values of sensor and reference capacitor should be selected in a unified range to reduce gain offset. In fact, there is no limit to the size of the measured capacitance. The sensor can be almost from 0fF to tens of nf. PICOCAP also supports the measurement of differential capacitance sensors with internal linear compensation.
4. Main characteristics of pcap01chip
Pcap0 1 chip is a single-chip capacitance measurement scheme, which has the following characteristics:
One chip can be used in many applications, and the measurement flexibility is very high:
A) Low measurement power consumption, minimum 2? At 10Hz. A
B) the measurement accuracy is up to 22 significant bits and 4 aF rms accuracy.
C) the measurement frequency can be up to 500 kHz.
Very wide capacitance measurement range, from a few fF to hundreds of nF.
Ultralow gain and offset drift
18 bit high resolution temperature measurement
48-bit DSP, 4k byte OTP, 4k byte SRAM
Internal or external clock oscillation
Up to 6 IO ports can be supported.
IIC, SPI, PWM, PDM interface
Wide power supply voltage range: 2. 1 V to 3.6 v.
Wide operating temperature range (-40℃ to+125℃)
QFN32 or QFN24 package
Schematic diagram of internal structure:
Pcap0 1 made a slight cap? The high precision advantage of the measurement principle makes the capacitance measurement reach an unprecedented level. According to the different sizes of sensors and reference capacitors and the different measurement modes selected, we have the following measurement data. This measurement data is a typical relationship between measurement noise accuracy and data output frequency. Our test is completed by using Pcap0 1 evaluation system, 10pF reference capacitor and 1pf span load capacitor. The voltage of the chip is V = 3.0 V:
Principle of capacitance measurement scheme pcap01src = "/uploadfile/dygl//2012/201121408 5520973.jpg".
As can be seen from the above table, we give two cases, namely, floating drift mode and grounding mode. When the drift mode is applied and fully compensated, the root mean square noise measured at 5Hz output is 6aF, and the effective bit is as high as 20.7 bits! When different settings of different measuring frequencies are selected, the relative relationship between accuracy and speed is given in the table. Of course, the effective resolution of the measurement will vary with the size of the basic capacitance.
When the compensation mode is used in high-precision measurement, it can make the measurement have very low gain and zero drift. Capacitors can be connected in ground and drift mode. The sensor and reference capacitor are selected to enter the discharge network through the internal integrated analog switch. In addition, due to the patented circuit and compensation algorithm, parasitic capacitance can be compensated internally. In this temperature range, the compensation result can only reach 0.5 ppm /K gain offset. This is much better than the internal offset of most sensors themselves.
Sensor connection mode:
For the measurement of capacitance sensor, the chip provides a very flexible connection mode, as follows:
In the chip, users can choose whether to use internal integrated discharge resistor or external discharge resistor to measure capacitance. The connection mode is shown in the following figure:
Principle of capacitance measurement scheme pcap01src = "/uploadfile/dygl//2012/201121408 5521352.
Wire compensation:
In capacitance measurement, the influence of conductor parasitic capacitance on the whole measurement can not be ignored. Especially when the wire is long, the influence of parasitic capacitance of the wire will have a fatal impact on the measurement results. Pcap0 1 can effectively compensate the parasitic capacitance of the sensor line;
Principle of capacitance measurement scheme pcap01src = "/uploadfile/dygl//2012/201121408 5521452.
Through the sensor connection mode, the parasitic capacitance of the wires connecting the two ends of the sensor can be compensated, and the influence of the wires on the measurement results can be eliminated. Then, if line compensation is needed, three measurements need to be made in drift mode, as shown below:
For high stability and high precision measurement, we suggest connecting the sensor in drift mode to obtain complete compensation. Of course, if the conductor is very short and the requirements for measurement performance and temperature performance are not strict, only internal compensation can be used, which can be applied to grounding and drift modes:
Temperature measuring device RDC:
Pcap0 1 has a very powerful temperature measuring unit inside, and users can choose to use an external temperature sensor for measurement, or use an internally integrated aluminum resistor as the temperature sensor resistance. The temperature coefficient Tk of internal resistance of aluminum is ≈ 2800 ppm/k, which can be fully met by general temperature measurement. Of course, if the requirements for temperature measurement are high, it is necessary to connect a high-precision temperature sensor (such as PT 1000) for measurement.
Principle of capacitance measurement scheme pcap01src = "/uploadfile/dygl//2012/201121408 5522367.jpg"
Using an external temperature sensor
Using an internal temperature sensor
48-bit powerful DSP processing unit;
There is a 48-bit signal processing unit inside the chip, which will process the information of CDC (capacitance measurement) and RDC (resistance measurement), obtain the measurement data, and give the results to the chip output port. The obtained rough data will be stored in internal RAM with OTP or SRAM, which can be used by customers to write their own programs. After the measurement is completed, the chip will definitely enter SRAM or OTP to execute internal programs. The simplest thing is to read and write the measurement results to the chip. Then a lot of work can be done in the program, and the functions of ordinary single chip microcomputer can be realized in the DSP processing unit inside the chip. Acam provides different versions of firmware for chips, which is suitable for different types of applications. For example, firmware for measuring temperature and humidity is provided. When you apply the chip to temperature and humidity measurement, you can easily correct and compensate the temperature and humidity, and there is an integrated calculation software inside, which is more convenient for customers to develop.
Pressure firmware is another integrated firmware for pressure sensor applications and other common applications. It has a mathematical algorithm of high-order polynomial approximation, linear compensation and temperature compensation algorithm. Besides the application of pressure sensor, these compensation algorithms can be called in many other sensor applications, and the implementation is very simple.
In addition, there are standard firmware, which can measure common capacitors and give results, and there are many effective communication interfaces and other necessary functions. For more detailed information about DSP and internal programming, please refer to the DSP technical manual of Pcap0 1. If you have any functional requirements, please contact the technical support staff of Shiqiang Telecom, the general agent of acam in Greater China, which will provide you with a relatively complete consultation and suitable internal firmware program.
5. Chip software and hardware design scheme
The above picture shows the typical hardware design scheme of Pcap0 1, which is suitable for ordinary capacitive sensors (the sensor is not marked in the picture). The output mode is SPI through communication mode. You can choose to use an external temperature sensor for temperature measurement, and of course you can also use an internal integrated temperature measuring resistor. The overall circuit design is very simple, and the number of required components is very small. It greatly reduces the development difficulty of the whole system.
5. Application areas:
Capacitance-to-digital converters have a wide range of applications, and the main application fields are as follows:
Temperature and humidity sensor, pressure sensor and liquid level sensor.
Displacement sensor, angle sensor, acceleration sensor, weighing scale, etc. ..
6. Concluding remarks
To sum up, the Pcap0 1 single chip scheme will make your overall scheme design simpler, and the capacitance measurement performance will be better, superior and more reliable. Innovative single-chip circuit and freely selected firmware with different compensation methods, such as linear compensation and temperature linear compensation, not only improve the measurement level of the circuit, but also further improve the measurement performance of the sensor itself.
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