Principle of single-chip capacitance measurement solution PCAP01

1 Introduction

For the measurement of capacitance sensors, the traditional circuit method has its insurmountable limitations. The complex analog circuit design and the difficult to expand capacitance measurement range will bring great resistance to development.

The PICOCAP® measurement principle patented by German acam company has provided a revolutionary breakthrough in capacitance measurement. In 2011, the company launched the latest single-chip capacitance measurement solution PCAP01 with an internal DSP microcontroller, which will bring capacitance measurement to an unprecedented level.

2. Overview

PCap01 is a single-chip solution for capacitance measurement and digital conversion with a single-chip microcomputer processing unit. The measurement range of this chip covers from a few fF to hundreds of nF, and it can be very easily configured to meet the needs of various applications. PCap01 is suitable for ultra-low power measurements as low as a few uA, high-performance measurements with high accuracy up to 21 effective bits, and fast measurements up to 500,000 times per second. This chip provides a perfect combination of high-precision measurement, low-power measurement and fast measurement applications. The sensor data can be calibrated inside the chip and then transmitted through the SPI or IIC data serial interface. In addition, the chip can also send PWM/PDM output voltage signals through the IO port. The remaining IO ports can be used as interrupt pins, horizontal alarm signal pins or ordinary IO ports.

PCap01 has a very small QFN package size and requires only a very small number of external components (at least 2 external dual-pass capacitors are required), making the entire system design very compact and cost-effective, suitable for a wide range of capacitance measurements.

3. Introduction to PICOCAP measurement principle

The PICOCAP measurement principle represents a revolutionary new approach to capacitance measurement. In this principle, a sensor capacitor and a reference capacitor are connected to the same discharge resistor, forming a low-pass filter.

The capacitor is first charged to the supply voltage and then discharged through the resistor. The discharge to a controllable threshold voltage level is recorded by the very high precision time-to-digital converter (TDC) inside the chip.

This measurement process is repeated on the sensor and reference capacitors, interleaved, using the same resistors. The result of the calculation is the ratio of the measurements, which is related to the temperature dependence of the resistors and the comparator. The values ​​of the sensor and reference capacitors should be chosen to be in the same range to reduce the gain offset. In practice, there is no limit to the size of the measured capacitor. The sensors can be from 0fF to tens of nF. PICOCAP also supports the measurement of differential capacitance sensors with internal linear compensation.

4. Main features of PCAP01 chip

The Pcap01 chip is a single-chip capacitance measurement solution with the following features:

One chip can be used for many applications, with high measurement flexibility:

a) Low measurement power consumption, only 2 µA at 10Hz

b) Measurement accuracy up to 22 effective digits, 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

Ultra-low gain and offset drift

18-bit high-resolution temperature measurement

48-bit DSP, 4k byte OTP, 4k byte SRAM

Internal or external clock oscillator

Supports up to 6 IO ports

IIC, SPI, PWM, PDM interface

Wide supply voltage range from 2.1 V to 3.6 V

Wide operating temperature range (-40℃ to +125℃)

QFN32 or QFN24 package

Internal structure schematic diagram:

Pcap01 takes advantage of the high precision of the PICOCAP® measurement principle to achieve an unprecedented level of capacitance measurement. Depending on the size of the sensor and reference capacitor, as well as the selected measurement mode, we have the following measurement data. This measurement data is a typical measurement noise accuracy vs. data output frequency. Our test was completed using the Pcap01 evaluation system and a 10pF reference capacitor and a 1pf span loading capacitor. The chip voltage is V = 3.0 V:

As can be seen in the table above, we have given two cases, floating mode and grounded mode. When the drift mode is applied, the RMS noise measured at 5Hz output is 6aF with full compensation, and the effective bits of the measurement are as high as 20.7 bits! The relative relationship between accuracy and speed is given in the table when different settings are selected for different measurement frequencies. Of course, the effective resolution of the measurement will vary depending on the size of the base capacitor.

When applying compensation mode for high precision measurements, very low gain and zero drift can be achieved. The capacitor can be connected to ground, drift mode. The sensor and reference capacitor are selected to the discharge network by the internal integrated analog switch. In addition, due to the patented circuit and compensation algorithm, parasitic capacitance can be compensated internally. The compensation result can reach only 0.5 ppm /K gain drift over the temperature range. This is much better than the internal drift of most sensors themselves.

Sensor connection method:

For the measurement of capacitive sensors, the chip provides a very flexible connection method. The typical connection method is shown below:

In the chip, users can choose whether to use the internal integrated discharge resistor to measure the capacitance or to connect an external discharge resistor for measurement. The connection method is shown in the figure below:

Lead wire compensation:

In capacitance measurement, the influence of the parasitic capacitance of the wire on the entire measurement cannot be ignored. Especially when the wire is long, the influence of the parasitic capacitance of the wire will have a fatal impact on the measurement result. In Pcap01, the parasitic capacitance of the sensor wire can be effectively compensated:

By connecting the sensors in the above way, the parasitic capacitance of the wires connecting the two ends of the sensors can be compensated, eliminating the influence of the wires on the measurement results. If wire compensation is desired, the three measurements in drift mode need to be performed as follows:

If you need high stability and high precision measurements, we recommend connecting the sensor in drift mode for full compensation. Of course, if the wires are very short and the temperature performance requirements for the measurement are not critical, you can also use only internal compensation, which can be applied in both ground and drift modes: