Challenges involved in the use of capacitive sensors: an overview

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1 Challenges involved in the use of capacitive sensors: an overview Martin Jaiser Capacitive sensing is an attractive technology: a capacitive sensor is easy to understand, and relatively easy and cheap to manufacture compared to other technologies. Just by configuring two physical parameters the geometry of the plates and the characteristics of the dielectric the designer can produce a measurable signal. Parameters measured by capacitive sensing technology today include pressure, humidity, proximity and acceleration. In vehicles, fuel level, rain and seat occupancy sensors are also based on capacitive principles. And of course it is impossible to imagine a sophisticated consumer electronics device today without a capacitive touchscreen display. But unfortunately, for all the popularity of these sensors their use poses numerous problems for the designer to solve. Capacitive sensors are affected by temperature variations and by EMI Their operation may be impaired or distorted by environmental artefacts such as humidity or by contaminants such as dust and dirt When relatively precise measurement of variations in capacitance is required, their implementation is not easy. This article examines the traditional ways in which capactive sensing is implemented today, highlighting their advantages and drawbacks. It then introduces two new architectures from ams, and compares their operation to that of the traditional methods. 1. Traditional ways of measuring capacitance 1.1 Time-based principles: By far the most common method used today to measure capacitance is to measure time/frequency. Usually an unknown capacitance is used to modify an oscillator circuit: the change in the frequency may be converted into a measure of capacitance. Alternatively, the effect of a known impedance on the charge or discharge curve of an unknown capacitance may be measured. The easy way to measure capacitances in the pf range is to measure the time constant using a known impedance. The time constants will be in the region of a few µs, assuming that the user chooses to avoid using a noisy, high-impedance resistor. Measuring such small time constants precisely calls for the use of GHz clocks. If using a PLL circuit instead, it will have to have extremely low jitter, which means that it will be expensive. Page 1 / 9

2 The alternative to the impedance method is to use an oscillator circuit (see Figure 1): on the one hand this is easy to design with low-cost components; on the other, it has multiple sources of measurement error, because it is affected not only by changes in temperature but also in supply voltage. Fig. 1: the time-based principle of capacitance measurement 1.2 Charge-balancing method: The principle of the charge-balancing method is to transfer the charge of a fully charged, known capacitance to an unknown capacitance: then calculating the charge remaining in the known capacitance reveals the magnitude of the unknown capacitance (see Figure 2). At its simplest, this involves charging the known capacitance, and transferring the charge simply by connecting the known and the unknown capacitances together using a switch. It sounds simple but the devil is in the detail. First, the switch needs an input capacitance very much less than the value of the capacitance to be measured. When measuring capacitances in the pf range, such a switch is hard to implement. What is more, this input capacitance must be very stable over temperature. Second, the impedance of the amplifier used to measure the state of charge must be extremely high: even an amplifier with impedance of >1GΩ would add error to the measurement. Such impedance values might be achievable in a laboratory environment, but in the field, exposed to dust and humidity over potentially years of operation, an amplifier with such characteristics cannot realistically be found. Page 2 / 9

3 One exception should be mentioned: at the nanoscale, MEMS accelerometers use the principle of balancing capacitances to achieve very high measurement accuracy. Fig. 2: simplified circuit for implementing the charge-balancing method 1.3 Bridge circuits: This method uses a circuit very similar to a classic Wheatstone bridge circuit. Just like the Wheatstone bridge, it involves a comparison between two voltage dividers: a known path, and a path including the capacitance to be measured (see Figure 3). The difference between the two centre voltages is amplified and digitised. The supply to the bridge should be an AC voltage of an appropriate frequency. At the board level, this method for measuring capacitance is rarely used. This is because it calls for three equal capacitances, with limited tolerance for variance between the three. This is expensive to realise in practice. At the chip level, however, this is much easier to implement. Capacitances in the pf range are common and do not need to be realised exactly: the proportion must be known accurately, and the absolute value must be known once only, but it does not need to be trimmed to a specific value. In addition, since the on-chip capacitances have very similar temperature characteristics, the circuit s capacitance measurements are stable over temperature. Page 3 / 9

4 Fig 3: use of a bridge circuit for measuring an unknown capacitance 2.0 New ways of measuring capacitance 2.1 The Capacitance-to-Digital Converter: Technically speaking, the term capacitance-to-digital converter (CDC) can be applied to any converter using a modified charge-balancing or switched capacitor-input circuit. In practice, usually only sigma-delta converters are called CDCs. The CDC is an inversion of the usual charge-balancing circuit. Normally, a charge-balancing input stage has two known (usually equal) capacitances, and a known reference voltage. The aim is to measure a second, unknown voltage. The CDC does the opposite: it uses two known (usually equal) voltages and a reference capacitance, and measures the remaining (unknown) capacitance (see Figure 4). Page 4 / 9

5 Fig. 4: the input structure of a CDC It is best to start the explanation of the circuit at the input of the integrator. It is here that the charge values are compared. In order for the circuit to be able to sense capacitance variances, the input of the comparator needs to be zero on an average basis. This means that the comparator must switch the reference voltage as often into the upper part of the circuit as needed to compensate for the charge value in the lower part of the circuit. The more often this needs to be done, the higher the value of the measured capacitance. (This simple explanation ignores the considerable body of signal and filter theory underpinning the operation of a sigma-delta converter.) The advantage of this circuit is that it benefits from the high resolution and accuracy of sigma-delta converters, which are combined with relatively low speed. As the measured capacitance is connected to the virtual ground of the input of the integrator, there is no need to take account of any parasitic capacitances parallel to ground. In any case, jamming signals from outside the chip can often be suppressed by the large digital filters used in sigma-delta converters. Furthermore, the reference capacitance can usually be made in such a way that it has a similar temperature gradient to Page 5 / 9

6 that of the measured capacitance. In many ways, then, the CDC is able to provide accurate measurements of capacitance that are free of distortion or error. There is a problem with the CDC, however: it has no way to detect parallel low-impedance paths, which can appear over time as a result of environmental artefacts such as humidity or dirt. Such contaminants distort the device s measurements by discharging the capacitance while it is being measured. As a result, a portion of the charge does not reach the integrator, and so when environmental contamination occurs, the measured value of the capacitance ends up being lower than it should be. 2.2 The PICO-CAP method In south west Germany near the city of Karlsruhe is based a company named acam Messelektronik, a subsidiary of sensor and semiconductor manufacturer ams (Unterpremstätten, Austria, The speciality of acam is the ultra-precise time-to-digital converter, which uses a ring oscillator built from an odd number of digital inverters. The smallest measurable time constant of this ring oscillator is the delay of a single such inverter, enabling the device to operate at measurement frequencies of some 10GHz. (These on-chip high-frequency operations never reach the board level, where they could cause severe EMI/RFI problems.) Using an acam time-to-digital converter, the measurement of small capacitances in the pf range is a simple matter of measuring the discharge time, using the so called PICO-CAP method. To compensate for any temperature or process variations, the user can compare the measured discharge time with the discharge time of a reference capacitance built in the same process as the measured capacitance. Both measurements will use the chip s internal clock, so clocking variations attributable to temperature changes have no effect. But the PICO-CAP method also has other benefits. It can easily detect whether the measured capacitance is really connected, or whether there is an open wire or short circuit, since the charging of the capacitance is via a high impedance in the initial portion of the charging process (see Figure 5). The final portion is performed using the measurement impedance. By looking at the charging curve, the system can detect whether it is measuring a short circuit, an open wire or the proper capacitance. Fig. 5: how the PICO-CAP method charges a capacitance Page 6 / 9

7 To eliminate the distorting effect of parasitic capacitances, PICO-CAP provides a method for detecting and measuring their magnitude. Internal switches provide access to these parasitics, switching the measurement capacitance in a specific order, in both single ended and differential mode to ground and supply (see Figure 6). Fig. 6: internal switches enable the detection and compensation of parasitic capacitances The PICO-CAP method for measuring capacitance is fast, provides high resolution and accuracy, and is relatively easy for the system designer to implement. Having said this, there can be problems with large capacitances, which can cause the time-to-digital converter to capture large unwanted signals. The main measuring element is a comparator, and if this is saturated there is no filter after it able to calculate the real signal. This means that unwanted parallel conductances cannot be measured and detected. Fortunately there is a measurement principle that is able to handle this effect: the frequency demodulation method. 2.3 The frequency demodulation method: Many applications are exposed to considerable interference from external sources. Capacitive sensors are as described above very susceptible to such disturbance. The reason for this is that the plates of a capacitor tend to work as antennas for interfering signals. In certain applications, such as seat occupancy sensing, the measured capacitance is relatively small, while the area of the sensor plates is large. What is more, such sensors are exposed to environmental factors which do not affect the capacitance itself, but which build up a parallel conductance which affects the charging and discharging of the capacitance. Most measurement methods suffer from error when this occurs. Now ams has developed a method for handling these problems, to be implemented in a new series of standard products. The method is ready for testing with sample test chips. The chip generates a measurement frequency which is applied via an amplifier stage to the measured capacitance. The current is measured and filtered into a very narrow band using a high performance band-pass filter (see Figure 7). This filtering ensures that only the frequency ranges around Page 7 / 9

8 the excitation signal s frequency are measured; any interfering frequencies in other bands are simply ignored. After filtering the measurement signal, a synchronous demodulator splits it into phase and quadrature signals (I and Q). So the final result passed to the ADC is in phase with the voltage (resistive) and 90 out of phase (capacitive). Therefore the conductance not only does not distort the result of the capacitive measurement, it is also known. As a result, its value may be used in special applications. Fig. 7: block diagram of the frequency demodulation method The ams frequency demodulation method, then, not only suppresses any interfering conductance, but actually makes it measurable. In addition, interfering frequencies may be suppressed, or the measurement frequency may be changed to avoid interference on the frequency band in use. Furthermore, the ams chip features a separate terminal which provides a driver to any shield on the cables connected to the capacitance. By exciting it with the measurement signal, the ams chip can eliminate the effect of the supply cable s capacitance. This new method developed by ams suppresses external sources of capacitance extremely effectively: experiments with steering wheels, in which a capacitive sensor is used to detect the proximity of the driver s hands, were unaffected by switching current pulses of up to 10A (supplying a heater circuit). Page 8 / 9

9 The new frequency demodulation method, then, captures highly accurate measurements of small capacitance values, while providing very high immunity to the external sources of interference to which large capacitive sensors are inherently vulnerable. Summary Capacitive sensors are widely used in the electronics industry: they are cheap to make, easy to understand and their implementation is not especially problematic. Having said that, difficulties can arise when precise measurement of changes in capacitance is required, as the capacitance values of the sensor are normally small in the pf range. Providing precise, robust and repeatable results over years of operation in the field calls for a good understanding of the nature of the sensors and of the available methods of measuring capacitance. This article describes and compares six methods, some widely used and others newly developed. It discusses the benefits and drawbacks of the different measurement principles and provides insight into the problems each suffers from, and ways to solve them. It also provides hints about the implementation of direct measurement, and advice about parasitics and other potential sources of distortion. Biography Martin Jaiser gained his diploma in electronic engineering from the University of Karlsruhe. He then held several positions in sales and field applications engineering at Analog Devices, Rambus and Elmos. Since 2012, he has been working as a field applications engineer specializing in automotive applications at ams AG, with a particular focus on position sensing, inductive and capacitive sensing, and NFC. For further information ams AG Martin Jaiser Field Applications Engineer Tel: +43 (0) Page 9 / 9

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