Sensors and Actuators A 97±98 (2002) 15±20 A smart wind sensor using thermal sigma-delta modulation techniques Ko® A.A. Makinwa*, Johan H. Huijsing Electronic Instrumentation Laboratory, DIMES, Delft University of Technology, Mekelweg 4, 2628 CD Delft, The Netherlands Received 28 June 2001; received in revised form 11 December 2001; accepted 11 December 2001 Abstract A smart wind sensor realized in a standard CMOS process combines a two-dimensional thermal ¯ow sensor and three auto-zeroed comparators on a single chip. The comparators form the basis of three thermal sigma-delta modulators that control and digitize the heat distribution in the chip. One modulator maintains the chip at a constant temperature above that of the ¯ow, while the other two cancel orthogonal components of a ¯ow-induced temperature gradient. The bit-stream outputs of the modulators are decimated off-chip and used to determine wind speed and direction. Wind tunnel tests show that the sensor is capable of measuring wind speed and direction with an accuracy of 4% and 28, respectively, over the range 2±18 m/s. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Flow sensor; Thermal sensor; Sigma-delta modulation 1. Introduction Wind speed and direction can be measured by two-dimensional (2-D) thermal ¯ow sensors realized in silicon. Such ``wind sensors'' typically consist of a heated chip [1] or membrane [2,3], which is non-uniformly cooled by the wind. The resulting ¯ow-induced temperature gradient in the sensor is measured by on-chip thermopiles and from this information both wind speed and direction can be determined. In typical wind sensors, the output of the thermopiles is small (in the millivolt range), which requires the use of precision (low-offset) interface circuitry. When implemented off-chip, however, such circuitry adds signi®cantly to the cost and complexity of the total sensor system. Furthermore, transporting such small signals off-chip signi®cantly increases their susceptibility to external interference, e.g. from mains and RF sources. For these reasons, efforts have been made to realize ``smart'' thermal ¯ow sensors with cointegrated interface electronics [4±6]. In these designs, a precision ampli®er boosts the thermopile signals to levels compatible with the input range of an analog-to-digital converter (ADC). Since, both a precision ampli®er and an ADC are required, however, this interface architecture requires considerable chip area to implement. * Corresponding author. Tel.: 31-15-278-5747; fax: 31-15-278-5755. E-mail address: k.makinwa@ieee.org (K.A.A. Makinwa). Alternatively, the heat distribution in the sensor can be controlled such that ¯ow-induced temperature differences are cancelled [7]. This technique has the added advantage that the heat distribution in the sensor will always be centered, even in the presence of possible thermal asymmetry introduced during the sensor's fabrication [8]. As described in [7], the heat distribution in the sensor can be controlled using thermal sigma-delta (TSD) modulation techniques. From the resulting heat distribution, both wind speed and direction could be accurately determined. Only three low-offset comparators and some control logic were required to implement the off-chip interface circuitry, resulting in an area-ef®cient architecture. Furthermore, the interface outputs are digital signals (bit-streams) which eliminates the need for an explicit ADC. In this paper, a smart wind sensor is described which consists of a wind sensor and three low-offset comparators realized on a single CMOS chip. The on-chip comparators, together with a few external components, are used to realize the TSD architecture proposed in [7]. The ®rst section of the paper describes the CMOS realization of the wind sensor. This is followed by a description of the interface electronics. Next, the design of the on-chip comparators is described: an auto-zeroed topology was used to achieve suf®ciently low offset in a CMOS process. Finally, the results of electrical measurements and wind tunnel tests on the sensor are presented. 0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 0 3 4 - 1 16 K.A.A. Makinwa, J.H. Huijsing / Sensors and Actuators A 97±98 (2002) 15±20 2. The CMOS wind sensor The wind sensor chip consists of a square silicon substrate on which four heaters, four thermopiles and a central diode have been integrated, as shown in Fig. 1. The interface electronics is located in the unused space in the middle of the chip. For full CMOS compatibility, mechanical robustness and low cost, the chip is not micromachined, i.e. it is wafer thick. Although extra sensitivity may be obtained by etching away part of the substrate, as for example in [2,3], experience with a commercial wind sensor shows that this is not required for meteorological applications [9]. The wind sensor chip is protected from direct contact with the air¯ow to be measured by gluing it to a thin (0.25 mm) ceramic disk, as shown in Fig. 2. The air¯ow is then passed over the opposite side of the disk. Since, ceramic is a good thermal conductor, a measurable temperature gradient will still be induced in the heated chip. Wire-bonded leads connect the chip to a ¯ex foil, which in turn is connected to external circuitry. An opaque encapsulant (glob top) shields the chip from light and protects the sensor and the fragile leads from moisture ingress and oxidation. The thermopiles sense orthogonal components of the ¯ow-induced temperature gradient. For increased sensitivity, thermopiles on opposite sides of the chip are connected in series since they measure the same component of the temperature gradient. Each thermopile consists of 12 pdiffusion/Al thermocouples and has an estimated sensitivity of 6 mV/K and a nominal resistance of 60 KO. The p arms of each thermopile are realized in a common n-well, the resulting pn junctions are then reverse-biased to electrically isolate the thermopiles from the other on-chip circuitry. The wind sensor chip is heated by four polysilicon resistors, each with a nominal resistance of 200 O. Since, the heaters are large (0.4 mm2) the mismatch in heater resistance is small, in the CMOS process the mismatch used was less than 0.5%. The central diode (implemented as a diode-connected substrate PNP transistor) is used to measure the average absolute temperature of the chip. When forward biased, its base-emitter voltage has a temperature coef®cient of approximately 2 mV/K. 3. Interface electronics 3.1. Operating principle Each component (dTns or dTew) of the ¯ow-induced temperature gradient may be accurately cancelled (and, thus measured) by a differential TSD modulator. The topology of one of these modulators is shown in Fig. 3. Here, Rs is the thermal resistance between the heaters, and R¯ow,n and R¯ow,s are ¯ow-dependent resistors that model the sensor's heat loss to the ¯ow. Intuitively viewed, the modulator attempts to drive dTns towards zero by applying heat pulses to either the north or south heaters. These pulses will be low-pass ®ltered by the various thermal resistances and the sensor's thermal capacitance Cth. Then dT ns 0, since the clock frequency greatly exceeds the ®lter's cut-off frequency and the differential heating power dPns Pn Ps balances the sensor's asymmetric heat loss to the ¯ow. The modulator's bit-stream output bns then represents the normalized differential power dPns/Pref, where Pref is the power dissipated during a heat pulse. In a similar manner, the east±west temperature difference is cancelled by a second TSD modulator which applies heat pulses to either the east or west heaters. Fig. 1. Schematic layout of a CMOS wind-sensor. Fig. 2. Schematic cross-section of the packaged wind-sensor chip. Fig. 3. Block diagram of the north±south differential TSD modulator. K.A.A. Makinwa, J.H. Huijsing / Sensors and Actuators A 97±98 (2002) 15±20 17 Fig. 5. Block diagram of the comparator. Fig. 4. Block diagram of the wind-sensor system. 3.2. System architecture A block diagram of the wind-sensor system is shown in Fig. 4. Each differential modulator consists of a latched comparator, connected to each thermopile pair, which drives the appropriate heaters via external switches (S2 or S3) from a reference voltage Vref. The resistors Rnom (with a resistance equal to the nominal heater resistance) stabilize heater power dissipation against variations in heater resistance due to temperature and process spread [5]. With S1 closed, two of the four heaters will always be ``on'' at any given time and, therefore, the total heat dissipated in the sensor will be constant. The differential modulators thus operate the sensor in a constant power (CP) mode. The sensor may also be operated in a constant temperature difference (CTD) mode, where it is maintained at a constant overheat with respect to the ¯ow. Compared to operation in CP mode, this results in improved transient response, since the sensor's temperature no longer has to change (and settle) in response to sudden changes in wind speed. To facilitate operation in CTD mode, a voltage proportional to the ¯ow temperature Tamb plus a constant overheat DT, is generated with the help of an off-chip diode. Comparator C1 then compares this voltage with the output of the onchip diode. This comparator, together with the thermal ®lter constituted by the entire chip, forms a third, common-mode TSD modulator that (via S1) interrupts the CP heating process in such a way as to maintain T chip T amb DT. 4. Low-offset comparator design 4.1. Design considerations For the differential modulators to operate correctly, the comparator's input offset must lie within the output range of the thermopile pairs. Since, silicon is a good thermal con- ductor, the on-chip temperature differences are small (milliKelvins) and so are the thermopile signals (microvolts). Thermal asymmetries introduced by the sensor's packaging also introduce offset, typically equivalent to a comparator offset of 20 mV [1]. In order to reduce comparator offset below this level, an auto-zero scheme was used. As an added bene®t, this scheme also eliminates 1/f noise. 4.2. Comparator architecture The block diagram of the low-offset comparator is shown in Fig. 5. It uses a classic architecture [10,11] in which the output-referred offset of the ®rst stage and the input-referred offset of the second stage are stored on the capacitors in an auto-zero phase. The main source of residual offset is then the mismatch in charge injection from the switches around the second stage. A fully differential topology was used to cope with the expected ground ``bounce'' produced by the large heater currents. The input stage of the comparators consists of large PMOS devices for low noise and low initial offset. An output latch generates TTL-compatible levels and holds the comparator's previous state during the auto-zero phase. 5. Results The wind sensor was implemented in a 1.6 mm CMOS process, Fig. 6. The interface electronics dissipates about 1 mW from a 5 V supply. In contrast, the heaters dissipate 0.6 W when the sensor is operated in CP mode and between 0.4 and 0.6 W (depending on wind speed) when the sensor is operated in CTD mode. Therefore, the heat produced by the on-chip electronics does not interfere signi®cantly with the sensor's operation. A power spectrum of the output of a differential modulator (CP mode, zero ¯ow) is shown in Fig. 7. The modulator clock frequency is 8192 Hz. The noise shaping produced by the thermal low-pass ®lter is clearly visible. At low frequencies, the ®lter's ®nite gain causes the quantization noise spectrum to ¯atten. No 1/f noise is visible, demonstrating the effectiveness of the auto-zero scheme. The non-zero dc component is due to mainly thermal 18 K.A.A. Makinwa, J.H. Huijsing / Sensors and Actuators A 97±98 (2002) 15±20 Fig. 6. Chip photo of the wind-sensor (4 mm 4 mm). Fig. 7. Hann windowed, 16 averaged, 32,768 point, power spectrum of the bit-stream bns. asymmetry in the sensor (it varies linearly with Pref). A resolution of 11-bits relative to Pref (300 mW) in a 1 Hz bandwidth was obtained. In contrast, the sensor has a bandwidth (for ¯ow variations) of about 0.1 Hz [1]. For the wind tunnel tests, the sensor was built into the aerodynamic housing of a commercial wind sensor [9]. The resulting assembly is shown in Fig. 8, the ceramic disk bearing the chip is mounted ¯ush with the upper surface of the small inner disk. The two larger outer disks guide the air¯ow over the inner disk in a well-de®ned manner. The output of the differential modulators was decimated externally by a 5000-tap sinc2 ®lter, followed by a 10-tap movingaverage ®lter. In CTD mode, the decimated modulator outputs are sinusoidal functions of wind direction, Fig. 9. The amplitude and offset of these functions are proportional to the square root of ¯ow speed, Fig. 10. Using these relationships, wind speed and direction were computed from the output of the differential modulators [7]. The results are shown in Fig. 11. It may be seen that the errors are random and less than 4% and 28 in wind speed and direction, respectively. These results are similar to the performance of similar sensors using external interface electronics [8,9]. This shows that the presence of the on-chip electronics does not affect the performance of the sensor. The sensor was also tested in CP mode, i.e. with the common-mode modulator disabled. In this mode, the decimated modulator outputs are also sinusoidal functions of wind direction. However, the sensor's amplitude characteristic, while remaining well de®ned and monotonic, is a more complex function of wind speed than it is in CTD mode [12]. However, the errors in the computed wind speed and direction are similar. Fig. 8. (a) Cross-sectional view of the wind-sensor; (b) external view of the wind sensor. K.A.A. Makinwa, J.H. Huijsing / Sensors and Actuators A 97±98 (2002) 15±20 19 6. Conclusions A ``smart'' wind-sensor has been realized in standard CMOS technology. The on-chip interface electronics uses thermal sigma-delta modulation techniques to control, and simultaneously digitize the two-dimensional ¯ow-dependent heat distribution in the sensor. This interface architecture is low power and area ef®cient, and does not interfere thermally with the sensor's operation or increase its chip area. The interface's bit-stream output is decimated off-chip and used to determine wind speed and direction. The results are accurate to within 4% and 28 in wind speed and direction, respectively, over the range 2±18 m/s. Fig. 9. Decimated modulator output dPns (*) and dPew () vs. flow direction at speeds of 2, 8 and 18 m/s. Acknowledgements The authors thank the Dutch Technology Foundation (STW) for their ®nancial support and Mierij Meteo B.V. for their assistance with the wind tunnel experiments. References Fig. 10. Amplitude and offset of dPns (*) and dPew () vs. the square root of flow speed. [1] B. van Oudheusden, J.H. Huijsing, An electronic wind meter based on a silicon flow sensor, Sens. Actuators A 21±23 (1990) 420±424. [2] J. Robadey, O. Paul, H. Baltes, Two-dimensional integrated gas flow sensors by CMOS IC technology, J. Micromech. Microeng. 5 (1995) 243±250. [3] B. van Oudheusden, A.W. van Herwaarden, High-sensitivity 2-D flow sensor with an etched thermal isolation structure, Sens. Actuators A 21±23 (1990) 425±430. [4] E. Yoon, K.D. Wise, An integrated mass flow sensor with on-chip CMOS interface circuitry, IEEE Trans. Electron. Devices 39 (6) (1992) 1376±1386. [5] Q. Huang, C. Menolfi, C. Hammerschmied, A MOSFET-only interface for integrated flow sensors, Proc. ISCAS (1996) 372±376. [6] F. Mayer, A Haberli, H. Jacobs, G. Ofner, O. Paul, H. Baltes, Singlechip CMOS Anemometer, Proc. IEDM (1997) 895±898. [7] K.A.A. Makinwa, J.H. Huijsing, A wind-sensor interface using thermal sigma-delta modulation techniques, Sens. Actuators A 92 (2001) 280±285. [8] B.W. van Oudheusden, Silicon thermal flow sensor with a twodimensional direction sensitivity, Measure. Sci. Technol. 1 (1990) 565±575. [9] Mierij Meteo B.V., Solid-state wind-sensor MMW-005, Product Data sheet, http://www.mierijmeteo.nl. [10] D.J. Allstot, A precision variable-supply CMOS comparator, IEEE J. Solid-state Circuits SC-17 (1982) 1080±1087. [11] B. Razavi, B. Wooley, Design techniques for high speed, highresolution comparators, IEEE J. Solid-state Circuits SC-27 (1992) 1916±1926. [12] K.A.A. Makinwa, J.H. Huijsing, Constant power operation of a twodimensional flow sensor using thermal sigma-delta modulation techniques, Proc. IMTC (2001) 1577±1580. Biographies Fig. 11. Errors in computed wind speed and direction at wind speeds of 2, 8 and 18 m/s. Kofi A.A. Makinwa was born on 3 April 1964. He studied at the Obafemi Awolowo University, Ile-Ife, Nigeria, where he received a BSc degree (first class honours) in 1985 and an MSc degree in 1988, both in electronic 20 K.A.A. Makinwa, J.H. Huijsing / Sensors and Actuators A 97±98 (2002) 15±20 engineering. He then proceeded to the Philips International Institute, Eindhoven, The Netherlands where he received an MEE degree (with distinction) in 1989. He began his working career in 1989 as a research scientist at Philips Research Laboratories in Eindhoven, a position he held until 1999. During this period, he developed electronic systems for interactive displays, and for optical and magnetic storage systems. He is currently at Delft University of Technology, Delft, The Netherlands where he is working towards a PhD on integrated smart wind sensors. He is author and co-author of 12 papers and holds 9 patents. Johan H. Huijsing was born on 21 May 1938. He received the MSc degree in electrical engineering from the Delft University of Technology, Delft, The Netherlands in 1969, and the PhD degree from this university in 1981 for his thesis on operational amplifiers. He has been an assistant and Associate Professor in electronic instrumentation at the Faculty of Electrical Engineering of the Delft University of Technology since 1969, where he is a Professor in the chair of Electronic Instrumentation since 1990. From 1982 to 1983, he was a Senior Scientist at Philips Research Laboratories in Sunnyvale, CA, USA. Since 1983, he is a consultant for Philips, Sunnyvale and since 1998 also a consultant for Maxim, Sunnyvale. The research of Johan H. Huijsing is focussed on the systematic analysis and design of operational amplifiers, analog-todigital converters and integrated smart sensors. He is author and co-author of some 200 scientific papers, 20 US patents and 6 books, and co-editor of 8 books. He is a fellow of the IEEE, for contributions to the design and analysis of analog integrated circuits. He was awarded the title of Simon Stevin Meester for applied Research by the Dutch Technology Foundation.