This paper describes a CMOS mixed-signal interface with a RF transmitter. This die is assembled in a Multi-Chip-Module (MCM) micro-system together with the micromachined soil moisture sensor to achieve a cost effective solution with accurate and reliable measurements for soil moisture in agriculture. The soil moisture probe, based on Dual-Probe Heat-Pulse (DPHP) method, is fabricated in bulkmicromachining technology. The DPHP method is based on the measurement of the maximum temperature rise at some distance
from the heater, after applying a heat-pulse. The measurement of the temperature rise is obtained by subtracting soil temperature from the probe temperature. The mixed-signal
interface is based on a pre-amplification stage and first-order sigma-delta modulator. The bit-stream output of the modulator
is then applied to a counter as a first order decimation filter thus providing a 12-bit readout sample. Prior to transmission, data
is encoded as a pulse width modulated (PWM) signal and then transmitted by means of an amplitude shift-keying (ASK) modulation.
The transmitter features a VCO phase locked to the quartz crystal reference of 13.56 MHz to achieve a carrier frequency of 433.92 MHz. A RF power amplifier based on class
E topology was chosen. The CMOS mixed-signal interface with a RF transmitter has been implemented in a single-chip using a
standard CMOS process (AMI 0.7 µm, n-well, 2 metals and 1poly)

Correia, J. H.

Couto, Carlos

Morais, Raul

Valente, A.

Universidade do Minho: RepositoriUM

A CMOS Mixed-Signal Interface with a RF Transmitter for aMicromachined Soil Moisture SensorRaul MORAIS1, Student Member, IEEE, A. VALENTE2, Student Member, IEEE,J. H. CORREIA3, Member, IEEE, and C. COUTO4, Senior Member, IEEE1,2UTAD, DE, Quinta de Prados, Vila Real, Portugal, e-mail : (rmorais1, avalente2)@utad.pt3,4UM, DEI, Campus de Azure´m, Guimara˜es , Portugal, e-mail: (jhc3, ccouto4)@dei.uminho.ptAbstract— This paper describes a CMOS mixed-signal in-terface with a RF transmitter. This die is assembled in aMulti-Chip-Module (MCM) micro-system together with the mi-cromachined soil moisture sensor to achieve a cost-effectivesolution with accurate and reliable measurements for soilmoisture in agriculture. The soil moisture probe, based onDual-Probe Heat-Pulse (DPHP) method, is fabricated in bulk-micromachining technology. The DPHP method is based on themeasurement of the maximum temperature rise at some dis-tance from the heater, after applying a heat-pulse. The mea-surement of the temperature rise is obtained by subtracting soiltemperature from the probe temperature. The mixed-signalinterface is based on a pre-amplification stage and first-ordersigma-delta modulator. The bit-stream output of the modulatoris then applied to a counter as a first order decimation filter thusproviding a 12-bit readout sample. Prior to transmission, datais encoded as a pulse width modulated (PWM) signal and thentransmitted by means of an amplitude shift-keying (ASK) mod-ulation. The transmitter features a VCO phase locked to thequartz crystal reference of 13.56 MHz to achieve a carrier fre-quency of 433.92 MHz. A RF power amplifier based on classE topology was chosen. The CMOS mixed-signal interface witha RF transmitter has been implemented in a single-chip using astandard CMOS process (AMI 0.7 µm, n-well, 2 metals and 1poly).Index Terms— Wireless, Sigma−Delta, Soil-Moisture Sen-sors.I. INTRODUCTIONIn the field of environmental control applications such asirrigation management systems many sensors are needed fortemperature, relative humidity, CO2 concentration, solar ra-diation and soil moisture.The economics factors and the desire to minimize re-source over-consumption have increased the requirementsfor irrigation management systems to manage water moreefficiently. This resulted on a wider spectrum of availableequipment for measuring soil water status [1].Advances in silicon micromachining in the last years haveencouraged a wide demand for silicon-based microsensors.Integrated microsensors, with on-chip interface circuitry, arecurrently replacing discrete sensors due to their inherent ad-vantages, namely, low cost, accuracy, high reliability and on-chip processing. To accomplish small, robust and inexpen-sive microsystems, it is desirable to integrate the soil mois-ture sensor with digital signal processing and wireless front-end. Therefore, this microsystem could be installed nearplant roots for measuring real plant water needs. With wire-Fig. 1. System Overviewless capabilities, it would be possible to implement a smartirrigation control system to provide only the sufficient water,thus minimizing over-consumption.II. SYSTEM OVERVIEWThe multi-chip module is basically made of two blocks, asdepicted in Fig. 1: the micromachined sensor and the mixed-signal interface with a RF transmitter.Since the bulk-micromachined sensor and electronics areimplemented in different technologies, at this point the twosystem modules are being implemented in a multi-chip mod-ule. In the near future they will be implemented in only onechip using standard CMOS process with post-processing mi-cromachining techniques.A. CMOS mixed-signal interface with RF transmitterThe implemented CMOS mixed-signal interface, outlinedin Fig. 2, includes a pre-amplification stage, a first-order,continuous-time, fully-differential sigma-delta modulator, acounter as a first-order decimation filter, a shift register, anda RF transmitter. The carrier is generated on-chip by a fre-quency synthesizer.The mixed-signal interface is responsible for controllingthe heat pulse duration, analog-to-digital conversion, anddata transmission. A 12-bit first-order sigma-delta converterperforms the analog-to-digital conversion. Before transmis-sion, data is encoded in a binary pulse width modulated sig-nal, and afterwards the carrier of 433.92 MHz is modulated0-7803-7912-8/03/$17.00 © 2003 IEEERFFirst-orderSigma-DeltaModulatorBias & ReferenceControl LogicCounterFIFO/SRRFTransmitterMicromachinedSoil MoistureSensorSubstrateHeaterProbeCMOS AMI 0.7Fig. 2. Schematic representation of the mixed-signal interfacein ASK (Amplitude Shift Keying). A RF Power amplifierdrives a small off-chip loop antenna.The CMOS mixed-signal interface with a RF transmit-ter has been implemented in a single-chip using a standardCMOS process (AMI 0.7 µm, n-well, 2 metals and 1 poly).B. Micromachined Sensor ModuleThe bulk-micromachined soil moisture sensor [2] is basedon the dual-probe heat-pulse (DPHP) method to determinethe volumetric heat capacity and hence the soil water con-tent. Dimensions are 30 mm long, 6 mm wide and 0.8 mmhigh. The probe pitch is 3 mm, allowing small-scale spatialmeasurements of water fractions of the soil, important forinferring soil moisture at plant root level.The sensor consists of two needle probes mounted in par-allel to provide a heater and a sensor probe. The heater usespolysilicon resistor for resistive heating (thermo resistive ef-fect - Joule heat). The temperature sensors (probe and refer-ence substrate) are made from micromachined p-n junctions(Phosphorus - Boron).The heat pulse is generated by applying a voltage to theheater for a fixed period of time. In the opposite rod, themaximum temperature rise above the substrate temperature(reference) is measured. Considering the expected range ofwater content (0 % to 40 %), and that common sensitivity ofintegrated junction-based thermal sensors is approximately−2.2mV/◦C, then a 1 % change in soil water content wouldcause a change in p-n junction direct voltage drop of about22 µV for each p-n junction.III. MIXED-SIGNAL INTERFACEThe maximum rise in temperature, ∆Tm, is measuredby subtracting the soil temperature signal from the substratetemperature (reference signal). This results in a very smallsignal with a full-scale of about 1 mV . The soil water con-tents must be measured with a resolution better than 1 %.This requirement leads to a merely 22 µV sensor readout.Signal bandwidth is typically of the order of few Hz. Thisextremely small signal must be amplified with high gain to anappropriate signal level where it can be converted to the dig-ital domain. The amplification of microvolt signals dictatesthe maximum amplifier offset allowed. To achieve an equiv-alent input offset of 10 µV a chopper stabilization techniqueCounterfchopSubstrateHeaterProbeChopperChopperControl LogicBCAfchopC ntrol LogicfsDigital WorldAFig. 3. Analog front-endwas employed [3]. The architecture of the mixed-signal in-terface is shown in Fig. 3, where B represents the substratetemperature input (reference), A the probe temperature inputand C the heater output control.The analog front-end consists of a chopper modulator, afully differential folded-cascode amplifier, a chopper demod-ulator and a low-pass filter. The input signal is modulatedvia a chopper modulator, which up-converts the signal to thechopping frequency. The modulated signal is then ampli-fied and de-modulated back to the baseband. The amplifierinput offset is by this way modulated at the chopping fre-quency and needs to be removed by filtering. The 1/f noiseis also removed if the chopping frequency is higher than theknee frequency. The low-pass filter is implemented with asecond-order gm-c filter. In this application, the cut-off fre-quency of the filter was chosen as 500 Hz. The choppingfrequency has been set to 20 kHz, which is higher than thenoise corner frequency. The chopper modulator, however, in-jects a residual offset in the signal due to non-idealities of theMOS switches, such as clock feedthrough and charge injec-tion. A fully differential topology is used to improve noiseimmunity, reduce common-mode interference and minimizeresidual offset injected in the signal by the chopper modula-tors.The amplified signal is now applied to a first-order Sigma-Delta (Σ −∆) converter, as depicted in Fig. 4. Sigma-Deltamodulators trade bandwidth for resolution, which makes thisarchitecture suitable for this application. In our case, havinga very low-frequency signal (< 10 Hz) and medium accu-racy (12 bit) a one bit, first order Σ −∆ converter has beenchosen because of the lower complexity, die area and powerconsumption when compared to higher order Σ−∆ convert-ers. A further advantage of the Σ−∆ architecture is the re-laxation of the component requirements used inside the loop.A high oversampling ratio of 512 was chosen to overcomethe problems associated with low performance circuits.However, this simpler architecture has severe drawbacksconcerning pattern noise associated to DC inputs and lowsignal-to-noise ratio (SNR) [6]. DC stability and noise re-jection are achieved here by switching the polarity of the dif-ferential input signal before the Σ − ∆ converter. Duringhalf the conversion time, the modulator’s output bit streamis applied to a counter (first order decimation filter). Duringthe second half of the conversion time, the counter input isRef+1-bitDACRef-Ref+1-bitDACRef-D QD QD0D1D11BitStreamCLK2CLK1InIn+OTAFig. 4. First-order sigma-delta converter.CrystalOscillator13.56MHzSerial DATAClass-EMatchingNetworkCMOS AMI 0.713.56MHzFrequencyLockedLoopPWMEncoderASKModulator433.92 MHzClass-EP.A.Fig. 5. RF transmitter diagram.switched to the modulator’s complementary output [4].IV. RF TRANSMITTER MODULEThe RF transmitter module includes a crystal-controlledfrequency synthesizer, a pulse-width modulation (PWM) en-coding circuit, an Amplitude Shift Keying (ASK) modulatorand a power amplifier as depicted in Fig. 5. For each sam-ple, data is assembled together with a preamble for receiversynchronization, encoded and transmitted. Each packet isconsisted by a 11-bit preamble, 12-bit data and 4-bit check-sum.One of the most important elements of the transmitter isthe oscillator block. The oscillator should be capable of op-erating at a stable and predictable frequency over time. Forthis reason, the frequency synthesizer was implemented asa frequency-locked loop [5], using a 13.56 MHz crys-tal reference oscillator to obtain the carrier frequency of433.92 MHz.Prior to transmission, data is encoded in pulse widths of25 % and 75 % duty-cycle representing the logic levels ’0’and ’1’ respectively, and then the PWM signal is transmittedby means of a amplitude shift keying modulation. The mod-ulated signal is then applied to a class-E power amplifier thatdrives a small off-chip loop antenna to transmit the signal. Inthis first prototype, the typical class-E matching network isoff-chip, as seen in Fig. 6. Since component values are suitedfor integration, they will be on-chip in the next prototype.CMOS AMI 0.7RF Driver&RF SwitchVddRFCL1C2C1C3L3RLOADFig. 6. Class-E power amplifier off-chip matching network.Fig. 7. SimulatedTm distribution at 35s.V. RESULTS AND DISCUSSIONThe micromachined sensor was simulated usingFEMLAB. For this purpose, sensor rods are consideredinfinitely long and so the simulation was taken in a transver-sal section of the rod. Fig. 7 shows the temperature rise(Tm) distribution 35 s after applying a heat-pulse ofq = 600 Jm−1 during 8 s.Soil samples of Almendra silt loam, which were wet to apredetermined water content and mixed, were packed into acylinder 77 mm in diameter by 70 mm long, with the soilmoisture sensor at the center. Measurements were taken andthen the soil was weighed and dried at 105 ◦C for 24 h to de-termine bulk density and water content (thermo-gravimetricmethod),θg =(wetweight)− (dryweight)dryweight. (1)Table I lists the thermo-gravimetric soil water content (θg)and the measured values; maximum temperature rise (∆Tm)and the heat applied per unit length of the line source (q).The values of (θv) are calculated using,θv =qeπr2Tm − (1.92× 106Xm + 2.50× 106Xo)4.18× 106 , (2)where, r, Xm and Xo are the rod radius (m), the mineral andorganic fractions of soil respectively.In soils with low organic matter, such as Almendra, Xo isneglected. The value of Xm is determined by dividing thesoil bulk density by the particle density. An average valueof 2.65 Mgm−3 is often used for particle density of soils.Therefore, the calculated values of soil water content (θv)using Eq. 2, are in good agreement with thermo-gravimetricTABLE IOBTAINED SOIL WATER CONTENTS.θg(m3m−3) ∆Tm(◦C) q(Jm−1) θv(m3m−3)0 1.90 602 00.1 1.35 605 0.1060.2 1.04 598 0.1900.3 0.85 601 0.3080.4 0.72 603 0.401Heat-pulse Measurements23,0023,2523,5023,7524,0024,2524,5024,751 11 21 31 41 51 61 71 81 91 101Time (s)Temperature(ºC)Temperature(ºC)Fig. 8. Tipical Temperature by time data.values. Fig. 8 shows a typical temperature response of heat-pulse measurements. The shadow zone indicates the time ofheat-pulse application.Fig. 9 shows the simulated step response of the fully dif-ferential folded cascode amplifier for a 500 µV input stepand a gain of 66 dB. Simulations have shown that it canbe expected an effective resolution of 12-bit for the Σ − ∆modulator with a conversion time of 9.66 ms for a clockfrequency of 423.75 kHz.Fig. 10 shows a binary pattern ′011′ PWM encoded signal(A) and transmitted signal (B) in a 50 Ω load.VI. CONCLUSIONSThe design and modeling of a MCM-based micro-systemfor soil moisture measurements using the Dual-Probe Heat-Pulse (DPHP) method were achieved. The micro-system in-cludes the soil moisture sensor, Σ−∆ converter, signal pro-20 40 60 80 100 120Vout(+)Vout( )Vout,diff 0.99978VVout,diff,Vout(),Vout(+)[V]Time [ S]0-1.0-0.50.00.51.01.52.02.53.0140Fig. 9. Amplifier step response to 500 µV differential input.Fig. 10. RF Outputcessing circuits (digital filtering and sensor interface) and aRF 433.92 MHz transmitter. The DPHP method showed tobe the most appropriate to measure humidity at different soildepths, and therefore, close to plant roots in a non-destructiveand automated manner. This is the first time that the DPHPmethod is implemented in a MCM-based micro-system andthe first integrated sensor for soil moisture.In this paper a CMOS mixed-signal interface with a RFtransmitter for a micromachined soil moisture sensor hasbeen proposed. Chip layout has been submitted to fabrica-tion through Europractice IC service. A set of experimentshas to be done for testing the reliability and precision of re-sults.Some enhancements are now being carried out, such as areceiver, among others. With bidirectional comunications ca-pabilities, it will be possible to implement a wireless networkof soil moisture sensors in a smart irrigation control system.REFERENCES[1] P. Charlesworth, Soil Water Monitoring, Irrigation Insights, 2000.[2] A. Valente, C. Couto and J. H. Correia, ”On-chip integrated siliconbulk-micromachined soil moisture sensor based on DPHP method,” inProceedings of Transducers’01 and Eurosensors XV, 2001, pp.316–319.[3] C. Menolfi and Q. Huang, ”A low-noise CMOS instrumentation ampli-fier for thermoelectric infrared detectors,” IEEE Journal of Solid-StateCircuits, vol.32, no.7 , July, 1997, pp. 968-976[4] Luca G. Fasoli, Frank R. Riedijk and Johan H. Huijsing, ”A Gen-eral Circuit for Resistive Bridge Sensors with Bitstream Output,” IEEETransactions on Instrumentation and Measurement, vol.46, no.4 , April,1997, pp. 954-960[5] Rafael J. Betancourt-Zamora, Ali Hajimiri and Thomas H. Lee, ”A1.5mW, 200MHz CMOS VCO for Wireless Biotelemetry,” First Inter-national Workshop on Design of Mixed-Mode Integrated Circuits andApplications, July 28-30, 1997, Cancun, Mexico.[6] J. C. Candy and G. C. Temes, Oversampling Delta-Sigma Converters,IEEE Press, New York: 1992, 1-25

A CMOS mixed-signal interface with a RF transmitter for a micromachined soil moisture sensor

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A CMOS mixed-signal interface with a RF transmitter for a micromachined soil moisture sensor

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