AN61-1 Application Note 61 August 1994 Practical Circuitry for ...

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Application Note 61A1T168kNOVASENSOR5+NPH-8-100AH †1041µFNON-POLAR–A2LT10770.1µF 200k*1% 10k1%OUTPUT6+0.1µF100k100µF2 1+3 L1 4698Ω1%R1**50Ω100Ω0.1%–100kA1LT10771N4148100kAA CELL+1µF6I LSW1V IN8 3FBLT1110AOSETSW2GND150Ω2 15 47LT1004-1.2* = NOMINAL VALUE. EACH SENSOR REQUIRES SELECTION** = TRIM FOR 150mV ACROSS A1-A2†= LUCAS NOVASENSOR FREMONT, CA (510) 490-9100L1 = COILTRONICS CTX50-1R21k0V TO 3.100V =0 TO 31.00"Hg.100Ω1N41481N5818430k39k100Ω+ 390µF16VNICHICONPLAN61 F16Figure 16. 1.5V Powered Barometric Pressure Signal Conditioner Floats Bridge Drive toEliminate Instrumentation Amplifier. Voltage Boosted Current Loop Drives TransducerA2AN61-12
Application Note 61Until recently, this type of accuracy and stability has onlybeen attainable with bonded strain gauge and capacitivelybasedtransducers, which are quite expensive. As such,semiconductor pressure transducer manufacturers whoseproducts perform at the levels reported are to be applauded.Although high quality semiconductor transducersare still not comparable to more mature technologies,their cost is low and they are vastly improved over earlierdevices.The circuit pulls 14mA from the battery, allowing about250 hours operation from one D cell.Quartz Crystal-Based ThermometerAlthough quartz crystals have been utilized as temperaturesensors (see Reference 5), there has been almost nowidespread adaptation of this technology. This is primarilydue to the lack of standard product quartz-based temperaturesensors. The advantages of quartz-based sensorsinclude simple signal conditioning, good stability and adirect, noise immune digital output almost ideally suited toremote sensing.Figure 17 utilizes an economical, commercially available(see Reference 6) quartz-based temperature sensor in athermometer scheme suited to remote data collection.10pF610M4 LTC4858 2 3 751NC 5VY1+33.5ppm/°C100k1000 FEET TWISTED-PAIR15pF5V86OUTLTC485 174100Ω 35 NC20°C TO 100°C = 261.900kHz – 262.800kHzY1 = MICRO CRYSTAL (SWISS) MT1/33.5ppm/°C0°C = 261.900kHz100°C = 262.800kHzFigure 17. Quartz Crystal Based Circuit ProvidesTemperature-to-Frequency Conversion. RS485Transceivers Allow Remote SensingAN61 • F17The LTC485 RS485 transceiver is set up in the transmitmode. The crystal and discrete components combine withthe IC’s inverting gain to form a Pierce type oscillator. TheLTC485’s differential line driving outputs provide frequencycoded temperature data to a 1000-foot cable run.A second RS485 transceiver differentially receives thedata and presents a single-ended output. Accuracy dependson the grade of quartz sensor specified, with 1°Cover 0°C to 100°C achievable.Ultra-Low Noise and Low Drift Chopped-FET AmplifierFigure 18’s circuit combines the extremely low drift of achopper-stabilized amplifier with a pair of low noise FETs.The result is an amplifier with 0.05µV/°C drift, offset within5µV, 100pA bias current and 50nV noise in a 0.1Hz to 10Hzbandwidth. The noise performance is especially noteworthy;it is almost 35 times better than monolithic chopperstabilizedamplifiers and equals the best bipolar types.FETs Q1 and Q2 differentially feed A2 to form a simple lownoise op amp. Feedback, provided by R1 and R2, setsclosed-loop gain (in this case 10,000) in the usual fashion.Although Q1 and Q2 have extraordinarily low noise characteristics,their offset and drift are uncontrolled. A1, achopper-stabilized amplifier, corrects these deficiencies.It does this by measuring the difference between theamplifier’s inputs and adjusting Q1’s channel current viaQ3 to minimize the difference. Q1’s skewed drain valuesensure that A1 will be able to capture the offset. A1 and Q3supply whatever current is required into Q1’s channel toforce offset within 5µV. Additionally, A1’s low bias currentdoes not appreciably add to the overall 100pA amplifierbias current. As shown, the amplifier is set up for anoninverting gain of 10,000 although other gains andinverting operation are possible. Figure 19 is a plot of themeasured noise performance.The FETs’ V GS can vary over a 4:1 range. Because of this,they must be selected for 10% V GS matching. This matchingallows A1 to capture the offset without introducing anysignificant noise.AN61-13
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AN61-1 Application Note 61 August 1994 Practical Circuitry for ...

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