Measurements readings never completely accurate must always corrected

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Measurements readings never completely accurate must always corrected

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Digital correction capacitive signals with Frame ASIC CAV414
Measurements readings never completely accurate must always corrected using suitable methods order keep within required error margins. modern approach calibrate compensate measured sensor signal with correction algorithms implemented processor. There integrated solutions currently market which principle allow electronic correction such above performed. However, these have usually been designed specific application suitable general number other reasons. This article illustrates simple inexpensive sensor systems assembled clearly distinguishing between analog digital components which permit electronic correction capacitive signals same time remedy weaknesses existing monolithic solutions.
Electronic correction capacitive signals application outlined here illustrates with CAV414 Frame ASIC from Analog Microelectronics GmbH which converts capacitive signals into analog output voltages; Figure simple, inexpensive RISC processor some considered circuitry sensor system compiled which enables capacitive measurement signal corrected electronically. Figure shows configuration application. term Frame ASIC describes class integrated analog signal evaluation circuits sensor technology. Frame ASIC name given whose input, output analog peripheral functions surround digital correction facilities sensor system processor controller) like frame [1].
COSC
Reference Oscillator
CAV414
VOUT
Integrator Integrator Signal Conditioning
GAIN
Voltage/Current Reference
VREF
LPOUT
RCX1 RCX2 RCOSC
Figure block diagram CAV414
analog microelectronics
Analog Microelectronics GmbH Fahrt 55124 Mainz Internet: http://www.analogmicro.de Phone: Fax: E-Mail: (0)6131/91 (0)6131/91 info@analogmicro.de
October 2000 1/11 Rev.
Digital correction capacitive signals with Frame ASIC CAV414
COSC
Reference Oscillator
CAV414
Integrator Integrator Signal Conditioning
Voltage/Current Reference
CREF
RCX1
RCX2
ROSC
Span Correction
Offset Correction
ATtiny11
CVDD
COMP2+ PWM1
RP1A
PWM2
RP3A RP2A CP1A RP3B RP2B CP1B CP2B CP2A RRES VTEMP
RP1B
COMP1-
RESET
Temperature Detection
Figure theoretical assembly application
analog microelectronics
October 2000 2/11
Digital correction capacitive signals with Frame ASIC CAV414
Signal conditioning CAV414 capacitive signals CAV414 industrial interface circuit capacitive sensors which includes full analog signal acquisition evaluation electronics various protective functions (against reverse polarity, example). addition, integrated reference voltage source allows further components, such processor, example, supplied with power. When measuring capaciV tance detects relative difV ference capacitance between this given reference capacitance. been optimised capacitances ranging from 10pF 2nF; differences full-scale (FS) capacitance between 100% reference capacitance. CAV414 Time provides signal proportional relatives difference capacitance Figure voltage oscillator which monitor, before converted into industrial output voltage. This signal internally temV perature-compensated. Details given data sheet [2]. number different applications assembled using CAV414 industrial voltage output combination with other from Analog Microelectronics GmbH. Adding AM402 Time AM422 setup, example, permits simple current loop signals Figure voltage integrators realised (see [4]).
OSC,HIGH OSC,LOW CLAMP
Reference Oscillator: CAV414 works according following principle. reference oscillator, whose frequency using capacitance COSC, drives symmetrical integrators CAV414 which phase-locked clock-synchronised. output signal amplitudes driven integrators determined capacitances (fixed reference capacitance) (capacitance measured). With high common-mode rejection high resolution, comparison integrator amplitudes produces relative difference capacitance between capacitances CX2. This integrator amplitude differential signal converted into direct voltage pass whose cutoff frequency amplification using external components. excitation current capacitances COSC with resistors RCX2 ROSC. described following section, this used
analog microelectronics
October 2000 3/11
Digital correction capacitive signals with Frame ASIC CAV414
electronic correction which entails calibrating offset span compensating relevant temperature coefficients. functional description gives necessary formulas calculate CAV414 with sensing element (capacitor capacitive sensor). formulas good approximation reality. reference oscillator works charging discharging external oscillator capacitor COSC internal parasitic capacitor COSC,PAR,INT external parasitic capacitor COSC,PAR,EXT (e.g. circuit board). external oscillator capacitor COSC COSC where fixed capacitor capacitive sensor. reference oscillator current IOSC determined external resistor ROSC reference voltage (see data sheet [2]): ROSC frequency reference oscillator fOSC given VOSC (COSC COSC COSC whereby VOSC difference internal threshold voltages (VOSC,HIGH VOSC,LOW) reference oscillator. VOSC defined internal resistors 2.1V oscillator voltage shown Figure Capacitive Integrators: principle operation basically same behaviour reference oscillator. difference time discharging capacitors which twice time charging clamped internal fixed voltage VCLAMP Figure signal voltage over capacitors shown. capacitive integrator current determined external resistor reference voltage (see data sheet [2]): capacitor charged maximum voltage (see Figure calculated follows VCLAMP voltages over capacitors subtracted resulting differential voltage referred reference voltage given
analog microelectronics
October 2000 4/11
Digital correction capacitive signals with Frame ASIC CAV414
,DIFF (VCX differential voltage VCX,DIFF goes directly into 2nd-order lowpass. 3dB-corner frequencies stages adjusted with external capacitors (CL1, CL2) internal resistors (R01, R02; typ. 20k). 3dB-corner frequencies have chosen depending reference oscillator frequency fOSC desired detection frequency fDET entire sensor system. following relation different types frequencies fulfilled cases: external capacitors desired corner frequency calculated follows output signal lowpass with ideal waveform becomes VLPOUT VDIFF with VDIFF (VCX differential output voltage VDIFF,0 small, amplified with external resistors while using internal non-inverting amplifier lowpass. maximum amplification differential output voltage VDIFF,0 limited maximum allowed input voltage range following instrumentation amplifier (VIA,IN,max 400mV). gain stage Hence follows output signal lowpass stage VLPOUT VDIFF with VDIFF VDIFF (VCX With instrumentation amplifier output stage, output signal becomes VOUT VDIFF with fixed gain adjustable gain
analog microelectronics
October 2000 5/11
Digital correction capacitive signals with Frame ASIC CAV414
Information processor used correction circuitry processor (ATtiny series from Atmel) necessarily designed sensor applications used digital corrective device. tiny size cost processor, however, make ideal sensor applications. before applied whole, number considerations circuitry must made. processor used here fully-static CMOS-RISC processor 8-pin package with 1kByte re-writable flash ROM, 32-byte RAM-similar register 1MHz clock frequency. processor does require external clock quartz internal oscillator. supply voltage range 2.7V.5.5V maximum (depending model processor used) with power consumption. types processor cope with operating temperature between -55°C +125°C. flash memory programmed using either extension port Atmel evaluation board, using special programming units from other suppliers using individual setups with digital signals. processor neither converter pulse width modulation outputs included hardware, inevitably these missing functions must realised using software. low-cost When measuring temperature Start discharge necessary compensation time measurement Recharge capacitor offset span temperature next measurement coefficients, here temperatureV dependent flow voltage semiCapacitor discharge curve conductor diodes used (approx. Voltage -2.4mV/K). Alternatively, other measured temperature sensors which supply suitable analog voltage signal applied. analog temZeit Duration time perature information measurement result) Comparator compares converted into digital signal voltage measured with that element that handled processor. ATtiny11 only integrated analog comparator (I/O-Pin) which turned into converter using suitable firmware (ramp principle). this analog voltage Square voltage wave measured (temperature signal) produced processor applied positive comparator input (pin Figure Figure low-cost principle negative input (pin capacitor connected which discharged through processor's pins resistor from given point time. soon capacitor voltage discharges temperature signal value, status
compcomp+ comp+ comp-
analog microelectronics
October 2000 6/11
Digital correction capacitive signals with Frame ASIC CAV414
processor relevant temperature calculated from discharge time elapsed. conversion temperature signal thus involves transforming input voltage into time proportional evaluating this. outputs alternative already mentioned, measurement signal actually corrected varying excitation currents capacitances COSC. order impact current sources analog signals required; these must supplied processor correction. processor itself does contain converter which emit signals such these pulse width modulation (PWM) outputs included hardware processor's pins programmed outputs. dimensions guide external components available from Analog Microelectronics GmbH Mathematica notebook.
last cycle
Initialise variables Measure temperature Rescale value Fetch corrective data output PWM1 PWM2
cycle (ca. 3ms)
next cycle
PWM1 output
pin7
duty cycle
PWM2 output
pin2
duty cycle
Time Measure temperature, process data Correct span Correct offset
Figure temporal diagram main programme switching outputs.
processor's outputs supply discrete voltages switched cycles within period software. average voltage produced back-end pass; this serves correction voltage proportional duty cycle (ratio switch-on period total pulse-repetition period). quantisation correction voltage level accuracy which achieved with this correspond quantisation switch-on/switch-off time. signals produced this manner thus supply correction voltages which used excitation currents offset span compensation CAV414. Figure gives programme cycle which split into three areas: Initialisation variables Temperature measurement using ADC; calculation relevant temperature value address from entries look-up table (correction values written into flash during calibration) Output span correction signal output offset correction signal output
analog microelectronics
October 2000 7/11
Digital correction capacitive signals with Frame ASIC CAV414
Storing calibration data flash place additional data EEPROM processor flash used here store correction data. correction data filed together with operating software programme memory. ATtiny11 used here supports readout data stored programme memory proper. Compensation sensor system compensation process sensor system main stages: entails setting gain room temperature other involves measuring calibration data offset span various temperature approximation points. Setting gain room temperature Initially, resistors connected circuitry (RL) connected (LPOUT) that input gain Where variable capacitance lowest value thus approximately identical reference capacitance (e.g. with proximity sensor which does have object front sensor area), output differential voltage calculated: VDIFF 0,min VLPOUT ,0,min Where variable capacitance highest value (e.g. with proximity sensor which object directly front sensor area), maximum output differential voltage calculated: VDIFF ,0,max VLPOUT 0,max Gain with using resistors that maximum output differential voltage VDIFF ,max (VDIFF 0,max VDIFF ,0,min value VDIFF ,max 400mV produced. resistors must within permissible range 100k 200k when setting gain. Calibration entire system Once sensor system been relevant output values room temperature next stage process ascertain correction data temperature compensation processor store this flash ROM. Measurements made either three different temperatures,
analog microelectronics
October 2000 8/11
Digital correction capacitive signals with Frame ASIC CAV414
pending accurate application required offset span values determined these temperature approximation points, with digital temperature value relevant approximation point also being recorded. compensate overall temperature error entire system system output voltage values VOUT recorded voltage values differential voltage VDIFF. With suitable programme these values used generate correction values look-up table. Once calibration process finished data stored flash ROM. Results using suggested setup Building purely analog circuit without digital correction described data sheet application instructions CAV414, without additional temperature compensation total error ±1.5% industrial temperature range (-25°C 85°C) achieved (with input capacitances offset span). such setup each system must corrected individually setting external resistors, depending reference capacitances used. this adjustment carried open board, faults occur during packaging. reference capacitances demonstrate dissimilar temperature behaviour, error becomes greater. applications where accuracy essence, compensation becomes extremely elaborate time-consuming. Figure demonstrates what easily achieved combining CAV414 with ATtiny11 RISC
Temperature drifts offset span Measurement VDIFF
0,15
Span (CX2 CX1)
Offset (CX2 CX1)
0,05
Error
-0,05
-0,1
-0,15
Temperature
Figure measurements with discrete capacitors input capacitances
analog microelectronics
October 2000 9/11
Digital correction capacitive signals with Frame ASIC CAV414
processor. Readings were taken from circuit illustrated Figure order simulate offset span measuring capacitance used offset used span. differential output CAV414 pins (voltage VDIFF VM-VLPOUT) calibrated zero offset value; span value 400mV same outputs. external components this particular application have been dimensioned that using ATtiny11 change offset ±30mV change output differential voltage span ±50mV corrected. device calibrated three different temperature approximation points (0°C, 30°C 60°C), requiring relatively little effort with regard measurement correction. temperature error corrected using linear approximation. measurements indicate that temperature drift system considerably improved. Compared setup which does have processor, using three-temperature correction accuracies ±0.3%FS obtained. further advantage this combination CAV4141 RISC processor that circuit corrected minimum error production packaged state. Possible faults arising during packaging device thus avoided. results show that using Analog Microelectronics GmbH concept modular signal evaluation outlined here (where analog digital functions kept separate), simple, inexpensive applications produced which through digital correction allow high degree accuracy achieved entire system. Future possibilities Development field processor technology steaming ahead. Atmel, example, announced that will soon launching successors ATtiny11, feature which will integrated converters. application described this article will thus only then able considerably simplified (less external components will required); second converter also enables sensor signal linearised. Bearing this mind, evident that Frame ASIC concept, coupled with ongoing development low-cost processors, will provide users with number ways conditioning sensor signals. this naturally also encompasses various possibilities mentioned information CAV414 (the data sheet [2], application instructions article [5]). example, evaluation differential capacitor combination CAV414 with AM402 generate industrial current loop signal (0/4.20mA) also applied full example [4]. capacitive sensor system used moisture sensors, inclinometers, measurement levels pressure sensor technology.
analog microelectronics
October 2000 10/11
Digital correction capacitive signals with Frame ASIC CAV414
Further information CAV414 Comprehensive information available Frame ASIC CAV414. following lists various documents with their relevant homepage link: Frame ASIC concept: Data sheet CAV414: Application instructions dimensions guide CAV414: Application instructions CAV414 with AM402 4.20mA application: Basic article CAN404 CAV414:
analog microelectronics
October 2000 11/11

Measurements readings never completely accurate must always corrected

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