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CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 1/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de PRINCIPLE FUNCTION Capacitance/Voltage converter IC with an adjustable, differential output, integrated temperature sensor Typical applications CAV424 is an analog linear transducer. The IC is suitable for all capacitive measurements which require a voltage output signal which is proportional to the change in the capacitance to be measured. It can be used for: ! Distance measurement ! Pressure sensing ! Humidity measurement ! Level sensing ! Measurement of strength ! As a capacity input circuit for microprocessors or as a stand-alone device CAV424 8mV/ K V = 5V + 5%CC V = 1,1 ... 3,9 VOUT Measurement capacitance Reference capacitance (10 pF ... 1 nF) Temperature
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 2/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de
CONTENTS
PRINCIPLE OF MEASUREMENT 4 HOW CAV424 WORKS 4 Oscillator function 5 Capacitive integrators 5 Signal conditioning 7 STANDARD DIMENSIONING 10 BOUNDARY CONDITIONS 10 DIMENSIONING PROCEDURE 10 INITIAL OPERATION 10 OUTPUT VOLTAGES 10 EXAMPLE APPLICATIONS 10 BLOCK DIAGRAM AND PINOUT 10 DELIVERY 10 ADDITIONAL EQUIPMENT 10 FURTHER READING 10
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 3/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de
FEATURES
! High detection sensitivity ! Wide capacitor measuring range: 5% – 100% relative to the reference capacitor, 0.5pF to 1nF. ! Detection frequency of up to 2kHz ! Adjustable output offset ! Adjustable full scale output signal ! Differential output ! High voltage immunity ! Temperature output signal ! Wide temperature range: -40°C...+105°C ! Supply voltage: 5V ± 5% ! Ratiometric output voltage ! Simple calibration (Excel program) ! RoHS compliant GENERAL DESCRIPTION CAV424 is an integrated C/V converter circuit which contains full signal conditioning electro- nics for almost any source of capacitive signal. For measurement capacitor CM CAV424 detects the relative change in capacitance !CM = CM,max– CM,min in relation to that of a given, fixed reference capacitor CR. The IC has been optimized for reference capacitors of between 10pF and 1nF where the change in capacitance !CM can be 5% to 100% of the basic capacitance CM,min. The differential voltage output has been specially designed for connection to an A/D converter. Together with the integrated temperature sensor and a processor calibratable systems can be assembled. A simple Excel program simplifies the dimensioning of CAV424. BLOCK DIAGRAMM Figure 1: Block diagram CAV424 CAV424 131510 VTEMP RCR RCM RCOSC VCC VM LPOUT GND CL1 CL2 RL COSC CR CM T Sensor Current reference Reference oscillator Integrator 1 Integrator 2 Signal conditioning
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 4/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de PRINCIPLE OF MEASUREMENT CAV424 is an integrated C/U converter circuit which contains full signal conditioning electronics for capacitive signal sources. CAV424 VCC UOUT IRIM CM CR Current sources I and I are integrated in CAV424M R Figure 2: Principle of capacitance measurement using CAV424 The principle of measurement with the CAV424 entails recording a change in capacitance in a sensor bridge comprising two adjustable current sources and two capacitors, the measurement capacitance (CM) of which can be altered by the amount !CM = CM,max– CM,min. The second capacitor is defined as a reference (CR, see Figure 2). CM,min is the basic capacitance of CM. The change in measurement capacitance is compared to the fixed reference capacitance CR and the resulting signal converted into an output voltage signal. HOW CAV424 WORKS The CAV424 IC functions according to the following principle. An adjustable oscillator, the frequency of which is set using capacitor COSC, drives two symmetrical integrators which are phase- locked and clock-synchronized (see Figure 3). The amplitudes of the two driven integrators are determined by capacitors CR and CM. With high common-mode rejection and a high resolution, the difference between the two amplitudes produces a signal which corresponds to the difference in capacitance between CR and CM (rectifier effect). This difference signal is then filtered in an ensuing active low pass. The resulting voltage signal passes on to an adjustable amplifier stage which sets the output signal to the required value.
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 6/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de The capacitive integrator currents ICR and ICM are set by external resistors RCM , RCR and reference voltage VM: CM M CM R VI " and CR M CR R VI " (3)*, (4)* Capacitors CM and CR are charged up to a maximum voltage of VCM and VCR respectively and can be calculated as follows: CLAMP MOSC CM CM VCf IV %##" 2 (5) CLAMP ROSC CR CR VCf IV %##" 2 (6) The two voltages VCM and VCR are subtracted from one another in the circuit's signal conditioning unit. Via this subtraction, which is tantamount to a rectification of the procedure, VCLAMP is eliminated and a direct voltage of VTPAS is produced as an output signal after filtering. Should ICR and ICM be the same for CM,min (i.e. should the reference capacitance be the same as the basic value of the measurement capacitance), on subtraction and filtering at the signal conditioning output a value of zero is obtained (see Figure 5). * The equations apply to RCX = 0 (see Figures 7 and 8). Should RCX "0 for the resistor due to better thermal coupling, alternative calculations are provided in the Excel spreadsheets Kali1_cav424.exc and Kali2_cavV424.xls. VCM time t VOSC T 2TT VCLAMP C CR < M I ICR CM = VCR Figure 4: Integrator oscillator voltage
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 7/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Signal conditioning The filtered and smoothed voltage has a value of VTPAS: & 'CMCRTPAS VVV (#" 8 3 (7) Signal VTPAS can be boosted using the follow-on internal operational amplifier, with the amplification GLP being determined by resistors RL1 and RL2. GLP is calculated as: L L LP R RG %" (8) With (7), this results in: & 'CMCRLPTPASLPDIFF VVGVGV (##"#" 8 For the output signal reference to ground (GND) it thus follows that: MDIFFLPOUT VVV %" (9) VLPOUT = f(CM, (CR), fosc, ICM, ICR), where the basic values of CM and CR must be placed in a fixed ratio. fosc or ICM, ICR act as parameters. CAV424 Signal conditioning VCC GND RCOSC VM LPOUT RLCL2CL1 CM CR COSC Integrator 1 Integrator 2 Reference- oszillator VTEMP 131510 T Sensor Current reference RCR RCM Figure 5: Block diagram and signal pattern
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 10/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de ELECTRICAL SPECIFICATIONS Tamb = 25°C, VCC = 5V (unless otherwise stated) Parameter Symbol Conditions Min. Typ. Max. Unit Supply Supply Voltage VCC Ratiometric range 4.75 5.00 5.25 V Temperature Specifications Operating Tamb -40 105 °C Storage Tst -55 125 °C Oscillator Oscillator Capacitor Range COSC COSC = 1.6 # CM 16 1600 pF Oscillator Frequency Range fOSC 1 130 kHz Oscillator Current IOSC ROSC = 250k) 9.5 10 10.75 *A Capacitive Integrator 1 and 2 Reference Capacitor Range CR 10 1000 pF Reference Capacitive Integrator Current IR RCR = 500k) 4.75 5 5.38 *A Measurement Capacitor Sensitivity $ CM $CM = (CM,max(CM,min )/CM,min 5 100 % Measurement Capacitor Range CM CM,min ! CM ! CM,max 10 2000 pF Measurement Capacitor Integrator Current IM RCM = 500k) 4.75 5 5.38 *A Detection Frequency fDET CL1 = CL2 =1nF 2 kHz Low Pass Stage Adjustable Gain GLP 1 10 Output Voltage VLPOUT VLPout = VDiff + VM , 1.1 VCC – 1.1 V Corner Frequency 1 fC1 R01 = 20k), CL1 =1nF 8 kHz Corner Frequency 2 fC2 R02 = 20k), CL2 =1nF 8 kHz Resistive Load at PIN LPOUT RLOAD 200 k) Capacitive Load at PIN LPOUT CLOAD 50 pF Output voltage shift VDIFF VM = 2.5V -1.4 1.4 V Temperature Coefficient VDIFF (together with Input Stages) dVDIFF /dT Tamb = -40 ... 105°C +100 ppm/°C Internal Resistor 1 and 2 R01, R02 20 k) Temperature Coefficient R01,02 dR01,02 /dT Tamb = -40 ... 105°C 1.9 10-3/°C Ratiometric Error of VLPOUT RAT@VDIFF* 0.11 %FS Voltage Reference VM Voltage VM Ratiometric to VCC 2.5 V VM vs. Temperature dVM /dT Tamb = -40...+105°C +20 +50 ppm/°C Current IVM Source 16 *A IVM Sink -16 *A Load Capacitance CVM 80 100 120 nF Ratiometric Error of VM RAT@VM** 0.007 %FS * RAT @ VDIFF = 2 [1.05 VDIFF(VCC = 5V) – VDIFF(VCC = 5.25V)]/[VDIFF(VCC = 5V) + VDIFF(VCC = 5.25V)] ** RAT @ VM = 2 [1.05 VM(VCC = 5V) – VM(VCC = 5.25V)]/[VM(VCC = 5V) + VM(VCC = 5.25V)]
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 11/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Parameter Symbol Conditions Min. Typ. Max. Unit Temperature Sensor VTEMP Voltage VTEMP RTEMP , 50M) 2.20 2.32 2.45 V Sensitivity dVTEMP/dT RTEMP , 50M) 8 mV/°C Thermal Nonlinearity RTEMP , 50M), end point method 0.5 %FS Table 1: Specifications for CAV424 Note: 1) The oscillator capacity has to be chosen using COSC = 1.6 # CM,Min 2) The capacitor range of CM and CR can be extended, whereby the system performance is reduced and the electrical limits are exceeded. 3) Currents flowing into the IC are negative. 4) RTEMP is the minimum load resistance at pin VTEMP. The system performance over temperature forces resistors RCR, RCM and ROSC to have the same temperature coefficient; this also requires that the components are placed very close together in the circuit. Capacitors CR, CM and COSC are also obliged to have the same temperature coefficient and a very close proximity on the circuit board. STANDARD DIMENSIONING For external elements which do not have to be altered dependent on the measurement capacity the following standard values apply: Parameter Symbol Min. Typ. Max. Unit Output Stage Resistor (1%) RL2 , RL3 100 k) Offset Resistor (1%) RB 100 k) Reference Voltage Capacity (VM = 2.5V) CVM 80 100 120 nF Filter Capacitance CRL1 2.2 nF Table 2: Standard values for external components BOUNDARY CONDITIONS Parameter Symbol Condition Min. Typ. Max. Unit Maximum Supply Voltage VCCmax 17 V Oscillator Frequency Range fOSC 1 130 kHz Reference Capacitive Integrator Current IR RCR = 500k) 5.38 *A Measurement Capacitor Integrator Current IM RCM = 500k) 5.38 *A Table 3: Boundary conditions
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 12/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de DIMENSIONING PROCEDURE Programs Kali1_cav424.xls and Kali2_cav424.xls can be used for dimensioning purposes. The dimensioning process takes the following scenarios into account: a) Kali1_cav424.xls – The integrator charging currents ICM and ICR are given and constant. Oscillator frequency fosc must be adjusted to suit the minimum value of measurement capacitance CM,min. b) Kali2_cav424.xls – Oscillator frequency fosc is given and determined and integrator charging currents ICM and ICR must be adjusted to suit the minimum value of measurement capacitance CM,min. In a) the dimensioning process assumes that in addition to measurement capacitors CM and CR parasitic capacitances in both the IC and measurement circuit also influence the signal pattern. When dimensioning on the basis of the given equations a deviation from the theoretical value in the output characteristic must thus be reckoned with. For this reason a calibration algorithm has been developed (Kali1_cav424.xls) which at constant integrator charging currents (ICM and ICR) calculates a suitable oscillator frequency of fosz depending on the minimum value of CM,min (basic capacitance). It then dimensions the resistors for the offset and signal span in such a way that the output signal adopts the required values. Compensation of the sensor system is carried out in two stages. In stage one a calibration operating point is defined, during which process oscillator frequency fosc is calculated depending on minimum measurement capacitance CM,min. To this end the minimum and maximum values (CM,min and CM,max) are entered in the Excel spreadsheet. The oscillator frequency, oscillator capacitance COSC and oscillator resistance ROSC are then output. In addition low pass filter capacitances CL1 and CL2 are calculated which are dependent on the oscillator frequency. It is sufficient if these values are computed once for the largest expected value of minimum basic capacitance CM,min (for example during one production batch) and the relevant capacitors added to the circuit. The maximum signal frequency is also determined by which the measurement capacitance is permitted to change. Taking the given and calculated external components and particularly predefined precision resistors RL1(mess) and RL2(mess) we can calculate the output voltage VLPOUT(mess). NB: RL1(mess) and RA(mess) must both be 100kOhm precision resistors with a tolerance of 0.1% maximum. Output signal values VLPOUT(mess) are now entered in stage two of the calibration program. Using the measured values the algorithm now calculates the setpoint for the two calibration resistors RL1 and RA which replace precision resistors RL1(mess) and RL2(mess) and must be individually mounted. Depending on the accuracy requirements of the setup their values should match those calculated as closely as possible.
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 13/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de Once the two calibration resistors RL1 and RA have been replaced by precision resistors RL1(mess) and RL2(mess) the system is calibrated to the required output value – with all parasitic effects and tolerances taken into account. In stage one of b), at a given fixed oscillator frequency calibration spreadsheet Kali2_cav424.xls calculates the values of integration currents ICM and ICR which can be achieved by setting resistors RCM and RCR. Using these values and the other external elements output voltage VLPOUT(mess) is measured and the value entered into the calibration program. The rest of stage two is identical to the calibration procedure described in a). INITIAL OPERATION Initial operation is described in detail in the description of the calibration program (see Kali1_cav424.xls and Kali2_cav424.xls). OUTPUT VOLTAGES The following applies for the output voltage (9): MDIFFLPOUT VVV %" If VM = 2.5V, according to the specifications the schematic shown in Figure 9 is generated. V (Volt) VLP OUT , max VLP OUT , min VM +Vdiff -Vdiff VCC 2,5 Figure 9: Output voltages
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 14/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de EXAMPLE APPLICATIONS Figure 10: Protective EMC circuitry for CAV 424 When measuring capacitance the electrodes are receptive to high-frequency disturbances such as aerials. Measures must thus be taken to protect these high impedance inputs. To protect against EMC resistors REMVM and REMVR are plugged into the supply lines servicing external capacitors CM and CR. Together with the parasitic and internal capacitances these act as low passes and thus suppress high-frequency disturbance factors. The following applies: CM EMVM I VR 1.0" and CR EMVR I VR 1.0" Further protective EMC measures are not required for industrial applications. As CAV424 has been manufactured using bipolar technology the IC is robust with regard to ESD. CAV424 Signalverarbeitung VCC RCOSCRCX2RCX1 VM LPOUT CM CR COSC Integrator 1 Referenz- Oszillator VTEMP T Sensor Stromreferenz 100mV 100mV REMVM REMVR CM CR
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 15/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de BLOCK DIAGRAM AND PINOUT 1 16 2 15 3 14 4 13 5 12 6 11 7 10 8 9 RCOSC RCR RCM RL LPOUT VM VTEMP N.C. CR CL1 CM CL2 COSC VCC GND N.C. Figure 12: CAV424 pinout Table 4: CAV424 pinout PIN NAME DESCRIPTION
1 RCOSC Oscillator current definition
2 RCR Current setting for integrator CR
3 RCM Current setting for integrator CM
4 RL Gain setting
5 LPOUT Output
6 VM Reference voltage 2.5V
7 VTEMP Temperature sensor
8 N.C. Not connected 9 N.C. Not connected
10 GND IC ground
11 VCC Supply voltage
12 COSC Oscillator capacitance
13 CL2 Low pass 2, corner frequency
14 CM Measurement capacitance
15 CL1 Low pass 1, corner frequency
16 CR Reference capacitance
Figure 11: Block diagram of CAV424
CAV424 - C/U transducer IC with adjustable output voltage Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz July 2007 - Rev 1.2 - Page 16/16 Phone: +49 (0)6131/91 073-0 Fax: +49 (0)6131/91 073-30 Internet: http://www.analogmicro.de Email: info@analogmicro.de DELIVERY CAV424 is available as: ! SO16 (n) ! Dice on 5" blue foil ! For sample batches CAV424 can be supplied on a DIL16 SO16 adapter (CAV424Adapt) ADDITIONAL EQUIPMENT For design purposes, by way of support Analog Microelectronics GmbH can also supply a breadboard (BBCAV424) which has been assembled for a set of parameters but which can also be used for individual measurements. FURTHER READING Please see our website for further information (www.analogmicro.de): [1] AN1008 application notes [2] PR1009 press release AMSYS reserves the right to amend any dimensions, technical data or other information contained herein without prior notification.