CAV444

CAV444 ANALOGMICRO | Alldatasheet

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Linear C/V-Converter for capacitive input signals Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz May 2014 – Rev. 3.0 Phone:+49 (0)6131/91 0730 -0 Fax: +49 (0)6131/91 073-30 Internet: www.analogmicro.de E–Mail: info@analogmicro.de Principle Function Capacitance\\Voltage-converter IC with linear transfer function and adjustable output Function CAV444 is an integrated circuit for capacitive sens ing applications. The IC converts the connected input measurement capacitance into an output voltag e, which is a linear function of the measure- ment capacitance. CAV444 can be used as stand-alone analog signal-pro cessing IC or as front-end for a micro proc- essor for electronically calibratable sensor systems. Typical Applications

• Humidity measurement
• Distance measurement
• Level sensing
• Material identification
• Object detection

Linear C/V-Converter for capacitive input signals May 2014 – Rev. 3.0 Page 3/12 www.analogmicro.de

FEATURES

• large measurement capacitance input range: 10pF up to 10nF
• ratiometric voltage output
• linear transfer function
• adjustable offset and gain
• fast response time
• measurement oscillator frequency: 1kHz-240kHz
• integrated temperature sensor
• supply voltage 5V ± 5%
• large temperature range: –40°C...+105°C
• easy-to-use calibration procedure (Excel–sheet)
• available in SO16 or as die
• RoHS conform BLOCK DIAGRAM GENERAL DESCRIPTION CAV444 is an integrated C/V converter suitable for the signal-processing in capacitive sensor sys- tems. Its output voltage is a linear function of th e connected measurement capacitance C M and rati- ometric to the supply voltage. The IC is completely analog leading to a fast re- sponse time and a resolution only limited by the signal to noise ratio. CAV444 provides the complete electronics needed for the conversion of capacitive input signals into voltage output signals, which can be amplified and offset adjusted using external trimming resistors. The IC can be used as stand-alone analog signal- processing IC or as front-end for a micro processor for electronically calibratable sensor systems. An Excel-sheet simplifies the external component’s dimensioning as well as the trimming of complete sensor systems. Figure 1: CAV444's block diagram

Linear C/V-converter for capacitive input signals May 2014 – Rev. 3.0 Page 4/12 www.analogmicro.de SPECIFICATIONS 1. Electric Specifications Tamb = 25°C, V CC = 5V, I CM = I CW = 20 µA (unless otherwise noted) Parameter Symbol Conditions Min. Typ. Max. Unit System Operating Temperature Tamb –40 105 °C Supply Voltage VCC 4.75 5.00 5.25 V Current consumption ICC Tamb = 25°C, G = 1 2 mA Measurement Capacitance 1) CM 10 10000 pF Output Voltage 2) VOUT 1.0 4.0 V Temperature Coefficient V OUT 3) TC VOUT dVOUT /(d T·VSpan ) @T amb = –40 ..85°C G = 1, C W = 1.5 nF, C M = 0.1 ..1nF ±100 ppm/°C Maximum Input Signal Fre- quency fSIG,max @ C M= 10pF, R CM = 125k Ω , CF1 = 2nF, C F2 = 2nF 3.5 kHz Minimal Response Time 5) tRES,min @ C M= 10pF, R CM = 125k Ω CF1 = 2nF, C F2 = 2nF 0.4 ms Measurement Oscillator Oscillator Frequency Range fM fM= V REF / (2* DVCM * R CM *C M ) 1 240 kHz Oscillator Voltage DVCM 2.1 2.15 2.2 V Oscillator Current Resistor RCM 50 125 kΩ Oscillator Charge Current ICM ICM = V REF / R CM 20 50 µ A Oscillator Charge Current Spread ICM @R CM = 125k Ω 19 20 21 µ A f/V-Converter Converter Capacitor Range CW CW = 1.4*C M,max *R CM / R CW @R CW =125k Ω , R CM = 125kΩ 14 14000 pF Minimum Converter Voltage VCW,min @PIN 16 1.15 1.2 1.25 V Maximum Converter Voltage VCW,max @PIN 16 4.1 V Converter Current Resistor RCw 100 1250 kΩ f/V-Capacitor Charge Current ICW ICW = V REF / R CW 2 25 µ A f/V-Capacitor Charge Current Spread I CW @R CW = 125k Ω 19 20 21 µ A f/V-Stage Biasing Resistor RA RA= 0.48* R CW , @R CW =125k Ω 60 kΩ Lowpass Stage Internal Low Pass Resistors R01 , R 02 20 kΩ Low Pass Filter Capacitor 6) CF1 ,C F2 2 1100 nF Corner Frequency 1 (3 dB) fCF 1 @C F1 =2nF 4 kHz Corner Frequency 2 (3 dB) fCF 2 @C F2 =2nF 4 kHz Offset Voltage LP-Stage Vofs LP -2 2 mV

Linear C/V-converter for capacitive input signals May 2014 – Rev. 3.0 Page 5/12 www.analogmicro.de Output Stage Adjustable Gain G 1 10 Output Current IOUT Source, Sink 7) -100 100 µ A Differential Output voltage VDIFF VDiff =V out - VREF -1.5 1.5 V Resistive Load at Pin V OUT RL 40 kΩ Capacitive Load at Pin V OUT CL 100 pF Input Offset Voltage VOFS RL = 100M Ω -2 +2 mV Voltage Reference V REF Reference Voltage VREF 2.49 2.5 2.51 V VREF vs. Temperature TC VREF dVREF /(d T·VREF ), Tamb = –40...+85°C ±50 ppm/°C Output Current IVREF Source, Sink 7) -100 100 µ A Load Capacitance CVREF 80 100 120 nF Temperature Sensor V TEMP Output Voltage VTEMP @ T amb = 25°C, RLoad ≥ 50M Ω 2.20 2.32 2.45 V Sensitivity S S = d VTEMP /d T, RLoad ≥ 50M Ω 8 mV/°C Resistive Load RLoad 10 MΩ Thermal Nonlinearity RLoad ≥ 50M Ω , Tamb = –25 ... 85°C 0.5 % Notes: 1) For linearity better than 0.3% (BFSL) it is recomm ended to use a maximum measurement capacitance CM,max not larger than ten times the particular minimum measurement capacitance CM,min . 2) If VCC ≠ 5 V , the maximum of VOUT is given by 0.8 ·VCC . 3) The temperature coefficient is normalized with VSpan = V Out (C M, max ) – V Out (C M, min ). TC VOUT is increased, if CM ( CW) is decreased. 4) The maximum input signal frequency fSIG,max is defined as the measurement capacitance’s change rate, at which the low pass filter reduces the output voltage by 6 dB. In general fSIG,max depends on the choice of values of CF1 and CF2 and can be increased if CF1 and CF2 are decreased (see note 6 below). 5) CAV444’s response time can be decreased if smaller low pass capacitors CF1 and CF2 are chosen. 6) The typical dimensioning of the low pass capacit ors CF1 and CF2 is based on the requirement that a ripple of less than 1 ‰ remains at the highest oscillator frequenc y. Smaller capacitances can be chosen but lead to a higher ripple. 7) Currents flowing into the IC have a negative sig n. 2. Absolute Maximum Values Parameter Symbol Condition Min. Typ. Max. Unit Maximum Supply Voltage VCCmax 6 V Maximum Oscillator Charge Current ICMmax 50 µ A Maximum f/V-Capacitor Charge Current ICWmax 25 µ A Storage temperature T -55 125 °C ESD Susceptibility 1) VESD HBM 2 kV Notes: 1) ESD Protection on all pins except pin 12, pin 14 an d pin 16. Table 1: CAV444’s electric specifications Table 2: Absolute Maximum Values

Linear C/V-converter for capacitive input signals May 2014 – Rev. 3.0 Page 6/12 www.analogmicro.de

APPLICATION INFORMATION

1. Functional Principle CAV444’s functional principle is described using Figure 2, where the functional blocks of the IC, the signal patterns inside the IC and a basic external wiring with necessary passive components are shown. The IC consists of six functional blocks: the measu rement oscillator, the f/V converter, the low pass filter, the output stage, the temperature sensor and the power supply. The power supply block drives all the other blocks and also generates a reference voltage of 2. 5 V. The temperature sensor block provides an outpu t voltage VTEMP proportional to the IC’s temperature. The capacita nce measurement path consists of the re- maining four blocks and is described below. CAV444 uses the following measurement principle: Th e input measurement capacitance C M, connected at pin 12, works as the capacitor of the measurement o scillator block. CM is charged and discharged periodi- cally with constant current by CAV444. At the measu rement oscillator’s output a triangular voltage wit h con- stant amplitude is generated. The frequency of this triangular voltage (measurement oscillator frequen cy fM) depends on the connected measurement capacitance CM. Using the f/V converter block and the following low pass filter block the triangular voltage is con verted into a DC voltage, which can be impedance tr ans- formed or amplified at the output stage. Furthermor e the output signal’s offset can be adjusted by usi ng the circuit in Figure 4. The output voltage VOUT at pin 5 (referenced to GND ) is ratiometric to the supply voltage and a linear function of CM.1 1 If the differential output voltage V DIFF = V OUT - V REF is evaluated and if no further offset adjustment is done, VDIFF is di- rectly proportional to C M. Figure 2: CAV444 with signal path and a basic circuit

Linear C/V-converter for capacitive input signals May 2014 – Rev. 3.0 Page 7/12 www.analogmicro.de 2. Transfer Function The system shown in Figure 2 can be used as basic circuit for the CAV444. The o utput voltage VOUT at pin 5 (referenced to GND) is given by: REF M CW W CM CM REF MLPAS MOUT VCRC RVVCVCV +⋅ ⋅∆ ⋅=+= 8 3)()( (1) with V LPAS = low-pass voltage VREF = reference voltage (with VREF = 2.5 V ) CW = f/V converter capacitance CM = measurement capacitance ∆VCM = voltage amplitude at the measurement capacitance (with ∆VCM ≈ 2.1 V ) RCM = oscillator current resistor RCW = reference current resistor RCM and RCW define the charge currents ICM (=V REF /R CM ) and ICW ( =V REF /R CW ) for the input capacitance CM and the converter capacitor CW. A typical dimensioning for these fixed resistors is RCM = R CW = 125 k Ω.2 CW’s dimensioning is given by: CW MCM W R CRC max , 4 . 1 ⋅⋅= (2) wherein CM,max is the particular used maximum measurement capacitance. Combining (1) and (2) leads to REF M M MOUT VC CCV +⋅= max ,16 9)( (3) The output voltage given by equation (3) is a linea r function of CM and is illustrated in Figure 3. A minimum measurement capacitance CM,min ≠ 0 leads to a minimum output voltage VOUT (C M,min ) > VREF . If instead of VOUT the differential voltage VDIFF between pin 5 and pin 6 is evaluated, the output s ignal is di- rectly proportional to CM. max ,16 9)()( M M REF MOUT MDIFF C CVCVCV ⋅=−= (4) 2 For an optimal and easy dimensioning of the external passive components Analog Microelectronics has developed the Excel-tool, Kali_CAV444 (see section “Further Literature”). Figure 3: Output signal V OUT as a function of the measurement capacitance C M

Linear C/V-converter for capacitive input signals May 2014 – Rev. 3.0 Page 8/12 www.analogmicro.de 3. C/V-converter with adjustable gain and offset For the adjustment of the output signal’s offset an d gain five resistors R1 .. R 5 have to be added to the net- work in Figure 2. The complete measurement circuit with CAV444 is illustrated in Figure 4. For the capacitive measurement system in Figure 4 the output voltage VOUT at pin 5 (referenced to GND ) is given by: REF LPAS OUT VBVGV ⋅+⋅= (5) with 5 35 44 3 5 45 3

11 RRRRRR

R R R RG ++ +⋅++= and 5 35 44 3 5 45 3 R RB ++ −⋅+= . (6) wherein R2, R 4 and R5 are fixed resistors, R 1 is the trimming resistor for the adjustment of gain and R 3 is the trimming resistor for the adjustment of offset. 3 Using the equations (1), (5) and (6) it is easy to see that the output voltage is again a linear funct ion of the measurement capacitance CM. REF M CW W CM CM MOUT VBCRC RVGCV ⋅+⋅ ⋅∆ ⋅⋅= 8 3)( (7) With the standard dimensioning for CW (see equation (2)) the output voltage is given by: REF M M MOUT VBC CGCV ⋅+⋅⋅= max ,16 9)( (8) 3 The final values of R1 and R3 depend on the desired output voltage as well as on the individual sensor setup. In addi- tion variances of the components (like manufacturing tolerances in RCM , RCW , CW, C M…) and the parasitic capacitances also influence the value of R1 and R3 (see section “Calibration Procedure”). Figure 4: CAV444 used as C/V-converter with adjustable gain and offset

Linear C/V-converter for capacitive input signals May 2014 – Rev. 3.0 Page 9/12 www.analogmicro.de 4. Standard Dimensioning A basic dimensioning of the passive external compon ents in Figure 4 with a measurement capacitance not larger than 1 nF is given in the table below. For t he application specific capacitance ranges an optim al di- mensioning can be calculated using the Excel-sheet Kali_CAV444. Parameter Symbol Min. Typ. Max. Unit Output Stage Resistors (1%) R2 , R 4 , R 5 100 kΩ Gain Resistor (0.1%) Calibration Start Value 1) R1 33 kΩ Offset Resistor (0.1%) Calibration Start Value 1) R3 100 kΩ f/V-Stage Biasing Resistor RA 60 kΩ Measurement Oscillator Current Resistor RCM 125 kΩ f/V-Stage Current Resistor RCW 125 kΩ Low Pass Filter Capacitors (depends on value of C M,max )3) CF1 ,C F2 1000 nF Reference Voltage capacitor ( VREF = 2,5V) 4) CVREF 80 100 120 nF f/V converter capacitor CW 1.4 * C M,max pF Notes: 1) The given values for the trimming resistors R1 and R3 are the initial values at the start of the trimmin g process. During the trimming process R1 and R3 will be set to individual values (calculated by th e Excel-sheet Kali_CAV444). 2) In many applications it is sufficient to use sta ndard components, like e-series capacitors and resi stors with low temperature coefficients ( ≤100 ppm). For RCM , RCW , RA, CVREF and CF1/F2 a variance of 5% of the given value is acceptable without a decrease in performance. For c omponents, which have to have lower variances, the toler- ances are given in round brackets in Table 3. 3) CF1 und CF2 depend on the minimum measurement capacitance CM,min and are calculated in such a way, that only a ripple of less than 1 ‰ remains at the highest os cillator frequency fM. 4) For best performance a high grade ceramic capaci tor has to be used for CVREF . 5. Calibration Procedure During the design of capacitive sensor systems it a lways has to be taken into account that there are p arasitic capacitances beside the measurement capacitance, wh ich influence the output signal and the calibration process. To simplify the consideration of these eff ects and to assist the user during the calibration of sensor systems based on the circuit in Figure 4, Analog Microelectronics developed a calibration st rategy, imple- mented in the Excel-sheet Kali_CAV444. This Excel-sheet is available at www.analogmicro.de . Using the Excel-sheet Kali_CAV444 the calibration o f capacitive sensor systems with CAV444 is done in two steps, a dimensioning and a trimming step. At first a dimensioning of the passive external components is cal- culated for the specific type of sensor implemented with CAV444. This dimensioning is mostly dependent on the capacitive measurement range of the sensor. Onl y the minimum and the maximum measurement capaci- tances as well as the desired differential output v oltage and the charge currents ICM and ICW are needed for the calculation. Based on this input values the pro gram calculates a suitable dimensioning of the exte rnal components, which can be used to assemble this type of sensor system. After the specific sensor systems have been built w ith the calculated dimensioning, a trimming of offs et and gain (using the trimming resistors R1 and R3) of the individual sensor systems has to be done. Therefore the output voltage has to be measured at the minimum an d the maximum measurement capacitance. Based on these values the Excel-sheet calculates the final t rimming resistor values for R1 and R3. After trimming R1 and R3 to the calculated values the sensor is ready for o peration and all parasitic capacitances as well as the tolerances of the used components are taken into account. Table 3: Standard values for external components at ICM = I CW = 20 µA, CM,max < 1nF

Linear C/V-converter for capacitive input signals May 2014 – Rev. 3.0 Page 10/12 www.analogmicro.de 6. Operation Instructions It is recommended to do the external component’s di mensioning using the Excel-sheet Kali_CAV444. In general the absolute maximum values in Table 2 should not be exceeded. For first investigations A nalog Mi- croelectronics offers the BBCAV444, a pre-assembled breadboard (see accessories) with easily adaptable measurement capacitance ranges, which can be used t o study the behavior of capacitance sensor heads as well. To realize the dimensioning calculated by the Excel -sheet networks of not more than two resistors or c apaci- tors should be used. In many applications it is suf ficient to use standard components, like e-series c apaci- tors, which fit to the calculated values within 5% (see section “Standard Dimensioning”) For the PCB layout it is recommended to place C M and CW as close to the IC as possible and to keep con- ducting lines from the IC to its external passive components short, leading to small parasitic capacitances. The parasitic capacitances inside the IC and in the sensor setup enlarge the used capacitances, especi ally the measurement and converter capacitance. This inf luences the capacitance measurement directly. There - fore it is important to keep the parasitic capacitances stable (e.g. no flexible wiring). A capacitor can be added in parallel to the measure ment capacitance to measure capacitances smaller th an the specified 10 pF. But in some cases the parasiti c capacitances in the measurement setup are already large enough, making the additional capacitance unn ecessary. Adding a capacitance in parallel to the m eas- urement capacitance can also be advantageous if the ratio of the used maximum and minimum measure- ment capacitances CM,max /C M,min is larger than ten to increase CAV444’s linearity. In real sensor applications the temperature behavio r is mainly influenced by CM’s, CW’s and further external component’s temperature coefficients. In general an optimal temperature behavior can be achieved by us ing CM and CW with equal temperature coefficients ( TC ). Because CAV444’s temperature coefficient increas es for small measurement capacitances it can be useful to shift the measurement capacitance range to high er values by connecting an additional capacitor in parallel to the measurement capacitance. In general ESD precautions are necessary during ass embly and handling of the device. It is essential t o ground machines and personnel properly. Notes: 1. If the voltage signal at pin 12 or 16 is measure d, the probe’s capacitance is added to the capacita nce at the spe- cific pin, which changes the output voltage signal. If measured at pin 12 the oscillator frequency is decreased and if measured at pin 16 the signal amplitude at the f /V-converter is reduced. To check the frequency and the signal amplitude after the f/V-converter without influencing the output signal it is possible to remove the low pass capaci- tor CF1 and measure at pin 15. 2. For level sensing applications it is important t o isolate the measurement electrodes from a conduct ive medium. Otherwise the output voltage is affected by the conductivity of the medium.

Linear C/V-converter for capacitive input signals May 2014 – Rev. 3.0 Page 11/12 www.analogmicro.de PACKAGE AND PINOUT The CAV444’s standard packaging is a SO16 (n) packa ge (for dimensions please see the packaging catalog DELIVERY FORMS CAV444 is available as: ORDER NUMBER DELIVERY FORM CAV444-0-SO16 CAV444 inside SO16 packaging CAV444-WAF sawn wafer on 8“ blue foil CAV444-Adapt CAV444 soldered to a SO16-DIL16 adapter ACCESSORIES To support developments using CAV444 Analog Microelectronics GmbH offers the Breadboard BBCAV444. ORDER NUMBER DELIVERY FORM BBCAV444 BBCAV444 - BreadBoard (PCB with CAV444) FURTHER LITERATURE 3. Application Notes on CAV444 (on request) 4. CAV444 - Die Size and Padout (on request) PIN NAME DESCRIPTION

1 RCM oscillator current adjustment

2 RCW f/V converter current adjustment

3 VB bias voltage connected to V CC

4 GAIN gain adjustment

5 VOUT output voltage

6 VREF reference voltage ca. 2.5V

7 VTEMP temperature sensor voltage output

8 N.C. not connected 9 N.C. not connected

10 GND IC ground

11 VCC supply voltage

12 CM measurement capacitance

13 CF 2 2nd order lowpass capacitor

14 RA stabilizing resistor f/V converter

15 CF 1 1st order lowpass capacitor

16 CW f/V converter capacitor

Table 4: Pin assignment CAV444 SO16 Package Figure 5: Pinout CAV444 SO16

Linear C/V-converter for capacitive input signals May 2014 – Rev. 3.0 Page 12/12 www.analogmicro.de NOTES Analog Microelectronics GmbH reserves the right to amend any dimensions, technical data or other information contained herein without prior notification.