CAV424 ETC

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Converter IC for Capacitive Signals CAV424 analog microelectronics January 2002 Analog Microelectronics GmbH Phone: +49 (0)6131/91 073 – 0 1/7 An der Fahrt 13, D – 55124 Mainz Fax: +49 (0)6131/91 073 – 30 Rev. 1.3 Internet: http://www. analogmicro.de E–mail: info@analogmicro .de

FEATURES

  • Ratiometric Supply Voltage: 5V ± 5%
  • Wide Operating Temperature Range: –40°C...+85°C
  • High Detection Sensitivity of Relative Capacitive Changes: 5% – 100%
  • Detection Frequency up to 2kHz
  • Differential Output Signal with Great Voltage Swing
  • Integrated Temperature Sensor
  • Adjustable with only two Resistors

APPLICATIONS

  • Industrial Process Control
  • Distance Measurement
  • Pressure Measurement
  • Humidity Measurement
  • Level Control GENERAL DESCRIPTION The CAV424 is an integrated C/V converter and contains the complete signal processing unit for capacitive signals on chi p. The CAV424 detects the relative capacitive change of a measuring capacity to a fixed reference capacity. The IC is optimised for capacities in the wide range of 10pF to 2nF with possible changes of capacity of 5% to 100% of the reference capacity. The dif- ferential voltage output signal can be directly connected to a following A/D converter or an- other signal conditioning IC from Analog Mi- croelectronics. Using the integrated temperature sensor, digital adjustab le systems can be built easily. DELIVERY
  • DIL16 packages
  • SO16(n) packages
  • Dice put on 5“ blue foil BLOCK DIAGRAM CA V424 Signal Conditioning VCC GND RCOSCRCX2RCX1 VM LPOUT RLCL2CL1 CX2 CX1 COSC T Sensor Integrator 1 Integrator 2 Reference Oscillator VTEMP 131510 Current Reference Figure 1: block diagram CAV424

Converter IC for Capacitive Signals CAV424 analog microelectronics January 2002 ELECTRICAL SPECIFICATIONS Tamb = 25°C, VCC = 5V (unless otherwise noted) Parameter Symbol Conditions Min. Typ. Max. Unit Supply Supply Voltage VCC ratiometric range 4.75 5.00 5.25 V Maximum Supply Voltage VCCmax 17 V Temperature Specifications Operating Tamb –40 85 °C Storage Tst –55 125 °C Junction Tj 150 °C Thermal Resistance Θja DIL16 plastic package 70 °C/W Θja SO16 (n) plastic package 140 °C/W Reference Oscillator Oscillator Capacitor Range COSC COSC = 1.6 ⋅ CX1 14 1800 pF Oscillator Frequency Range fOSC 1 130 kHz Oscillator Current IOSC ROSC = 250kΩ 9.5 10 10.75 µA Capacitive Integrator 1 and 2 Capacitor Range 1 CX1 10 1000 pF Capacitive Integrator Current 1 IX1 RCX1 = 500kΩ 4.75 5 5.38 µA Capacitor Detection Sensitivity ∆ CX ∆ CX = (CX2 − CX1 )/CX1 5 100 % Capacitor Range 2 CX2 CX2 = CX1 ⋅ (1 + ∆ CX ) 10.5 2000 pF Capacitive Integrator Current 2 IX2 RCX2 = 500kΩ 4.75 5 5.38 µA Detection Frequency fDET CL1 = CL2 =1nF 2 kHz Lowpass Adjustable Gain GLP 11 0 Output Voltage VLPOUT 1.1 VCC – 1.1 V Corner Frequency 1 fC1 R01 = 20kΩ, CL1 =1nF 8k H z Corner Frequency 2 fC2 R02 = 20kΩ, CL2 =1nF 8k H z Resistive Load at PIN LPOUT R LOAD 200 kΩ Capacitive Load at PIN LPOUT C LOAD 50 pF Temperature Coefficient VDIFF (together with Input Stages) dVDIFF /dTV DIFF = VLPOUT - VM , Tamb = –40 ... 85°C ±100 ppm/°C Internal Resistor 1 and 2 R01, R02 20 kΩ Temperature Coefficient R01,02 dR01,02 /dTT amb = –40 ... 85°C 1.9 10 -3/°C Ratiometric Error of VLPOUT RAT@VDIFF*0 . 1 1 % F S * RAT @ VDIFF = 2 [1.05 VDIFF(VCC = 5V) – VDIFF(VCC = 5.25V)]/[VDIFF(VCC = 5V) + VDIFF(VCC = 5.25V)]

Converter IC for Capacitive Signals CAV424 analog microelectronics January 2002 Parameter Symbol Conditions Min. Typ. Max. Unit Voltage Reference VM Voltage VM 2.5 V VM vs. Temperature d VM /dTT amb = –40...+85°C ±20 ±50 ppm/°C Current IVM Source 16 µA IVM Sink –16 µA Load Capacitance CVM 80 100 120 nF Ratiometric Error of VM RAT@VM 0.007 %FS Temperature Sensor VTEMP Voltage VTEMP RTEMP ≥ 50MΩ 2.20 2.32 2.45 V Sensitivity d VTEMP/dT RTEMP ≥ 50MΩ 8m V / ° C Thermal Nonlinearity RTEMP ≥ 50MΩ, end point method 0.5 %FS RAT @ VM = 2 [1.05 VM(VCC = 5V) – VM(VCC = 5.25V)]/[VM(VCC = 5V) + VM(VCC = 5.25V)] Note: 1) The oscillator capacity has to be chosen in the following way: COSC = 1.6 ⋅ CX1 2) The capacitor range of CX1 and CX2 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 BOUNDARY CONDITIONS Parameter Symbol Min. Typ. Max. Unit Current Definition of Ref. Oscillator RCOSC 235 250 265 kΩ Current Adjustment of Cap. Integrator 1 RCX1 475 500 525 kΩ Current Adjustment of Cap. Integrator 2 RCX2 475 500 525 kΩ Output Stage Resistor Sum RL1 + RL2 90 200 kΩ Reference Voltage 2.5V (only for internal use) CVM 80 100 120 nF Lowpass Capacitance 1 CL1 100⋅CX1 200⋅CX1 Lowpass Capacitance 2 CL2 100⋅CX1 200⋅CX1 Oscillator Capacitance COSC COSC =1.55⋅CX1 COSC =1.60⋅CX1 COSC =1.65⋅CX1 Note: The system performance over temperature forces that the resistors RCX1, RCX2 and ROSC have the same temperature co- efficient and a very close placement of them in the circuit. The capacities CX1, CX2 and COSC are also forced to have the same temperature coefficient and a very close placement of them in the circuit. FUNCTIONAL DESCRIPTION The CAV424 functions according to the following principle. A variable reference oscillator, whose fre- quency is set via capacitance COSC, drives two symmetrical integr ators which are phase-locked and clock-synchronised. The amplitudes of the two driven integrators are determined by capacitances CX1 and CX2, where CX1 is designated as the (measurement signal) reference capacitance and CX2 as the measurement signal capacitance. W ith high common-mode rejection ratio and a high resolution, com-

Converter IC for Capacitive Signals CAV424 analog microelectronics January 2002 parison of the two amplitudes pro- duces a signal which corresponds to the change in capacitance of C X1 and CX2 relative to one another. This dif- ference signal is rectified in an ensu- ing low pass. The filtered DC signal is transferred to the differential, ad- justable output stage. Individual cir- cuit variables, such as filter constants and amplification, can be set with just a few external components. By using the integrators and their capacitances C X1 and CX2, swings in capacitance of 5% to 100% in relation to the measurement reference capacitance can be measured. As CX1 can be varied in a range of 10 pF to 1 nF, the range of measurement for the measurement signal capacitance is 0-10.5 pF to 0-2 nF. The way a capacitive sensor functions whose signal can be conditioned with a CAV424 is described in detail in the following section. S imple dimensional requirements are given, permitting a sensor system to be assembled. The CAV424 reference oscillator The reference oscillator charges up and then discharges the external oscillator capacitance C OSC, the internal parasitic capacitance of the IC, C OSC,PAR,INT, and the external parasitic capacitance COSC,PAR,EXT (from a printed board assembly, for example). Oscillator capacitance C OSC is dimensioned as follows:

16.1 XOSC CC ⋅= ,

where CX1 is the fixed capacitance (reference capacitance) of a capacitive sensing element. The reference oscillator current IOSC is determined via external resistance ROSC and reference voltage VM: I V ROSC M OSC The frequency of the reference oscillator fOSC is given by () EXTPAROSCINTPAROSCOSCOSC OSC OSC CCCV If ,,,,2 ++⋅∆⋅= , VOSC,HIGH Time t VOSC,LOW VOSC T 2T3T T Figure 2: oscillator voltage curve VCX1 VOSC T 2T3T T VCX2 VCLAMP Time t Figure 3: integrator voltage curve

Converter IC for Capacitive Signals CAV424 analog microelectronics January 2002 where ∆VOSC is the difference between the thresholds ( VOSC,HIGH and VOSC,LOW) of the internal reference oscillator. ∆VOSC is defined via internal resist ances and has a value of 2.1V @ VCC = 5V. The oscillator voltage curve is shown in Figure 2. Capacitive integrators The built-in capacitive integrators function in much the same way as the reference oscillator. One difference lies in the discharge time, which here is twice as long as the charge-up period. Furthermore, the discharge voltage is clamped to an internal fixed voltage, VCLAMP. The signal voltage of capacitances CX1 and CX2 is outlined in Figure 3. The capacitive integrator current ICX is set by external resistance RCX and reference voltage VM: I V RCX M CX Capacitance CX is charged up to maximum voltage VCX and can be calculated as follows: CLAMP EXTPARXINTPARXXOSC CX CX VCCCf IV +++⋅⋅= ,,,,2 The two voltages across capacitances CX1 and CX2 are subtracted from one another. Applied to the reference voltage VM the resulting differential voltage is: ()VV V VCX DIFF CX CX M, =−+ 12 Differential voltage VCX,DIFF is applied to a second-order low- pass filter. The 3dB cut-off frequencies of the two stages, fC1 and fC2, are defined by external capacitances CL1 and CL2 and internal resistances R01 and R02 (typically 20kΩ). The 3dB cut-off frequencies must be selected with regard to the reference oscillator frequency fOSC and the required detection frequency of the overall sensor system ( fDET). Here, the following inequality of the various frequencies must be adhered to: ff fDET C OSC<< < The external capacitance for the required cut-off frequency fC amounts to C RfL C = ⋅⋅ 2 0π The output signal of the low-pass filter tracing the ideal curve shown in Figure 3 is calculated as MDIFFLPOUT VVV += 0, with () 210, CXCXDIFF VVV −⋅= Should the differential output voltage VDIFF,0 be too small it can be amp lified using the non-inverting output amplifier, with the degree of amplification being determined by resistances RL1 and RL2. The amplification of the stage is L L LP R RG +=

Converter IC for Capacitive Signals CAV424 analog microelectronics January 2002 It thus follows that the output signal of the low-pass stage is MDIFFLPOUT VVV += with () 210, CXCXLPDIFFLPDIFF VVGVGV −⋅⋅=⋅= In order to reduce the number of external compone nts needed for the sensor system a temperature acquisition sensor was integrated. With the aid of a processor, this sensor can be used to compensate for the temperature error of the entire sensor system, for example. FUNCTIONAL DIAGRAM VCC CL1 CL2 COSC CX1 CX2 CA V424 131510 ROSCRCX2RCX1 VDIFF GND RL1 RL2 CVM Signal Conditioning T Sensor Integrator 1 Integrator 2 Reference Oscillator Current Reference Figure 4: functional diagram CAV424 Adjustment: The zero-adjustment is made by the resistors RCX1 or RCX2 for the case that the varying capacitance CX2 has nearly the same (and its smallest) value as the fixed capacitance CX1 (reference capacitance). There- fore one of this resistors is varied until the differential voltage MLPOUTDIFF VVV −= is zero: 0=DIFFV

Converter IC for Capacitive Signals CAV424 analog microelectronics January 2002 Application Example: The following values are given:

  • fixed capacitance CX1: 50pF
  • varying capacitance CX2: 50 ... 100pF Calculation: With the equations given in the boundary conditions, the following values for the devices can be cal- culated:
  • COSC: 80pF
  • CL1: 10nF
  • CL2: 10nF PINOUT CX1 CL1 CX2 CL2 COSC VCC N.C. GND RCOSC RCX1 RCX2 RL LPOUT VM VTEMP N.C. 8 9 Figure 5: pinout CAV424 DELIVERY The CAV424 is available in version:
  • 16 pin DIL
  • SO 16 (n) (maximum power dissipation PD = 300mW)
  • Dice on 5“ blue foil PIN NAME DESRIPTION 1 RCOSC Current Definition of Ref. Oscillator 2 RCX1 Current Adjustment of Cap. Integrator 1 3 RCX2 Current Adjustment of Cap. Integrator 2

4 RL Gain Adjustment

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 Capacitor of Reference Oscillator

13 CL2 Corner Frequency of Lowpass 2

14 CX2 Integrator Capacitor 2

15 CL1 Corner Frequency of Lowpass 1

16 CX1 Integrator Capacitor 1

The information provided herein is believed to be reliable; however, Analog Microelectronics assumes no responsibility for inaccuracies or omissions. Analog Microelectronics assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user's own risk. Prices and specifications are subject to change without notice. No patent rights or licences to any of the circuits described herein are implied or granted to any third party. Analog Microelectronics does not authorise or warrant any Analog Microelectronics product use in life support devices and/or systems.