Converter IC for Capacitive Signals CAV424 FEATURES GENERAL DESCRIPTION • 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 The CAV424 is an integrated C/V converter and contains the complete signal processing unit for capacitive signals on chip. 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 differential voltage output signal can be directly connected to a following A/D converter or another signal conditioning IC from Analog Microelectronics. Using the integrated temperature sensor, digital adjustable systems can be built easily. APPLICATIONS • • • • • Industrial Process Control Distance Measurement Pressure Measurement Humidity Measurement Level Control DELIVERY • DIL16 packages • SO16(n) packages • Dice put on 5“ blue foil BLOCK DIAGRAM RCX1 RCX2 RCOSC VTEMP 7 3 2 1 VCC CAV424 11 T Sensor Current Reference COSC 12 Reference Oscillator 6 VM CX1 16 Signal Conditioning Integrator 1 CX2 14 Integrator 2 5 LPOUT 10 GND 15 CL1 13 CL2 4 RL Figure 1: block diagram CAV424 analog microelectronics Analog Microelectronics GmbH An der Fahrt 13, D – 55124 Mainz Internet: http://www.analogmicro.de Phone: +49 (0)6131/91 073 – 0 Fax: +49 (0)6131/91 073 – 30 E–mail: [email protected] January 2002 1/7 Rev. 1.3 Converter IC for Capacitive Signals CAV424 ELECTRICAL SPECIFICATIONS Tamb = 25°C, VCC = 5V (unless otherwise noted) Parameter Symbol Conditions Min. Typ. Max. Unit 4.75 5.00 5.25 V 17 V 1.4 mA Supply Supply Voltage VCC Maximum Supply Voltage VCCmax Quiescent Current ICC ratiometric range Tamb = –40 ... 85°C, GLP = 1 0.6 1.0 Temperature Specifications Operating Tamb –40 85 °C Storage Tst –55 125 °C Junction Tj Thermal Resistance Θja DIL16 plastic package 70 150 °C/W °C Θja SO16 (n) plastic package 140 °C/W Oscillator Capacitor Range COSC COSC = 1.6 ⋅ CX1 Oscillator Frequency Range fOSC Oscillator Current IOSC Reference Oscillator 14 1800 pF 1 130 kHz 10.75 µA 1000 pF 5.38 µA ROSC = 250kΩ 9.5 10 Capacitive Integrator 1 and 2 Capacitor Range 1 CX1 10 Capacitive Integrator Current 1 IX1 RCX1 = 500kΩ 4.75 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 Detection Frequency fDET CL1 = CL2 =1nF 5 5 5.38 µA 2 kHz Lowpass Adjustable Gain GLP Output Voltage VLPOUT Corner Frequency 1 fC1 R01 = 20kΩ, CL1 =1nF Corner Frequency 2 fC2 R02 = 20kΩ, CL2 =1nF Resistive Load at PIN LPOUT RLOAD Capacitive Load at PIN LPOUT CLOAD Temperature Coefficient VDIFF (together with Input Stages) dVDIFF /dT Internal Resistor 1 and 2 R01, R02 Temperature Coefficient R01,02 dR01,02 /dT Ratiometric Error of VLPOUT RAT@VDIFF* 1 10 1.1 VCC – 1.1 V 8 kHz 8 kHz 200 kΩ 50 VDIFF = VLPOUT - VM , Tamb = –40 ... 85°C Tamb = –40 ... 85°C pF ±100 ppm/°C 20 kΩ 1.9 10-3/°C 0.11 %FS * RAT @ VDIFF = 2 [1.05 VDIFF(VCC = 5V) – VDIFF(VCC = 5.25V)]/[VDIFF(VCC = 5V) + VDIFF(VCC = 5.25V)] analog microelectronics January 2002 2/7 Converter IC for Capacitive Signals Parameter Symbol Conditions CAV424 Min. Typ. Max. Unit Voltage Reference VM Voltage VM VM vs. Temperature dVM /dT Tamb = –40...+85°C Current IVM Source IVM Sink Load Capacitance CVM Ratiometric Error of VM RAT@VM** 2.5 V ±20 80 100 ±50 ppm/°C 16 µA –16 µA 120 nF 0.007 %FS Temperature Sensor VTEMP Voltage VTEMP RTEMP ≥ 50MΩ Sensitivity dVTEMP/dT RTEMP ≥ 50MΩ Thermal Nonlinearity 2.20 2.32 RTEMP ≥ 50MΩ, end point method 2.45 V 8 mV/°C 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 500 Current Adjustment of Cap. Integrator 2 RCX2 475 RL1 + RL2 90 Reference Voltage 2.5V (only for internal use) CVM 80 100 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 Output Stage Resistor Sum 525 kΩ 200 kΩ 120 nF COSC =1.65⋅CX1 Note: The system performance over temperature forces that the resistors RCX1, RCX2 and ROSC have the same temperature coefficient 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 frequency is set via capacitance COSC, drives two symmetrical integrators 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. With high common-mode rejection ratio and a high resolution, com- analog microelectronics January 2002 3/7 Converter IC for Capacitive Signals CAV424 parison of the two amplitudes proV duces a signal which corresponds to V the change in capacitance of CX1 and CX2 relative to one another. This difference signal is rectified in an ensuing low pass. The filtered DC signal V is transferred to the differential, adjustable output stage. Individual circuit variables, such as filter constants T Time t 3T T 2T and amplification, can be set with just 2 4 a few external components. By using Figure 2: oscillator voltage curve the integrators and their capacitances CX1 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. OSC OSC,HIGH OSC,LOW The way a capacitive sensor functions whose signal can be conditioned with a CAV424 is described in detail in the following section. Simple 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 COSC, the internal parasitic capacitance of the IC, COSC,PAR,INT, and the external parasitic capacitance COSC,PAR,EXT (from a printed board assembly, for example). Oscillator capacitance COSC is dimensioned as follows: VOSC VCX1 VCX2 VCLAMP T 2 COSC = 1.6 ⋅ C X 1 , 3T 4 T 2T Time t Figure 3: integrator voltage curve 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 OSC = VM ROSC The frequency of the reference oscillator fOSC is given by f OSC = I OSC 2 ⋅ ∆VOSC ⋅ (COSC + COSC , PAR , INT + COSC , PAR , EXT ) analog microelectronics , January 2002 4/7 Converter IC for Capacitive Signals CAV424 where ∆VOSC is the difference between the thresholds (VOSC,HIGH and VOSC,LOW) of the internal reference oscillator. ∆VOSC is defined via internal resistances 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: V I CX = M RCX Capacitance CX is charged up to maximum voltage VCX and can be calculated as follows: I CX VCX = + VCLAMP 2 ⋅ f OSC ⋅ (C X + C X , PAR , INT + C X , PAR , EXT ) The two voltages across capacitances CX1 and CX2 are subtracted from one another. Applied to the reference voltage VM the resulting differential voltage is: VCX ,DIFF = (VCX 1 − VCX 2 ) + V M 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: f DET < f C << f OSC The external capacitance for the required cut-off frequency fC amounts to 1 CL = 2π ⋅ R0 ⋅ f C The output signal of the low-pass filter tracing the ideal curve shown in Figure 3 is calculated as 3 VLPOUT = VDIFF ,0 + VM with VDIFF , 0 = ⋅ (VCX 1 − VCX 2 ) 8 Should the differential output voltage VDIFF,0 be too small it can be amplified using the non-inverting output amplifier, with the degree of amplification being determined by resistances RL1 and RL2. The amplification of the stage is R GLP = 1 + L1 RL 2 analog microelectronics January 2002 5/7 Converter IC for Capacitive Signals CAV424 It thus follows that the output signal of the low-pass stage is VLPOUT = VDIFF + VM with 3 VDIFF = GLP ⋅ VDIFF ,0 = GLP ⋅ ⋅ (VCX 1 − VCX 2 ) 8 In order to reduce the number of external components 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 RCX1 7 ROSC RCX2 3 2 1 VCC CAV424 11 T Sensor Current Reference COSC 12 Reference Oscillator 16 Integrator 1 14 Integrator 2 6 CVM Signal Conditioning VDIFF CX1 5 CX2 10 15 CL1 13 CL2 4 RL1 RL2 GND 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). Therefore one of this resistors is varied until the differential voltage VDIFF = VLPOUT − VM is zero: VDIFF = 0 analog microelectronics January 2002 6/7 Converter IC for Capacitive Signals CAV424 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 calculated: • COSC: 80pF • CL1: 10nF • CL2: 10nF PINOUT RCOSC 1 16 CX1 RCX1 2 15 CL1 RCX2 3 14 CX2 RL 4 13 CL2 LPOUT 5 12 COSC VM 6 11 VTEMP 7 10 VCC GND N.C. 8 9 N.C. Figure 5: pinout CAV424 DELIVERY PIN NAME 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 RCOSC RCX1 RCX2 RL LPOUT VM VTEMP N.C. N.C. GND VCC COSC CL2 CX2 CL1 CX1 DESRIPTION Current Definition of Ref. Oscillator Current Adjustment of Cap. Integrator 1 Current Adjustment of Cap. Integrator 2 Gain Adjustment Output Reference Voltage 2.5V Temperature Sensor Not Connected Not Connected IC Ground Supply Voltage Capacitor of Reference Oscillator Corner Frequency of Lowpass 2 Integrator Capacitor 2 Corner Frequency of Lowpass 1 Integrator Capacitor 1 The CAV424 is available in version: • 16 pin DIL • SO 16 (n) (maximum power dissipation PD = 300mW) • Dice on 5“ blue foil 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. analog microelectronics January 2002 7/7