CAV424

Converter IC for Capacitive Signals 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 CAV424 GENERAL DESCRIPTION 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 VTEMP 7 2 RCX1 RCX2 RCOSC 3 1 11 CAV424 T Sensor Current Reference VCC COSC 12 Reference Oscillator 6 CX1 16 Integrator 1 Integrator 2 Signal Conditioning 5 VM CX2 14 LPOUT 10 15 13 4 GND Figure 1: block diagram CAV424 CL1 CL2 RL 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: info@analogmicro.de January 2002 1/7 Rev. 1.3 Converter IC for Capacitive Signals ELECTRICAL SPECIFICATIONS Tamb = 25°C, VCC = 5V (unless otherwise noted) Parameter Supply Supply Voltage Maximum Supply Voltage Quiescent Current Temperature Specifications Operating Storage Junction Thermal Resistance Tamb Tst Tj –40 –55 DIL16 plastic package SO16 (n) plastic package COSC = 1.6 ⋅ CX1 ROSC = 250kΩ 70 140 85 125 150 VCC VCCmax ICC Tamb = –40 ... 85°C, GLP = 1 0.6 1.0 ratiometric range 4.75 5.00 Symbol Conditions Min. Typ. CAV424 Max. Unit 5.25 17 1.4 V V mA °C °C °C °C/W °C/W Θja Θja Reference Oscillator Oscillator Capacitor Range Oscillator Frequency Range Oscillator Current Capacitive Integrator 1 and 2 Capacitor Range 1 Capacitive Integrator Current 1 Capacitor Detection Sensitivity Capacitor Range 2 Capacitive Integrator Current 2 Detection Frequency Lowpass Adjustable Gain Output Voltage Corner Frequency 1 Corner Frequency 2 Resistive Load at PIN LPOUT Capacitive Load at PIN LPOUT Temperature Coefficient VDIFF (together with Input Stages) Internal Resistor 1 and 2 Temperature Coefficient R01,02 Ratiometric Error of VLPOUT GLP VLPOUT fC1 fC2 RLOAD CLOAD dVDIFF /dT R01, R02 dR01,02 /dT RAT@VDIFF* Tamb = –40 ... 85°C VDIFF = VLPOUT - VM , Tamb = –40 ... 85°C ±100 20 1.9 0.11 R01 = 20kΩ, CL1 =1nF R02 = 20kΩ, CL2 =1nF 200 50 1 1.1 10 VCC – 1.1 8 8 V kHz kHz kΩ pF ppm/°C kΩ 10-3/°C %FS CX1 IX1 ∆ CX CX2 IX2 fDET RCX1 = 500kΩ ∆ CX = (CX2 − CX1 )/CX1 CX2 = CX1 ⋅ (1 + ∆ CX ) RCX2 = 500kΩ CL1 = CL2 =1nF 10 4.75 5 10.5 4.75 5 5 1000 5.38 100 2000 5.38 2 pF µA % pF µA kHz COSC fOSC IOSC 14 1 9.5 10 1800 130 10.75 pF kHz µA * 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 Voltage Reference VM Voltage VM vs. Temperature Current VM d VM / d T IVM IVM Load Capacitance Ratiometric Error of VM Temperature Sensor VTEMP Voltage Sensitivity Thermal Nonlinearity VTEMP dVTEMP/dT RTEMP ≥ 50MΩ RTEMP ≥ 50MΩ RTEMP ≥ 50MΩ, end point method 2.20 2.32 8 0.5 CVM RAT@VM** Tamb = –40...+85°C Source Sink 80 100 0.007 2.5 ±20 ±50 16 –16 120 Symbol Conditions Min. Typ. CAV424 Max. Unit V ppm/°C µA µA nF %FS 2.45 V mV/°C %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 Current Definition of Ref. Oscillator Current Adjustment of Cap. Integrator 1 Current Adjustment of Cap. Integrator 2 Output Stage Resistor Sum Reference Voltage 2.5V (only for internal use) Lowpass Capacitance 1 Lowpass Capacitance 2 Oscillator Capacitance Symbol RCOSC RCX1 RCX2 RL 1 + RL 2 CVM CL 1 CL 2 COSC Min. 235 475 475 90 80 100⋅CX1 100⋅CX1 COSC =1.55⋅CX1 100 200⋅CX1 200⋅CX1 COSC =1.60⋅CX1 COSC =1.65⋅CX1 Typ. 250 500 500 Max. 265 525 525 200 120 Unit kΩ kΩ kΩ kΩ nF 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 OSC 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,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: COSC = 1.6 ⋅ C X 1 , where CX1 is the fixed capacitance (reference capacitance) of a capacitive sensing element. VOSC VCX1 VCX2 VCLAMP T 2 3T 4 T 2T Time t Figure 3: integrator voltage curve 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 = 2 ⋅ ∆VOSC ⋅ (COSC + COSC , PAR , INT + COSC , PAR , EXT ) I OSC , 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