CAV444 C/V transmitter IC with adjustable output voltage for capacitive input signals

Transcription

1 PRINCIPLE FUNCTION Capacitance/Voltage converter IC with an adjustable, differential output and temperature detection V = V % CC ± Measurment capacitor (8 pf bis. nf) CAV V OUT =, ±,V Temperature 8mV/ K Typical applications CAV is an integrated capacitance-to-voltage transducer. The IC is particularly suitable for all measurements designed to convert a capacitive input signal into a voltage that is direct proportional to the change in the capacitance to be measured. It can be used for: Measurement of humidity Level sensing Material identification Object detection The IC can be used as an input circuit for microprocessors or as a stand-alone IC An der Fahrt 3, D Mainz Mai 00 - Rev. - Page /

2 CONTENTS PRINCIPLE FUNCTION FEATURES 3 GENERAL DESCRIPTION 3 PRINCIPLE OF MEASUREMENT HOW CAV WORKS TRANSFER FUNCTION (FULL-SCALE OUTPUT SIGNAL) TRANSFER FUNCTION (WITH ADDITIONAL OFFSET ADJUSTMENT) 6 OUTPUT VOLTAGES 8 THE DIMENSIONING PROCESS 8 INITIAL OPERATION 8 STANDARD DIMENSIONING 0 BOUNDARY CONDITIONS 0 APPLICATIONS BLOCK DIAGRAM AND PINOUT DELIVERY ADDITIONAL EQUIPMENT FURTHER READING NOTES An der Fahrt 3, D Mainz Mai 00 - Rev. - Page /

3 FEATURES Differential output signal Wide capacitor measuring range: 8 pf to. nf Linear transfer behavior Adjustable offset voltage Adjustable full-scale signal Detection frequency: Hz to.9 khz Measurement oscillator frequency: khz to 30 khz Wide dynamic range detection Wide temperature range: -0 C...+8 C Simple calibration (Excel program) RoHS compliant BLOCK DIAGRAM VTEMP 7 GND 0 RA VCC VB 3 GENERAL DESCRIPTION CAV is an integrated C/V transducer that contains full signal conditioning electronics for the linear conversion of capacitive input signals into a suitable differential output voltage. It also has an additional temperature detector. The output signal is proportional to the change in capacitance C M = C M, max C M, min. A differential voltage referenced to internal reference voltage V REF is generated as an output signal. This output voltage has been specially designed for connection to a following A/D converter. As this is an analog circuit, its resolution are only limited by noise. Together with the integrated temperature sensor of the CAV and a processor, electronically calibratable systems can be assembled. A simple Excel software program eases the dimensioning of CAV.. Temperature- Sensor CAV Supply 6 VREF CM RCM Measurement oszillator f/v converter Lowpass filter Output stage VOUT 6 3 CW RCW CF CF GAIN Figure : Block diagram of CAV An der Fahrt 3, D Mainz Mai 00 - Rev. - Page 3/

4 PRINCIPLE OF MEASUREMENT CAV is an integrated C/V (capacitance-to-voltage) converter circuit that contains full signal conditioning and evaluation electronics for linear, capacitive signal sources. The principle of measurement behind CAV is the conversion of a change in capacitance (measurement capacitor, C M ) into a linear, differential output voltage. Measurement capacitor C M can be altered by the amount C M = C M, max C M,min (C M,min is the basic capacitance of C M ). HOW CAV WORKS The CAV IC functions according to the following principle. The measurement capacitor is the capacitor of an internal measurement oscillator. This generates the clock pulse with which the measurement capacitor is charged and discharged. The number of clock pulses provided depends on the measurement capacitor. These are converted into a DC voltage signal in the f/v converter and in the backend lowpass filter. The filtered DC voltage signal travels to an adjustable amplifier stage that enables the output signal to be set to the required value. VTEMP 7 GND 0 RA R A VCC VB 3 Temperature- Sensor CAV Supply 6 VREF C M CM RCM Measurement oszillator f/v converter 6 Lowpass filter 3 CW RCW CF CF Outputstage GAIN VOUT Figure : Block diagram of CAV with signal patterns An der Fahrt 3, D Mainz Mai 00 - Rev. - Page /

5 TRANSFER FUNCTION (FULL-SCALE OUTPUT SIGNAL) V CC VTEMP 7 GND 0 R A RA V B VCC VB 3 C M CM RCM Outputstage Temperature- Sensor Measurement oszillator CAV f/v converter Lowpass filter Supply 6 VREF VOUT C VREF V REF V OUT R CM 6 CW RCW CF C W R CW C F 3 CF C F GAIN R R V REF Figure 3: CAV with circuitry for full-scale adjustment Transfer function V OUT* for the CAV full-scale output signal is expressed as: V * = V * + V OUT DIFF REF where V REF = reference voltage and V The following applies to G LP : DIFF and to the output voltage after the lowpass: V with V CM =. V. LP TPAS () * = G V () R G LP = + (3) R TPAS 3 CM VCM R = 8 C R W CW CM () An der Fahrt 3, D Mainz Mai 00 - Rev. - Page /

6 Equation () implies that signal V TPAS is amplified by an internal operational amplifier in the output stage, where gain G LP can be determined by resistors R and R. Resistor R A is used to set the supply for the f/v converter. Pin VB is for internal biasing and must be connected up to supply voltage V CC. With equations (3) and () incorporated into (), the transfer function for the full-scale signal () is accrued as: R CM VCM RCM V OUT = VDIFF + VREF = GLP VTPAS + VREF = + + VREF R 3 * * CW R () 8 CW CM,max where C W =. The resistors are defined by the respective load currents and have fixed values.6 of R CM = 0k and R CW = 00k. We can see that the output voltage is a linearly dependent function of measurable variable C M (V OUT* = f(c M )), as all other variables are fixed by the dimensioning process. V OUT V REF,V C M C M Min C M Max C M Figure : Output signal V OUT* referenced to ground In Figure we can recognize that the output signal is raised by V REF. In order to be able to fix the offset, the network must be extended. TRANSFER FUNCTION (WITH ADDITIONAL OFFSET ADJUSTMENT) When setting the output signal the adjustability of the offset must be taken into account with the transfer function (). With regard to Figure, the transfer function is calculated as: V OUT = V + V = A V + B V (6) DIFF REF TPAS REF An der Fahrt 3, D Mainz Mai 00 - Rev. - Page 6/

7 with the setup coefficient for span as A and the setup coefficient for the offset as B: R ( RR + R ( R + R )) + R3(( R + R ) R + RR + R ( R + R )) A = and (7) R ( R R + R ( R + R )) 3 B = ( R + R ) R R + R (R R 3 R ( R R + ( R 3 + R ( R + R + R ) R )) + R R + R R ) (8) In the transfer equation resistors R and R 3 used to set the span and offset are variable. They are calculated in the Excel program Kali_CAV.xls. R, R and R are fixed 00k resistors. We can see that the output voltage is a linearly dependent function of measurable variable C M (V OUT = f(c M )), as all other variables are fixed by the dimensioning process and equation (). V CC VTEMP 7 GND 0 R A RA V B VCC VB 3 C M CM RCM Outputstage Temperature- Sensor Measurement oszillator CAV f/v converter Lowpass filter Versorgung 6 VREF C VREF VOUT V REF V OUT V CC R CM 6 CW RCW CF C W R CW C F 3 CF C F GAIN R R R R 3 R V REF GND Figure : Full circuit with adjustable full-scale output signal and adjustable offset signal An der Fahrt 3, D Mainz Mai 00 - Rev. - Page 7/

8 OUTPUT VOLTAGES The following applies to the output voltage: V = V + V OUT DIFF REF V OUT (Volt) V CC, V OUT, max +V DI FF V REF -V DIFF 0 V OUT, min Figure 6: Maximum output voltage VOUT THE DIMENSIONING PROCESS Excel program Kali_CAV.xls should be used for dimensioning purposes. The dimensioning process for CAV assumes that in addition to measurement capacitor C M and the f/v converter capacitor C W, parasitic capacitances in both the IC and measurement circuit also influence the signal pattern. For this reason the offset and full scale are calibrated based on the installed system, where all parasitic capacitances and exemplary variations in the components used have been taken into account. Taking transfer function (6) as its basis, the Excel spreadsheet Kali_CAV.xls [] first computes suitable values for a measurement operating point. The circuit output is measured at this point and these measurements then entered into the program in stage two of the procedure. The algorithm calculates the two adjusting resistors required to calibrate the system. Once these have been placed in the circuit, the calibration of both offset and span is complete. INITIAL OPERATION Initial operation is described in detail in the description of the calibration program (see Kali_CAV.xls). An der Fahrt 3, D Mainz Mai 00 - Rev. - Page 8/

9 ELECTRICAL SPECIFICATIONS T amb = C, V CC = V (unless otherwise stated) Parameter Symbol Conditions Min. Typ. Max. Unit Supply Supply Voltage V CC Ratiometric range V Quiescent Current I CC T amb = C, G LP = ma Temperature Specifications Operating T amb -0 0 C Storage T st - C Measurement Oscillator Measurement Capacitor Range C M I CM = 0µA 8 00 pf Oscillator Frequency Range f M 30 khz Oscillator Current I CM R CM = 0kΩ µa Detection Frequency f SIG.9 khz f/v Converter Converter Capacitor Range C W C W = C M,max /.6 I CW = µa. 37 pf Capacitive Charge Current I CW R CW = 00 kω.7.38 µa Lowpass Stage Adjustable Gain G LP 0 Output Voltage V OUT V out = V Diff + V REF. V CC. V Corner Frequency f CF R 0 = 0 kω, C F = nf 8 khz Corner Frequency f CF R 0 = 0 kω, C F = nf 8 khz Resistive Load at pin V OUT R L 00 kω Capacitive Load at pin V OUT C L 0 pf Output Voltage Shift V DIFF VM =. V -.. V Temperature Coefficient V DIFF (together with Input Stages) dv DIFF /dt T amb = C ±00 ppm/ C Internal Resistors and R 0, R 0 0 kω Temperature Coefficient R 0,0 dr 0,0 /dt T amb = C / C Ratiometric Error of V OUT RAT@V DIFF * 0. % FS Voltage Reference V REF Voltage V REF Ratiometric to V CC. V V REF vs. Temperature dv REF /dt T amb = C ±0 ±0 ppm/ C Current I VREF Source 6 µa I VREF Sink -6 µa Load Capacitance C VREF nf Ratiometric Error of V REF RAT@V REF * % FS * V DIFF = [.0 V DIFF (V CC = V) V DIFF (V CC =.V)]/[V DIFF (V CC = V) + V DIFF (V CC =.V)] ** V M = [.0 V M (V CC = V) V M (V CC =.V)]/[V M (V CC = V) + V M (V CC =.V)] An der Fahrt 3, D Mainz Mai 00 - Rev. - Page 9/

10 Parameter Symbol Conditions Min. Typ. Max. Unit Temperature Sensor V TEMP Voltage V TEMP R TEMP 0M.0.3. V Sensitivity dv TEMP /dt R TEMP 0M 8 mv/ C Thermal Nonlinearity R TEMP 0M, end point method 0. % FS Table : Electrical specifications for CAV Notes: ) Currents flowing into the IC are negative. ) R TEMP is the minimum possible load resistance at pin VTEMP. In order to achieve as good a temperature behavior as possible, it is essential that resistors R CM and R CW have the same temperature coefficients and that they are placed very close together in the circuit. STANDARD DIMENSIONING Parameter Symbol Min. Typ. Max. Unit Output Stage Resistors (%) R, R, R 00 kω Full-Scale Resistor (0.%), Calibration Start Value* R 33 kω Offset Resistor (0.%), Calibration Start Value* R 3 00 kω f/v-stage Biasing Resistor R A 0 kω Measurement Oscillator Resistor R CM 0 kω f/v-stage Filter Resistor R CW 00 kω Filter Capacitors (vary with value of C M,min )** C F,C F nf Reference Voltage Capacitor (V REF =.V) C VREF nf Table : Standard values for external components *) R and R 3 are the initial values given at the start of the calibration process (Kali_CAV.xls). During calibration, these are replaced by precisely computed, individual values. **) C F and C F are dimensioned by the calibration program. BOUNDARY CONDITIONS Parameter Symbol Condition Min. Typ. Max. Unit Maximum Supply Voltage V CCmax 7 V Oscillator Frequency Range f OSC 30 khz f/v Converter Current I CW R CW = 00 kω.38 µa Measurement Oscillator Current I CM R CM = 0 kω 0.7 µa Table 3: Boundary conditions An der Fahrt 3, D Mainz Mai 00 - Rev. - Page 0/

11 APPLICATIONS Example applications are generated by dimensioning the circuit in Figure with the help of the calibration program Kali_CAV.xls [] BLOCK DIAGRAM AND PINOUT PIN NAME DESCRIPTION RCM Current setting for the measurement oscillator RCW Current setting for the f/v converter 3 VB Bias voltage V CC GAIN Gain setting VOUT Output voltage 6 VREF Reference voltage. V 7 VTEMP Temperature sensor output 8 N.C. Not connected 9 N.C. Not connected 0 GND IC ground VCC Supply voltage CM Measurement capacitor/measurement oscillator capacitor 3 CF Lowpass capacitor, corner frequency RA Stabilizing resistor for f/v converter CF Lowpass capacitor, corner frequency 6 CW f/v converter capacitor RCM RCW VB GAIN VOUT VREF VTEMP N.C CAV CW CF RA CF CM VCC GND N.C. Figure 0: CAV Pin out Table : CAV Pin out DELIVERY CAV is available as: An SO6 (n); see data sheets: package For sample batches CAV can be supplied on a DIL6 SO6 adapter (CAVAdapt) Package dimensions: see An der Fahrt 3, D Mainz Mai 00 - Rev. - Page /

12 ADDITIONAL EQUIPMENT For design purposes, by way of support Analog Microelectronics can also supply a starter kit which consists of a breadboard (BBCAV) (which has been assembled for a specific set of parameters but which can also be used for individual measurements), a description and the spreadsheet Kali_CAV. FURTHER READING Please see our website for further information ( [] - Kali_CAV.pdf NOTES An der Fahrt 3, D Mainz Mai 00 - Rev. - Page /