CAV424 Datasheet, PDF (12/16 Pages) List of Unclassifed Manufacturers – Converter IC for Capacitive Signals

CAV424 - C/U transducer IC
with adjustable output voltage
DIMENSIONING PROCEDURE
Programs Kali1_cav424.xls and Kali2_cav424.xls can be used for dimensioning purposes.
The dimensioning process takes the following scenarios into account:
a) Kali1_cav424.xls – The integrator charging currents I_CM and I_CR are given and constant.
Oscillator frequency fosc must be adjusted to suit the minimum value of measurement
capacitance C_M,min.
b) Kali2_cav424.xls – Oscillator frequency fosc is given and determined and integrator
charging currents I_CM and I_CR must be adjusted to suit the minimum value of measurement
capacitance C_M,min.
In a) the dimensioning process assumes that in addition to measurement capacitors C_M and C_R
parasitic capacitances in both the IC and measurement circuit also influence the signal pattern.
When dimensioning on the basis of the given equations a deviation from the theoretical value in the
output characteristic must thus be reckoned with.
For this reason a calibration algorithm has been developed (Kali1_cav424.xls) which at constant
integrator charging currents (I_CM and I_CR) calculates a suitable oscillator frequency of fosz
depending on the minimum value of C_M,min (basic capacitance). It then dimensions the resistors for
the offset and signal span in such a way that the output signal adopts the required values.
Compensation of the sensor system is carried out in two stages. In stage one a calibration operating
point is defined, during which process oscillator frequency fosc is calculated depending on
minimum measurement capacitance C_M,min. To this end the minimum and maximum values (C_M,min
and C_M,_max) are entered in the Excel spreadsheet. The oscillator frequency, oscillator capacitance
C_OSC and oscillator resistance R_OSC are then output.
In addition low pass filter capacitances C_L1 and C_L2 are calculated which are dependent on the
oscillator frequency. It is sufficient if these values are computed once for the largest expected value
of minimum basic capacitance C_M,min (for example during one production batch) and the relevant
capacitors added to the circuit. The maximum signal frequency is also determined by which the
measurement capacitance is permitted to change.
Taking the given and calculated external components and particularly predefined precision resistors
R_L1(mess) and R_L2(mess) we can calculate the output voltage V_LPOUT(mess).
NB: R_L1(mess) and R_A(mess) must both be 100kOhm precision resistors with a tolerance of 0.1%
maximum.
Output signal values V_LPOUT(mess) are now entered in stage two of the calibration program. Using the
measured values the algorithm now calculates the setpoint for the two calibration resistors R_L1 and
R_A which replace precision resistors R_L1(mess) and R_L2(mess) and must be individually mounted.
Depending on the accuracy requirements of the setup their values should match those calculated as
closely as possible.
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
Phone: [phone]
Fax: [phone]
Internet:
http://www.analogmicro.de
Email: [e-mail]
July 2007 - Rev 1.2 - Page 12/16