CAV424 Datasheet, PDF (6/16 Pages) List of Unclassifed Manufacturers – Converter IC for Capacitive Signals

CAV424 - C/U transducer IC
with adjustable output voltage
V_OSC
V_CLAMP
V_CM
V_CR
T
T
2
Figure 4: Integrator oscillator voltage
C_R < C_M
I_CR = I_CM
2T ^{time t}
The capacitive integrator currents I_CR and I_CM are set by external resistors R_{CM ,} R_CR and reference
voltage V_M:
I _CM
"
V_M
R_CM
and
I _CR
"
V_M
R_CR
(3)*, (4)*
Capacitors C_M and C_R are charged up to a maximum voltage of V_CM and V_CR respectively and can
be calculated as follows:
V_CM
"
2#
I _CM
f_OSC # C_M
% V_CLAMP
(5)
V_CR
"
2#
I _CR
f_OSC # C_R
% V_CLAMP
(6)
The two voltages V_CM and V_CR are subtracted from one another in the circuit's signal conditioning
unit. Via this subtraction, which is tantamount to a rectification of the procedure, V_CLAMP is
eliminated and a direct voltage of V_TPAS is produced as an output signal after filtering.
Should I_CR and I_CM be the same for C_M,min (i.e. should the reference capacitance be the same as the
basic value of the measurement capacitance), on subtraction and filtering at the signal conditioning
output a value of zero is obtained (see Figure 5).
* The equations apply to R_CX = 0 (see Figures 7 and 8). Should R_CX "0 for the resistor due to better
thermal coupling, alternative calculations are provided in the Excel spreadsheets Kali1_cav424.exc
and Kali2_cavV424.xls.
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
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Internet:
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July 2007 - Rev 1.2 - Page 6/16