CAV424 Datasheet, PDF (4/7 Pages) List of Unclassifed Manufacturers – Converter IC for Capacitive Signals

Converter IC for Capacitive Signals
CAV424
parison of the two amplitudes pro-
duces a signal which corresponds to ^VOSC
the change in capacitance of C_X1 and
V_OSC,HIGH
C_X2 relative to one another. This dif-
ference signal is rectified in an ensu-
ing low pass. The filtered DC signal
is transferred to the differential, ad-
V_OSC,LOW
justable output stage. Individual cir-
cuit variables, such as filter constants
and amplification, can be set with just
T 3T _T
2
4
2T ^{Time t}
a few external components. By using
the integrators and their capacitances ^{Figure 2: oscillator voltage curve}
C_X1 and C_X2, swings in capacitance of 5% to 100% in relation to the measurement reference capacitance
can be measured. As C_X1 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.
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 ^VOSC
V_CX1
oscillator capacitance C_OSC, the
internal parasitic capacitance of the
V_CX2
IC, C_OSC,PAR,INT, and the external
parasitic capacitance C_OSC,PAR,EXT V_CLAMP
(from a printed board assembly, for
example). Oscillator capacitance C_OSC
is dimensioned as follows:
C_OSC = 1.6 ⋅ C_X1 ,
T 3T _T
2
4
Figure 3: integrator voltage curve
where C_X1 is the fixed capacitance
(reference capacitance) of a capacitive sensing element.
2T ^{Time t}
The reference oscillator current I_OSC is determined via external resistance R_OSC and reference voltage
V_M:
I_OSC
=
V_M
R_OSC
The frequency of the reference oscillator f_OSC is given by
( ) ^fOSC
=
2 ⋅ ∆V_OSC
⋅
C_OSC
I _OSC
+ C_OSC,PAR,INT
+ C_OSC,PAR,EXT
,
analog microelectronics
January 2002
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