CAV414 datasheet(5/6 Pages) AME

5 / 6 page

Converter IC for Capacitive Signals

CAV414

analog microelectronics

January 2001

5/6

FUNCTIONAL DESCRIPTION

A reference oscillator with a frequency adjusted by the capacity COSC drives two symmetrically built

integrators synchronously to its clock and its phase. The capacitors CX1 and CX2 determine the am-

plitude of the two driven integrators. The difference of the integrator amplitudes gives the relative

change of the capacities CX1 and CX2 to each other with high common mode rejection and high

resolution. The difference signal is conditioned by a lowpass filter. The corner frequency and gain

of it can be adjusted with a few external components. The output of the lowpass filter is connected

to an instrumentation amplifier and an output stage. These two stages transform the signal into an

adjustable voltage.

Adjustment:

The zero-adjustment is made by the resistors RCX1 or RCX2 for the case that the varying capacitance

CX2 has nearly the same (and its smallest) value as the fixed capacitance CX1 (reference capacitance).

Therefore one of this resistors is varied until the differential voltage

LPOUT

DIFF

−

is zero:

DIFF

Application Example:

The following values are given:

• fixed capacitance C

X1:

50pF

• varying capacitance C

X2:

50 ... 100pF

Calculation:

With the equations given in the boundary conditions, the following values for the devices can be

calculated:

• C

OSC:

80pF

• C

L1:

10nF

• C

L2:

10nF

If the signal VDIFF is amplified, it has to fulfil the unequation:

VDIFF ≤ 400mV

Detailed calculations are shown in a separately available Application Note.

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