CAV414 Datasheet, PDF (5/6 Pages) Analog Microelectronics – Converter IC for Capacitive Signals

Converter IC for Capacitive Signals CAV414
FUNCTIONAL DESCRIPTION
A reference oscillator with a frequency adjusted by the capacity C_OSC drives two symmetrically built
integrators synchronously to its clock and its phase. The capacitors C_X1 and C_X2 determine the am-
plitude of the two driven integrators. The difference of the integrator amplitudes gives the relative
change of the capacities C_X1 and C_X2 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 R_CX1 or R_CX2 for the case that the varying capacitance
C_X2 has nearly the same (and its smallest) value as the fixed capacitance C_X1 (reference capacitance).
Therefore one of this resistors is varied until the differential voltage
V_DIFF = V_LPOUT −V_M
is zero:
V_DIFF = 0
Application Example:
The following values are given:
• fixed capacitance C_X1:
• varying capacitance C_X2:
50pF
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 V_DIFF is amplified, it has to fulfil the unequation:
V_DIFF ≤ 400mV
Detailed calculations are shown in a separately available Application Note.
analog microelectronics
January 2001
5/6