CAV424

C/V-converter for single and differential capacitive input signals

July 2014 – Rev. 3.0

Page 7/14

www.analogmicro.de

FUNCTIONAL DESCRIPTION

1. Oscillator

To generate a clock for the integrator blocks, the oscillator block charges and discharges the external oscilla-

tor capacitance COSC periodically with a constant current given by:

OSC

REF

OSC

R

V

I

=

(1)

where VREF is the reference voltage and ROSC is the oscillator current resistor.

The time dependent voltage at the oscillator capacitance is illustrated in Figure 3. COSC is charged with IOSC

beginning at the oscillator’s lower threshold VOSC,LOW until the upper threshold VOSC,HIGH is reached. Then the

current’s direction is reversed and COSC is discharged until the lower threshold is reached again.

This leads to a triangular voltage signal at COSC with the frequency fOSC:

OSC

OSC

OSC

OSC

OSC

OSC

C

R

C

V

I

f

⋅

⋅

=

⋅

∆

⋅

=

2

.

4

5

.

2

2

(2)

with ∆VOSC = 2.1 V and VREF = 2.5 V (@ VCC = 5 V).

2. Measurement and Reference Integrators

The measurement and reference integrator blocks are the core part of the measurement path. At these

symmetrically built integrators the connected capacitances’ values are converted into a voltage signal. Both

integrators are driven by the oscillator block and charge and discharge the connected capacitances with

constant current.

As illustrated in Figure 4, for half a period the measurement capacitance CM is charged with ICM and the ref-

erence capacitance CR is charged with ICR, which are given by

CM

REF

CM

R

V

I

=

and

CR

REF

CR

R

V

I

=

(3)

where RCM and RCR are the measurement and reference integrator current resistors. The capacitances CM

and CR are charged to a maximum voltage of VCM and VCR respectively, which can be calculated as follows:

CLAMP

M

OSC

CM

CM

V

C

f

I

V

+

⋅

⋅

=

2

and

CLAMP

R

OSC

CR

CR

V

C

f

I

V

+

⋅

⋅

=

2

(4)

with VCLAMP = 1.2 V, which is the minimum integrator voltage.

Figure 3: Oscillator voltage signal over time