CAV424

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

July 2014 – Rev. 3.0

Page 6/14

www.analogmicro.de

FUNCTIONAL PRINCIPLE

CAV424’s functional principle is described using Figure 2, where the functional blocks of the IC, the signal

patterns inside the IC and a basic external circuit with necessary passive components are illustrated.

The IC consists of seven functional blocks: the oscillator, the measurement integrator, the reference integra-

tor, the low pass filter, the output stage, the temperature sensor and the power supply. The power supply

block drives all the other blocks and also generates a reference voltage of 2.5 V. The temperature sensor

block provides an output voltage VTEMP proportional to the IC’s temperature. The capacitance measurement

path consists of the remaining five blocks and is described below.

CAV424 uses a differential measurement principle to evaluate the measurement capacitance CM. Using two

symmetric integrator blocks CM is measured against a reference capacitance CR. Both capacitances are

charged and discharged synchronously with constant current by their separate integrator blocks. The charge

currents for CM and CR can be adjusted using RCM and RCR respectively. The time constant for the charge

and discharge process is given by the oscillator block, whose frequency can be adjusted using COSC and

ROSC. The measurement and the reference integrator both generate a sawtooth output voltage with an ampli-

tude proportional to 1/CM and 1/CR respectively. At the low pass stage the difference of these sawtooth out-

put voltages is converted into a DC voltage. The output voltage VOUT at pin 5 is ratiometric to the supply volt-

age and is a linear function of 1/CM. It can be referenced to GND or to pin 6 to obtain the differential output

voltage VDIFF = VOUT – VREF. Using the circuit in Figure 6 the output signal’s offset and gain can be adjusted

too.

Figure 2: CAV424 with signal path and a basic circuit