CAV424

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

July 2014 – Rev. 3.0

Page 11/14

www.analogmicro.de

3. Standard Dimensioning

A basic dimensioning of the passive external components in Figure 6 with a measurement capacitance not

larger than 1 nF is given in the table below. For the application specific capacitance ranges or if CR is given

by the measurement setup an optimal dimensioning can be calculated using the Excel-sheet Kali_CAV424.

Notes:

1)

The optimal value of CF1 and CF2 depends on the oscillator frequency. The given value can be used for CM,min

from 5 pF to 1000 pF, but leads to a slow response time.

2)

R1 = R3 = 100 kΩ are used to measure the output voltages at CM,min and CM,max at the start of the trimming proc-

ess. During the trimming process R1 and R3 will be set to individual values (calculated by the Excel-sheet

Kali_CAV424).

3)

For best performance a high grade ceramic capacitor has to be used for CVREF.

4)

Usually it is sufficient to use standard components with low temperature coefficients (≤ 100 ppm). For RCM, RCR,

ROSC, CVREF and CF1/F2 a variance of 5% of the given values is acceptable without a decrease in performance.

For components, which have to have lower variances, the tolerances are given in round brackets in Table 3.

4. Calibration Procedure

During the design of capacitive sensor systems it always has to be taken into account that there are parasitic

capacitances beside the connected measurement capacitance, which influence the output signal and the

calibration process. To simplify the consideration of these effects during the calibration of sensor systems,

Analog Microelectronics developed a calibration strategy, implemented in the Excel-sheet Kali_CAV424.

This Excel-sheet, based on the circuit in Figure 6, is available free of charge at www.analogmicro.de

.

Using this strategy the calibration of sensor systems with CAV424 is done in two steps, a dimensioning and

a trimming step. At first a dimensioning of the external components is calculated for the specific type of sen-

sor implemented with CAV424. This dimensioning is mostly dependent on the capacitive measurement

range of the sensor. Based on a few input values characterizing the capacitive sensor system the program

calculates a suitable dimensioning of the remaining external components.

After the specific sensor systems have been built with the calculated dimensioning a trimming of offset and

gain (using the trimming resistors R1 and R3) of the individual sensor systems has to be done. Therefore the

output voltage has to be measured at the minimum and the maximum measurement capacitance. Based on

these values the Excel-sheet calculates the final trimming resistor values for R1 and R3. After trimming R1

and R3 to the calculated values the sensor is ready for operation and all parasitic capacitances as well as the

tolerances of the used components are taken into account.

Parameter

Symbol

Min.

Typ.

Max.

Unit

Reference Capacitor

CR

CM,min

pF

Oscillator Capacitor

COSC

4.4*CM,min

pF

Measurement Integrator Current Resistor

RCM

1000

kΩ

Reference Integrator Current Resistor

RCR

1000

kΩ

Oscillator Current Resistor

ROSC

125

kΩ

Low Pass Filter Capacitors

1)

CF1, CF2

500

nF

Output Stage Resistor (1%)

R2, R4, R5

100

kΩ

Full-Scale Resistor (0.1%) Calibration Meas. Value

2)

R1(start)

100

kΩ

Offset Resistor (0.1%) Calibration Meas. Value

2)

R3(start)

100

kΩ

Reference Voltage Capacitor (VREF = 2.5V)

3)

CVREF

80

100

120

nF

Table 3: Standard dimensioning at ICM = ICR = 2.5 µA, IOSC = 20 µA, CM,max < 1 nF, CR = CM,min.