CAV424

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

July 2014 – Rev. 3.0

Page 12/14

www.analogmicro.de

5. Operation Instructions

For first investigations Analog Microelectronics offers the BBCAV424, a pre-assembled breadboard (see

section “Accessories”) with easily adaptable measurement capacitance ranges, which can be used to study

the behavior of capacitance sensor heads as well.

Capacitive measurement heads with a single capacitive output signal have to be connected to pin 14 (CM).

In this case the reference capacitance is typically chosen as a ceramic capacitor with CR = CM,min, where

CM,min.is the specific minimum measurement capacitance.

If a differential capacitive measurement head with a measurement and an integrated reference capacitance

is used, a differential measurement can be made by connecting the measurement capacitance to pin 14

(CM) and the reference capacitance to pin 16 (CR).

For both cases it is recommended to do the external component’s dimensioning using the Excel-sheet

Kali_CAV424. To consider possible production variances in the capacitive measurement heads it is

recommended to do the dimensioning using the specific minimal value of CM,min and the maximal value of

CM,max. If a reference capacitance is integrated in the measurement head, the minimal reference capacitance

value should be used for CR as well.

To realize the dimensioning, calculated by the Excel-sheet, networks of not more than two resistors or

capacitors should be used. In many applications it is sufficient to use standard components, like e-series

capacitors, which fit to the calculated values within 5% (see section “Standard Dimensioning”).

For the PCB layout it is recommended to keep conducting lines from the IC to its external passive

components short, leading to small parasitic capacitances. Furthermore the conducting lines for CM and CR

should be routed symmetrically.

The parasitic capacitances in the sensor setup enlarge the used capacitances, especially the measurement,

the reference and the oscillator capacitance and influence the capacitance measurement directly. It is

important to keep the parasitic capacitances as small and stable as possible. If flexible wiring has to be used,

then only shielded wires should be used for CM and CR and the wires should be kept as short as possible.

The parasitic capacitances inside the IC have to be considered as well if small capacitances have to be

measured. Typically a parasitic capacitance of 5 pF is added to the connected capacitance at pin 12, 14 and

16, when the SO16(n) package is used.

CAV424’s differential measurement principle can be used to reduce the susceptibility against

electromagnetic disturbances. To use the benefits of this measurement principle CM and CR should be placed

close to each other and their wiring should be kept symmetrical.

In real sensor applications the temperature behavior is mainly influenced by CM’s, CR’s and COSC’s and

further external component’s temperature coefficients (TCs). In general an optimal temperature behavior can

be achieved by using COSC with a low TC and CM and CR, which have equal TCs.

In general ESD precautions are necessary during assembly and handling of the device. It is essential to

ground machines and personnel properly.

Take care that the absolute maximum values in Table 2 are not exceeded, when the device is put into

operation.

Notes:

1.

If the voltage signal at pin 12, 14 or 16 is measured using an oscilloscope, the probe’s capacitance is added to

the capacitance at the specific pin, which changes the output voltage signal. To check the frequency and the dif-

ferential signal amplitude without influencing the output signal it is possible to remove the low pass filter capacitor

CF1 and measure at pin 15.

2.

For level sensing applications it is important to isolate the measurement electrode from a conductive medium.

Otherwise the output voltage is affected by the medium’s conductivity.