CAV424

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

July 2014 – Rev. 3.0

Page 10/14

www.analogmicro.de

2. CAV424 used as C/V-converter with adjustable gain and offset

For the adjustment of the output signal’s offset and gain five resistors R1 .. R5 have to be added to the net-

work in Figure 2. The complete measurement circuit with CAV424 is illustrated in Figure 6.

For the capacitive measurement system in Figure 6 the output voltage VOUT at pin 5 (referenced to GND) is

given by:

REF

M

CM

R

CR

OSC

OSC

OSC

OUT

V

B

C

R

C

R

R

C

V

G

V

⋅

+













⋅

−

⋅

⋅













⋅

⋅

∆

⋅

⋅

=

1

1

8

3

(11)

with

5

3

5

4

4

3

5

4

5

3

5

1

2

1

1

R

R

R

R

R

R

R

R

R

R

R

R

R

R

G

+

+

+

⋅

+

+

=

and

5

3

5

4

4

3

5

4

5

3

5

1

1

R

R

R

R

R

R

R

R

R

R

R

R

B

+

+

−

⋅

+

=

.

(12)

wherein R2, R4 and R5 are fixed resistors,

R1 is the trimming resistor for the adjustment of gain and

R3 is the trimming resistor for the adjustment of offset.

Using the equations (11) and (12) it is easy to see that the output voltage is again a linear function of the

reciprocal measurement capacitance 1/CM.

Inserting (9) into (11) leads to the following easy transfer function,

REF

M

M

M

OUT

V

B

C

C

G

C

V

⋅

+













−

⋅

⋅

=

min

,

1

44

.

0

)

(

.

(13)

wherein VOUT only depends on CM,min and CM.

Figure 6: CAV444 used as C/V-converter with adjustable gain and offset