AD7747_17 datasheet(23/29 Pages) AD

Part #	AD7747

Description	24-Bit Capacitance-to-Digital Converter

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Manufacturer	AD [Analog Devices]

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AD7747

Rev. 0 | Page 22 of 28

The CAPDAC can be understood as a negative capacitance

connected internally to the CIN pin. There are two independent

CAPDACs, one connected to the CIN(+) and the second con-

nected to the CIN(−). The relation between the capacitance

input and output data can be expressed as

() (

))

(

)

(

−

+

−

≈

CAPDAC

C

CAPDAC

C

DATA

Y

X

The CAPDACs have a 6-bit resolution, monotonic transfer

function, are well matched to each other, and have a defined

temperature coefficient. The CAPDAC full range (absolute

value) is not factory calibrated and can vary up to ±20% with

the manufacturing process. See the Specifications section and

Figure 16 of the typical performance characteristics.

SINGLE-ENDED CAPACITIVE CONFIGURATION

The AD7747 can be used for interfacing to a single-ended

capacitive sensor. In this configuration the sensor should be

connected to one of the AD7747 CIN pins, for example CIN(+)

and the other pin should be left open circuit. Note that the

CAPDIFF bit in the Cap Setup register must be set to 1 at all

times for the correct operation.

It is recommended to guard the unused CIN input with the

active shield to ensure the best performance in terms of noise,

offset, and offset drift.

The CDC (without using the CAPDACs) measure the positive

(or the negative) input capacitance in the range of 0 pF to 8 pF

(see Figure 27).

CAPDIFF = 1

0...8pF

CDC

SHLD

CIN(+)

CIN(–)

CX

0...8pF

CAPDAC(+)

OFF

CAPDAC(–)

OFF

0x800000

TO

0xFFFFFF

DATA

Figure 27. CDC Single-Ended Input Configuration

The CAPDAC can be used for programmable shifting of the

input range. The example in Figure 28 shows how to use the full

±8 pF CDC span to measure capacitance between 0 pF to 16 pF.

CAPDIFF = 1

±8pF

CDC

CAPDAC(+)

8pF

CAPDAC(–)

0pF

SHLD

CIN(+)

CIN(–)

CX

0...16pF

0x000000

TO

0xFFFFFF

DATA

Figure 28. Using CAPDAC in Single-Ended Configuration

Figure 29 shows how to shift the input range further, up to

25 pF absolute value of capacitance connected to the CIN(+).

CAPDIFF = 1

±8pF

CDC

CAPDAC(+)

17pF

CAPDAC(–)

0pF

SHLD

CIN(+)

CIN(–)

0x000000

TO

0xFFFFFF

CX

9...25pF

(17pF ± 8pF)

DATA

Figure 29. Using CAPDAC in Single-Ended Configuration

DIFFERENTIAL CAPACITIVE CONFIGURATION

When the AD7747 is used for interfacing to a differential

capacitive sensor, each of the two input capacitances, CX and CY,

must be less than 8 pF (without using the CAPDACs) or must

be less than 25 pF and balanced by the CAPDACs. Balancing

by the CAPDACs means that both CX − CAPDAC(+) and

CY − CAPDAC(−) are less than 8 pF.

If the unbalanced capacitance connected to CIN pins is higher

than 8 pF, the CDC introduces a gain error, an offset error, and

nonlinearity error.

See the examples shown in Figure 30, Figure 31, and Figure 32.

CAPDIFF = 1

±8pF

CDC

CX

0...8pF

CY

CAPDAC(+)

OFF

CAPDAC(–)

OFF

SHLD

CIN(+)

CIN(–)

0x000000

TO

0xFFFFFF

DATA

0...8pF

Figure 30. CDC Differential Input Configuration

0x000000

TO

0xFFFFFF

DATA

CAPDIFF = 1

±8pF

CDC

CAPDAC(+)

17pF

CAPDAC(–)

17pF

CX

13...21pF

(17pF ± 4pF)

CY

13...21pF

SHLD

CIN(+)

CIN(–)

(17pF ± 4pF)

Figure 31. Using CAPDAC in Differential Configuration

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