AD7152 datasheet(19/24 Pages) AD

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AD7152/AD7153

Rev. 0 | Page 19 of 24

CAPDAC

The CDC full-scale input range of the AD7152/AD7153 can

be set to ±0.25 pF, ±0.5 pF, ± 1 pF, and ±2 pF in differential

mode or 0.5 pF, 1 pF, 2 pF, and 4 pF in single-ended mode.

For simplicity, the following text and figures use the maximum

full scale of ±2 pF and +4 pF.

The parts can accept a higher capacitance on the input and the

common-mode or offset capacitance (unchanging component)

can be balanced by programmable on-chip CAPDACs.

DATA

CDC

EXC

CIN(+)

CIN(–)

CAPDAC(+)

CAPDAC(–)

Figure 28. Using a CAPDAC

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

connected to the CIN(–).

In differential mode, the relationship between the capacitance

input and output data can be expressed as

DATA ≈ (CX − CAPDAC(+)) − (CY − CAPDAC(−))

In single-ended mode, the relationship between the capacitance

input and output data can be expressed as

DATA ≈ CX −(CAPDAC(+) + CAPDAC(−))

The CAPDACs have a 5-bit resolution each, 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

Figure 18

, and Figure 19).

The CAPDACs are shared by the two capacitive channels on the

AD7152. If the CAPDACs need to be set individually, the host

controller software should reload the CAPDAC values to the

AD7152 before executing a conversion on a different channel.

SINGLE-ENDED CAPACITIVE INPUT

When configured for a single-ended mode (the CAPDIFF bit in

the Channel 1 Setup or Channel 2 Setup registers is set to 0), the

AD7152/AD7153 CIN(−) pin is disconnected internally. The

CDC (without using the CAPDACs) can measure positive input

capacitance in the range of 0 pF to 4 pF (see Figure 29).

0x0000 ... 0xFFF0

DATA

CAPDIFF = 0

0pF TO 4pF

CDC

EXC

CIN(+)

CIN(–)

0pF TO 4pF

CAPDAC(+)

OFF

CAPDAC(–)

OFF

Figure 29. CDC Single-Ended Input Mode

The CAPDAC can be used for programmable shifting of the

input range.

Figure 30 shows how to shift the input range up to 9 pF absolute

value of capacitance connected to the CIN(+) using

the CAPDAC(+) only.

0x0000 ... 0xFFF0

DATA

CAPDIFF = 0

0pF TO 4pF

CDC

EXC

CIN(+)

CIN(–)

5pF TO 9pF

CAPDAC(+)

5pF

CAPDAC(–)

OFF

Figure 30. Using CAPDAC in Single-Ended Mode

Figure 31 shows how to shift the input range up to 14 pF

absolute value of capacitance connected to the CIN(+) using

both CAPDAC(+) and CAPDAC(−).

0x0000 ... 0xFFF0

DATA

CAPDIFF = 0

0pF TO 4pF

CDC

EXC

CIN(+)

CIN(–)

10pF TO 14pF

CAPDAC(+)

5pF

CAPDAC(–)

5pF

Figure 31. Using CAPDAC in Single-Ended Mode

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