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參數(shù)資料
| 型號： | EVAL-AD7747EBZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 15/28頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVAL FOR AD7747 |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 傳感器類型： | 觸摸，電容式 |
| 接口： | I²C |
| 電源電壓： | 2.7 V ~ 5.25 V |
| 嵌入式： | 否 |
| 已供物品： | 板，纜線，CD |
| 已用 IC / 零件： | AD7747 |
| 產(chǎn)品目錄頁面： | 781 (CN2011-ZH PDF) |
| 相關(guān)產(chǎn)品： | AD7747ARUZ-REEL7-ND - IC CONV CAP TO DIG 24BIT 16TSSOP AD7747ARUZ-REEL-ND - IC CONV CAP TO DIG 24BIT 16TSSOP AD7747ARUZ-ND - IC CONV 24BIT CAP-DGTL 16-TSSOP |
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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
05
46
9-
01
6
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
05
46
9-
01
7
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
05
46
9-
01
8
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
05
46
9-
0
19
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
05
46
9-
0
20
CX
13...21pF
(17pF ± 4pF)
CY
13...21pF
SHLD
CIN(+)
CIN(–)
(17pF ± 4pF)
Figure 31. Using CAPDAC in Differential Configuration
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