AD7156 Analog Devices, AD7156 Datasheet - Page 17

• Manufacturer Part Number: AD7156
• Manufacturer: Analog Devices

Specifications of AD7156

• Resolution (bits): 12bit
• # Chan: 2
• Sample Rate: 100SPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: Capacitive
• Ain Range: 0.5 pF,1 pF,2 pF,4 pF
• Pkg Type: CSP

DATA REGISTERS
Ch 1 Address Pointer 0x01, Address Pointer 0x02
Ch 2 Address Pointer 0x03, Address Pointer 0x04
16 Bits, Read Only
Default Value 0x0000
Data from the last complete capacitance-to-digital conversion
reflects the capacitance on the input. Only the 12 MSBs of the
data registers are used for the CDC result. The 4 LSBs are
always 0, as shown in Figure 36.
The data register is updated after a finished conversion on the
capacitive channel, with one exception: when the serial interface
read operation from the data register is in progress, the data
register is not updated and the new capacitance conversion
result is lost.
The stop condition on the serial interface is considered to be
the end of the read operation. Therefore, to prevent incorrect
data reading through the serial interface, the two bytes of a
data register should be read sequentially using the register
address pointer autoincrement feature of the serial interface.
The nominal AD7156 CDC transfer function (an ideal transfer
function excluding offset and/or gain error) maps the input
capacitance between zero scale and full scale to output data
codes between 0x3000 and 0xD000 only (see Table 8).
Table 8. AD7156 Capacitance-to-Data Mapping
Data
0x0000
0x3000
0x5800
0x8000
0xA800
0xD000
0xFFF0
1
An ideal part with no offset and gain error, values shown in picofarad for 2 pF capacitance input range.
MSB
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2
Input Capacitance
Under range (below 0 pF)
Zero scale (0 pF)
Quarter scale (+0.5 pF)—auto-DAC step down
Midscale (+1 pF)
Three-quarter scale (+1.5 pF)—auto-DAC step up
Full scale (+2 pF)
Over range (above +2 pF)
DATA HIGH
1
12-BIT CDC RESULT
Figure 36. CDC Data Register
BIT 1 BIT 0 BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2
Rev. 0 | Page 17 of 28
DATA LOW
For an ideal part, linear, with no offset error and no gain error,
the input capacitance can be calculated from the output data
using the following equation:
where Input_Range = 4 pF, 2 pF, 1 pF, or 0.5 pF.
The following is the same equation written with hexadecimal
numbers:
With offset error and gain error included, the equation is:
Or the same equation with hexadecimal numbers:
C
C
C
⎛
⎜
⎝
C
⎛
⎜
⎝
1
1
(
(
(
(
+
+
pF)
pF)
pF)
pF)
Gain
Gain
=
=
=
=
Data
Data
Data
Data
100
100
_
_
Error
Error
40
0
40
0
%
%
xA
xA
−
−
−
−
,
,
960
960
0
12
12
x 0
x 0
000
000
(%)
BIT 1 BIT 0
(%)
,
,
288
3000
288
3000
⎞
⎟
⎠
⎞
⎟
⎠
LSB
+
+
×
×
Offset
Offset
×
×
Input
Input
Input
Input
_
_
_
_
Error
Error
_
_
Range
Range
Range
Range
(
(
pF
pF
(pF)
(pF)
(pF)
(pF)
)
)
×
×
AD7156