AD7151
Rev. 0 | Page 16 of 28
DATA REGISTER
Address Pointer 0x01, 0x02
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 (most
significant bits) of the data register are used for the CDC result.
The 4 LSBs (least significant bits) are always 0, as shown in
Figure 34.
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2
12-BIT CDC RESULT
BIT 1 BIT 0 BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2
DATA HIGHMSB DATA LOW LSB
BIT 1 BIT 0
0
07086-044
Figure 34. CDC Data Register
The nominal AD7151 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 0xCFF0 only (see
Table 8).
Table 8. AD7151 Capacitance-to-Data Mapping
Data Input Capacitance
0x0000 Not valid, underrange
0x3000 Zero-scale (0 pF)
0x8000 Mid-scale (+1 pF)
0xCFF0 Full-scale (+2 pF)
0xFFF0 Not valid, overrange
The input capacitance can be calculated from the output data
using the following equation:
RangeInput
Data
C _
40944
12288
)pF( ×
−
=
where Input_Range = 4 pF, 2 pF, 1 pF, or 0.5 pF.
The following is the same equation written with hexadecimal
numbers:
RangeInput
F
Data
C _
09x0
3000x0
)pF( ×
−
=
The data register is updated after a finished conversion, 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 the data
register should be read sequentially using the register address
pointer auto-increment feature of the serial interface.
AVERAGE REGISTER
Address Pointer 0x05, 0x06
16 Bits, Read-Only, Default Value 0x0000
This register shows the average calculated from the previous
CDC data. The 12-bit CDC result corresponds to the 12 MSBs
of the average register.
The settling time of the average can be set by programming the
ThrSettling bits in the setup register. The average register is
overwritten directly with the CDC output data, that is, the
history is forgotten if the timeout is enabled and elapses.
FIXED THRESHOLD REGISTER
Address Pointer 0x09, 0x0A
16 Bits, Read/Write, Factory Preset 0x0886
A constant threshold for the output comparator in the fixed
threshold mode can be set using this register. The 12-bit CDC
result corresponds to the 12 MSBs of the threshold register. The
fixed threshold register shares the address pointer and location
on-chip with the sensitivity and timeout registers. The fixed
threshold register is not accessible in the adaptive threshold
mode.
SENSITIVITY REGISTER
Address Pointer 0x09
8 Bits, Read/Write, Factory Preset 0x08
The sensitivity register sets the distance of the positive
threshold above the data average, and the distance of the
negative threshold below the data average, in the adaptive
threshold mode.
NEGATIVE
THRESHOLD
POSITIVE
THRESHOLD
DATA AVERAGE
OUTPUT ACTIVE
TIME
SENSITIVITY
DAT
SENSITIVITY
7086-024
Figure 35. Threshold Sensitivity
The sensitivity is an 8-bit value and is mapped to the lower eight
bits of the 12-bit CDC data, that is, it corresponds to the 16-bit
data register as shown in
Figure 36.
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3
SENSITIVIT
BIT 2 BIT 1 BIT 0
BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2
12-BIT CDC RESULT
BIT 1 BIT 0 BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2
DATA HIGH DATA LOW
BIT 1 BIT 0
07086-025
Figure 36. Relation Between Sensitivity Register and CDC Data Register
