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參數(shù)資料
| 型號： | EVAL-AD5245EBZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 5/20頁 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD FOR AD5245 |
| 標準包裝： | 1 |
| 主要目的： | 數(shù)字電位器 |
| 嵌入式： | 否 |
| 已用 IC / 零件： | AD5245 |
| 主要屬性： | 1 通道，256 位置 |
| 次要屬性： | I²C 接口 |
| 已供物品： | 板 |
AD5245
Rev. B | Page 13 of 20
THEORY OF OPERATION
The AD5245 is a 256-position digitally controlled variable
resistor (VR) device.
An internal power-on preset places the wiper at midscale
during power-on, which simplifies the fault condition recovery
at power-up.
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between Terminals A and
B is available in 5 k, 10 k, 50 k, and 100 k. The nominal
resistance (RAB) of the VR has 256 contact points accessed by
the wiper terminal, plus the B terminal contact. The 8-bit data
in the RDAC latch is decoded to select one of the 256 possible
settings.
A
W
B
A
W
B
A
W
B
034
36-
03
4
Figure 35. Rheostat Mode Configuration
Assuming that a 10 k part is used, the wiper’s first connection
starts at the B terminal for Data 0x00. Because there is a 50
wiper contact resistance, such a connection yields a minimum
of 100 (2 × 50 ) resistance between Terminals W and B. The
second connection is the first tap point, which corresponds to
139 (RWB = RAB/256 + 2 × RW = 39 + 2 × 50 ) for Data 0x01.
The third connection is the next tap point, representing 178
(2 × 39 + 2 × 50 ) for Data 0x02, and so on. Each LSB data
value increase moves the wiper up the resistor ladder until the
last tap point is reached at 10,100 (RAB + 2 × RW).
D5
D4
D3
D7
D6
D2
D1
D0
RDAC
LATCH
AND
DECODER
RS
A
W
B
03
43
6-
0
3
5
Figure 36. AD5245 Equivalent RDAC Circuit
The general equation determining the digitally programmed
output resistance between W and B is
W
AB
WB
R
D
R
×
+
×
=
2
256
)
(
(1)
where:
D is the decimal equivalent of the binary code loaded in the
8-bit RDAC register.
RAB is the end-to-end resistance.
RW is the wiper resistance contributed by the on resistance of
the internal switch.
In summary, if RAB = 10 k and the A terminal is open
circuited, then the following output resistance RWB is set for the
indicated RDAC latch codes.
Table 6. Codes and Corresponding RWB Resistance
D (Dec.)
RWB ()
Output State
255
9,961
Full Scale (RAB – 1 LSB + RW)
128
5,060
Midscale
1
139
1 LSB
0
100
Zero Scale (Wiper Contact Resistance)
Note that in the zero-scale condition, a finite wiper resistance of
100 is present. Care should be taken to limit the current flow
between W and B in this state to a maximum pulse current of
no more than 20 mA. Otherwise, degradation or possible
destruction of the internal switch contact can occur.
Similar to the mechanical potentiometer, the resistance of the
RDAC between the Wiper W and Terminal A also produces a
digitally controlled complementary resistance, RWA. When these
terminals are used, the B terminal can be opened. Setting the
resistance value for RWA starts at a maximum value of resistance
and decreases as the data loaded in the latch increases in value.
The general equation for this operation is
W
AB
WA
R
D
R
×
+
×
=
2
256
)
(
(2)
For RAB = 10 k and the B terminal open circuited, the
following output resistance RWA is set for the indicated RDAC
latch codes.
Table 7. Codes and Corresponding RWA Resistance
D (Dec.)
RWA ()
Output State
255
139
Full Scale
128
5,060
Midscale
1
9,961
1 LSB
0
10,060
Zero Scale
Typical device-to-device matching is process lot dependent and
can vary by up to ±30%. Because the resistance element is
processed in thin film technology, the change in RAB with
temperature has a very low 45 ppm/°C temperature coefficient.
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