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AD5161BRM5 Datasheet(PDF) 15 Page - Analog Devices

Part No. AD5161BRM5
Description  256-Position SPI/I2C Selectable Digital Potentiometer
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD5161BRM5 Datasheet(HTML) 15 Page - Analog Devices

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Data Sheet
AD5161
Rev. B | Page 15 of 20
THEORY OF OPERATION
The AD5161 is a 256-position digitally controlled variable
resistor (VR)1 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 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ. The final two
or three digits of the part number determine the nominal resistance
value, e.g., 10 kΩ = 10; 50 kΩ = 50. 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. Assume a 10 kΩ
part is used, the wiper’s first connection starts at the B terminal
for data 0x00. Since there is a 60 Ω wiper contact resistance, such
connection yields a minimum of 60 Ω resistance between
Terminals W and B. The second connection is the first tap point,
which corresponds to 99 Ω (RWB = RAB/256 + RW = 39 Ω + 60 Ω)
for data 0x01. The third connection is the next tap point,
representing 177 Ω (2 × 39 Ω + 60 Ω) 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 9961 Ω (RAB – 1 LSB + RW).
Figure 42 shows a simplified diagram of the equivalent RDAC
circuit where the last resistor string will not be accessed;
therefore, there is 1 LSB less of the nominal resistance at full
scale in addition to the wiper resistance.
B
RDAC
LATCH
AND
DECODER
W
A
RS
RS
RS
RS
SD BIT
D7
D6
D4
D5
D2
D3
D1
D0
Figure 42. AD5161 Equivalent RDAC Circuit
1 The terms digital potentiometer, VR, and RDAC are used interchangeably.
The general equation determining the digitally programmed
output resistance between W and B is
W
AB
WB
R
R
D
D
R
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, and
RW
is the wiper resistance contributed by the on resistance of
the internal switch.
In summary, if RAB = 10 kΩ and the A terminal is open circuited,
the following output resistance RWB will be set for the indicated
RDAC latch codes.
Table 9. Codes and Corresponding RWB Resistance
D (Dec.)
RWB (Ω)
Output State
255
9,961
Full Scale (RAB – 1 LSB + RW)
128
5,060
Midscale
1
99
1 LSB
0
60
Zero Scale (Wiper Contact Resistance)
Note that in the zero-scale condition a finite wiper resistance of
60 Ω 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
R
D
D
R
256
256
)
(
(2)
For RAB = 10 kΩ and the B terminal open circuited, the
following output resistance RWA will be set for the indicated
RDAC latch codes.
Table 10. Codes and Corresponding RWA Resistance
D (Dec.)
RWA (Ω)
Output State
255
99
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
may vary by up to ±30%. Since 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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