AD5241(RevB) 查看數據表（PDF） - Analog Devices
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AD5241 Datasheet PDF : 16 Pages
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6
0
FFH
80H
–6
–12
40H
20H
–18
–24
10H
08H
–30
04H
–36
02H
–42
01H
–48
–54
100
1k
10k
FREQUENCY – Hz
100k
TPC 13. AD5242 100 kΩ Gain vs. Frequency vs. Code
AD5241/AD5242
6
FFH
0
–6
80H
–12
40H
–18
20H
10H
–24
08H
–30
04H
–36
02H
–42
01H
–48
–54
100
1k
10k
FREQUENCY – Hz
100k
TPC 14. AD5242 1 MΩ Gain vs. Frequency vs. Code
OPERATION
The AD5241/AD5242 provide a single-/dual-channel, 256-
position digitally controlled variable resistor (VR) device. The
terms VR, RDAC, and programmable resistor are commonly
used interchangeably to refer to digital potentiometer.
To program the VR settings, refer to the Digital Interface sec-
tion. Both parts have an internal power ON preset that places
the wiper in midscale during power-on, which simplifies the
fault condition recovery at power-up. In addition, the shutdown
SHDN Pin of AD5241/AD5242 places the RDAC in an almost
zero power consumption state where Terminal A is open circuited
and Wiper W is connected to Terminal B, resulting in only
leakage current being consumed in the VR structure. During
shutdown, the VR latch contents are maintained when the RDAC
is inactive. When the part is returned from shutdown, the stored
VR setting will be applied to the RDAC.
SHDN
D7
D6
D5
D4
D3
D2
D1
D0
RDAC
LATCH
AND
DECODER
A
SWSHDN
R
SW
N
2–1
R
SW
N
2–2
W
R
SW1
R RAB/2N
R
SW0
DIGITAL CIRCUITRY
B OMITTED FOR CLARITY
Figure 4. Equivalent RDAC Circuit
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between Terminals A and
B is available in 10 kΩ, 100 kΩ, and 1 MΩ. The final two or
three digits of the part number determine the nominal resistance
value, e.g., 10 kΩ = 10; 100 kΩ = 100; 1 MΩ = 1 M. The
nominal resistance (RAB) of the VR has 256 contact points
accessed by the Wiper Terminal, plus the B Terminal con-
tact. 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
00H. Since there is a 60 Ω wiper contact resistance, such con-
nection yields a minimum of 60 Ω resistance between Terminals
W and B. The second connection is the first tap point that cor-
responds to 99 Ω (RWB = RAB/256 + RW = 39 + 60) for data
01H. The third connection is the next tap point representing
138 Ω (39 × 2 + 60) for data 02H, and so on. Each LSB data
value increase moves the wiper up the resistor ladder until the
last tap point is reached at 10021 Ω [RAB – 1 LSB + RW].
Figure 4 shows a simplified diagram of the equivalent RDAC
circuit where the last resistor string will not be accessed; there-
fore, there is 1 LSB less of the nominal resistance at full scale in
addition to the wiper resistance.
The general equation determining the digitally programmed
resistance between W and B is:
( ) RWB
D
=
D
256
×
R AB
+
RW
(1)
where:
D is the decimal equivalent of the binary code between 0
and 255, which is loaded in the 8-bit RDAC register.
RAB is the nominal end-to-end resistance.
RW is the wiper resistance contributed by the on resistance
of the internal switch.
Again, if RAB = 10 kΩ and the A Terminal can be either open
circuit or tied to W, the following output resistance at RWB will
be set for the following RDAC latch codes.
REV. B
–9–
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