AD5123 View Datasheet(PDF) - Analog Devices
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AD5123 Datasheet PDF : 28 Pages
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Data Sheet
AD5123/AD5143
RDAC ARCHITECTURE
To achieve optimum performance, Analog Devices, Inc., has
patented the RDAC segmentation architecture for all the digital
potentiometers. In particular, the AD5123/AD5143 employ a
three-stage segmentation approach, as shown in Figure 36. The
AD5123/AD5143 wiper switch is designed with the transmission
gate CMOS topology and with the gate voltage derived from
VDD and VSS.
A
STS
The nominal resistance between Terminal A and Terminal B,
RAB, is 10 kΩ or 100 kΩ, and has 128/256 tap points accessed by
the wiper terminal. The 7-bit/8-bit data in the RDAC latch is
decoded to select one of the 128/256 possible wiper settings. The
general equations for determining the digitally programmed
output resistance between Terminal W and Terminal B are
AD5123:
RWB
(D)
D
128
RAB
RW
From 0x00 to 0x7F (1)
RH
RH
RM
RM
RL
AD5143:
RWB
(D)
D
256
R AB
RW
From 0x00 to 0xFF (2)
where:
D is the decimal equivalent of the binary code in the 7-bit/8-bit
RDAC register.
RAB is the end-to-end resistance.
W
RW is the wiper resistance.
7-BIT/8-BIT
ADDRESS
DECODER
RL
RM
RH
RM
RH
SBS
In potentiometer mode, similar to the mechanical potentiometer,
the resistance between Terminal W and Terminal A also produces
a digitally controlled complementary resistance, RWA. RWA also
gives a maximum of 8% absolute. RWA starts at the maximum
resistance value and decreases as the data loaded into the latch
increases. The general equations for this operation are
AD5123:
B
RAW
(D)
128
128
D
RAB
RW
From 0x00 to 0x7F (3)
Figure 36. AD5123/AD5143 Simplified RDAC Circuit
Top Scale/Bottom Scale Architecture
In addition, the AD5123/AD5143 include new positions to
reduce the resistance between terminals. These positions are
called bottom scale and top scale. At bottom scale, the typical
wiper resistance decreases from 130 Ω to 60 Ω (RAB = 100 kΩ).
At top scale, the resistance between Terminal A and Terminal W is
decreased by 1 LSB, and the total resistance is reduced to 60 Ω
(RAB = 100 kΩ).
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation—±8% Resistor Tolerance
AD5143:
RAW
(D)
256 D
256
RAB
RW
From 0x00 to 0xFF (4)
where:
D is the decimal equivalent of the binary code in the 7-bit/8-bit
RDAC register.
RAB is the end-to-end resistance.
RW is the wiper resistance.
If the part is configured in linear gain setting mode, the resistance
between Terminal W and Terminal A is directly proportional
to the code loaded in the associate RDAC register. The general
equations for this operation are
The AD5123/AD5143 operate in rheostat mode when only two
terminals are used as a variable resistor. The unused terminal can
be floating, or it can be tied to Terminal W, as shown in Figure 37.
A
W
A
W
A
W
B
B
B
Figure 37. Rheostat Mode Configuration
AD5123:
R AW
(D)
D
128
R AB
RW
From 0x00 to 0x7F (5)
AD5143:
R AW
(D)
D
256
R AB
RW
From 0x00 to 0xFF (6)
where:
D is the decimal equivalent of the binary code in the 7-bit/8-bit
RDAC register.
RAB is the end-to-end resistance.
RW is the wiper resistance.
Rev. A | Page 25 of 28
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