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AD5235 Datasheet(PDF) 7 Page - Analog Devices

Part No. AD5235
Description  Nonvolatile Memory, Dual 1024 Position Digital Potentiometers
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD5235 Datasheet(HTML) 7 Page - Analog Devices

 
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PRELIMINARY TECHNICAL DATA
Nonvolatile Memory Digital Potentiometers
AD5235
REV PrD 6 NOV, 2000
7
Information contained in this Product Concept data sheet describes a product in the early definition stage. There is no guarantee that the
information contained here will become a final product in its present form. For latest information contact Walt Heinzer/Analog Devices, Santa Clara,
CA. TEL(408)562-7254; FAX (408)727-1550; walt.heinzer@analog.com
Detail Potentiometer Operation
The actual structure of the RDAC is designed to emulate the
performance of a mechanical potentiometer. The RDAC contains a
string of connected resistor segments, with an array of analog
switches that act as the wiper connection to several points along the
resistor array. The number of points is the resolution of the device.
The AD5235 has 1024 connection points allowing it to provide
better than 0.5% set-ability resolution. Figure 3 provides an
equivalent diagram of the connections between the three terminals
that make up one channel of the RDAC. The SWA and SWB will
always be ON while one of the switches SW(0) to SW(2
N-1) will
be ON one at a time depends upon the resistance step decoded from
the Data Bits. Note that there are two 50
Ω wiper resistances, R
W.
The resistance contributed by RW must be accounted for when
calculating the output resistance. RW is the sum of the resistances of
SWA + SWX and SWB + SWX for A-to-Wiper and B-to-Wiper
respectively.
A
X
R
S
R
S
R
S
W
X
B
X
RDAC
WIPER
REGISTER
&
DECODER
R
S = R AB /2
N
SW
A
SW
B
SW(2
N -1)
SW(2
N -2)
SW(0 )
SW(1 )
DIGITAL
CIRCUITRY
OMITTED FOR
CLARITY
Figure 3. Equivalent RDAC structure
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between terminals A and B
are available with values of 25K
Ω, and 250KΩ. The final digits of
the part number determine the nominal resistance value, e.g., 25K
= 25; 250K
Ω = 250. The nominal resistance (R
AB) of the AD5235
VR has 1024 contact points accessed by the wiper terminal, plus
the B terminal contact. The 10-bit data word in the RDAC latch is
decoded to select one of the 1024 possible settings. The wiper's
first connection starts at the B terminal for data 00H. This B–
terminal connection has a wiper contact resistance, RW of 50
Ω,
regardless of what the nominal resistance RAB is. The second
connection (25K
Ω part) is the first tap point where R
WB
=74
Ω [R
WB
=RAB/1024 + RW = 24Ω+50Ω)] for data 01H. The third connection
is the next tap point representing RWB =49+50=99
Ω for data 02
H.
Each LSB data value increase moves the wiper up the resistor
ladder until the last tap point is reached at RWB=25025
Ω. See figure
3 for a simplified diagram of the equivalent RDAC circuit.
The general equation, which determines the digitally programmed
output resistance between Wx and Bx, is:
RWB(Dx) = (Dx/2
N) * R
AB + RW
eqn. 1
Where N is the resolution of the VR, Dx is the data contained in
the RDACx latch, and RAB is the nominal end-to-end resistance.
Since N=10 and RW=50Ω for AD5235, eqn. 1 becomes:
RWB(Dx) = (Dx/1024) * RAB + 50Ω
eqn. 2
For example, when VB = 0V and A–terminal is open circuit the
following output resistance values will be set for the following
RDAC latch codes (applies to RAB=25KΩ potentiometers):
Dx
RWB
Output State
(DEC)
(
Ω)
1023
25025
Ω Full-Scale
512
12500
Ω Mid-Scale
1
74
1 LSB
0
50
Zero-Scale (Wiper contact resistance)
Note that in the zero-scale condition a finite wiper resistance of
50
Ω is present. Care should be taken to limit the current flow
between W and B in this state to no more than 20mA to avoid
degradation or possible destruction of the internal switch contact.
Figure 4. Symmetrical RDAC Operation
Like the mechanical potentiometer the RDAC replaces, the
AD5235 part is totally symmetrical. The resistance between the
wiper W and terminal A also produces a digitally controlled
resistance RWA. Figure 4 shows the symmetrical programmability
of the various terminal connections. When these terminals are used,
the B–terminal should be tied to the wiper. Setting the resistance
value for RWA starts at a maximum value of resistance and
decreases as the data loaded in the latch is increased in value. The
general equation for this operation is:


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