AD5235 Analog Devices, AD5235 Datasheet - Page 7

• Manufacturer Part Number: AD5235
• Description: 8-Bit Dual Nonvolatile Memory Digital Potentiometer
• Manufacturer: Analog Devices

Nonvolatile Memory Digital Potentiometers
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 SW
always be ON while one of the switches SW(0) to SW(2
be ON one at a time depends upon the resistance step decoded from
the Data Bits. Note that there are two 50 wiper resistances, R
The resistance contributed by R
calculating the output resistance. R
SW
respectively.
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between terminals A and B
are available with values of 25K , and 250K . The final digits of
the part number determine the nominal resistance value, e.g., 25K
= 25; 250K = 250. The nominal resistance (R
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 00
terminal connection has a wiper contact resistance, R
regardless of what the nominal resistance R
connection (25K part is the first tap point where R
=R
is the next tap point representing R
Each LSB data value increase moves the wiper up the resistor
REV PrD 6 NOV, 2000
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[phone]; FAX [phone]; [e-mail]
AB
A
+ SW
/1024 + R
X
and SW
Figure 3. Equivalent RDAC structure
W
DIGITAL
CIRCUITRY
OMITTED FOR
CLARITY
REGISTER
DECODER
R
= 24 +50 )] for data 01
S
WIPER
RDAC
B
= R
&
+ SW
AB
/2
X
N
for A-to-Wiper and B-to-Wiper
R
R
R
S
S
S
W
PRELIMINARY TECHNICAL DATA
must be accounted for when
W
WB
is the sum of the resistances of
=49+50=99 for data 02
SW(2
SW(2
SW
SW
SW(1 )
SW(0 )
A
B
N
N
AB
-1)
-2)
H
. The third connection
is. The second
AB
W
A
B
A
X
X
X
and SW
H
) of the AD5235
. This B–
W
WB
of 50 ,
=74 [R
N
-1) will
B
will
W
H
.
.
WB
7
ladder until the last tap point is reached at R
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:
R
Where N is the resolution of the VR, Dx is the data contained in
the RDACx latch, and R
Since N=10 and R
R
For example, when V
following output resistance values will be set for the following
RDAC latch codes (applies to R
Dx
(DEC)
1023
512
1
0
Note that in the zero-scale condition a finite wiper resistance of
50 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.
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 R
of the various terminal connections. When these terminals are used,
the B–terminal should be tied to the wiper. Setting the resistance
value for R
decreases as the data loaded in the latch is increased in value. The
general equation for this operation is:
WB
WB
(Dx) = (Dx/2
(Dx) = (Dx/1024) * R
R
(
25025
12500
74
50
WA
WB
WA
Figure 4. Symmetrical RDAC Operation
starts at a maximum value of resistance and
. Figure 4 shows the symmetrical programmability
N
W
) * R
Output State
Full-Scale
Mid-Scale
1 LSB
Zero-Scale (Wiper contact resistance)
=50 for AD5235, eqn. 1 becomes:
B
= 0V and A–terminal is open circuit the
AB
AB
AB
+ R
is the nominal end-to-end resistance.
+ 50
W
AB
=25K potentiometers):
WB
AD5235
=25025 . See figure
eqn. 1
eqn. 2