Datasheet

AD5263
Rev. 0 | Page 14 of 28
OPERATION
The AD5263 is a quad-channel, 256-position, digitally
controlled, variable resistor (VR) device.
To program the VR settings, refer to the interface sections of the
previous pages. The part has an internal power-on preset that
places the wiper at midscale during power-on, which simplifies
the fault condition recovery at power-up. In addition, the shut-
down
SHDN
pin of AD5263 places the RDAC in an almost zero
power consumption state where Terminal A is open circuited
and the wiper W is connected to Terminal B, resulting in only
leakage current consumption in the VR structure. During shut-
down, the VR latch settings are maintained or new settings can
be programmed. When the part is returned from shutdown, the
corresponding VR setting will be applied to the RDAC.
03142-0-044
Bx
Wx
Ax
SD BIT
D7
D6
D4
D5
D2
D3
D1
D0
RDAC
LATCH
AND
DECODER
R
S
R
S
R
S
R
S
Figure 44. AD5263 Equivalent RDAC Circuit
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between Terminals A and
B is available in 20 kΩ, 50 kΩ, and 200 kΩ. The final two or
three digits of the part number determine the nominal
resistance value, e.g., 20 kΩ = 20; 50 kΩ = 50; 200 kΩ = 200. The
nominal resistance (R
AB
) 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. Assuming a 20 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 a connection yields a
minimum of 2 × 60 Ω resistance between Terminals W and B.
The second connection is the first tap point, and corresponds to
198 Ω (R
WB
= R
AB
/256 + R
W
= 78 Ω + 2 × 60 Ω) for data 0x01.
The third connection is the next tap point representing 216 Ω
(R
WB
= 78 Ω × 2 + 2 × 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 19,982 Ω (R
AB
– 1 LSB + 2 × R
W
).
Figure 44 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.
The general equation determining the digitally programmed
output resistance between Terminals W and B is
W
AB
WB
RR
D
DR ×+×= 2
256
)(
(1)
where:
D is the decimal equivalent of the binary code loaded in the
8-bit RDAC register.
R
AB
is the end-to-end resistance.
R
W
is the wiper resistance contributed by the ON resistance of
one internal switch.
In summary, if R
AB
= 20 kΩ and the A terminal is open-
circuited, the following RDAC latch codes result in the
corresponding output resistance, R
WB
.
Table 7. Codes and Corresponding R
WB
Resistances
D (dec) R
WB
(Ω) Output State
255 19,982 Full-Scale (R
AB
– 1 LSB + R
W
)
128 10,120 Midscale
1 198 1 LSB
0 120 Zero-Scale (Wiper Contact Resistance)
Note that in the zero-scale condition a finite wiper resistance of
120 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, R
WA
. When these
terminals are used, the B terminal can be opened. Setting the
resistance value for R
WA
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
ABWA
RR
D
DR ×+×
−
= 2
256
256
)(
(2)
For R
AB
= 20 kΩ and the B terminal open-circuited, the
following RDAC latch codes result in the corresponding output
resistance R
WA
:
Table 8. Codes and Corresponding R
WA
Resistances
D (dec) R
WA
(Ω) Output State
255 198 Full-Scale
128 10,120 Midscale
1 19,982 1 LSB
0 20,060 Zero-Scale