Datasheet

Data Sheet AD5253/AD5254
Rev. C | Page 25 of 32
R
AB
(%)
D (Code in Decimal)
03824-0-043
0
25
50
75
100
0
10 32
48 63
R
WA
R
WB
Figure 43. AD5253 R
WA
(D) and R
WB
(D) vs. Decimal Code
Since the digital potentiometer is not ideal, a 75 Ω finite wiper
resistance is present that can easily be seen when the device is
programmed at zero scale. Because of the fine geometric and
interconnects employed by the device, care should be taken to
limit the current conduction between W and B to no more than
±5 mA continuous for a total resistance of 1 kΩ or a pulse of
±20 mA to avoid degradation or possible destruction of the
device. The maximum dc current for AD5253 and AD5254 are
shown in Figure 21 and Figure 22, respectively.
Similar to the mechanical potentiometer, the resistance of the
RDAC between 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. The R
WA
starts at a maximum value and decreases as the data loaded into
the latch increases in value (see Figure 43. The general equation
for this operation is
AD5253: RWA(D) = [(64 – D)/64] × RAB + 75 Ω (3)
AD5254: RWA(D) = [(256 – D)/256] × RAB + 75 Ω (4)
The typical distribution of R
AB
from channel-to-channel
matches is about ±0.15% within a given device. On the other
hand, device-to-device matching is process-lot dependent with
a ±20% tolerance.
PROGRAMMABLE POTENTIOMETER OPERATION
If all three terminals are used, the operation is called potenti-
ometer mode (see Figure 44); the most common configuration
is the voltage divider operation.
03824-0-044
A
B
W
V
I
V
C
Figure 44. Potentiometer Mode Configuration
If the wiper resistance is ignored, the transfer function is simply
AD5253:
B
AB
W
VV
D
V +×=
64
(5)
AD5254:
B
AB
W
V
V
D
V +×=
256
(6)
A more accurate calculation that includes the wiper resistance
effect is
A
W
AB
W
AB
N
W
V
R
R
R
R
D
DV
2
2
)
(
+
+
=
(7)
where 2
N
is the number of steps.
Unlike in rheostat mode operation, where the tolerance is high,
potentiometer mode operation yields an almost ratiometric
function of D/2
N
with a relatively small error contributed by the
R
W
terms. Therefore, the tolerance effect is almost cancelled.
Similarly, the ratiometric adjustment also reduces the
temperature coefficient effect to 50 ppm/°C, except at low value
codes where R
W
dominates.
Potentiometer mode operations include other applications such
as op amp input, feedback-resistor networks, and other voltage-
scaling applications. The A, W, and B terminals can, in fact, be
input or output terminals, provided that |V
A
|, |V
W
|, and |V
B
| do
not exceed V
DD
to V
SS
.