AD5251 Analog Devices, AD5251 Datasheet - Page 23

• Manufacturer Part Number: AD5251
• Description: (AD5251 / AD5252) Dual 64-and 256-Position I2C Nonvolatile Memory Digital Potentiometers
• Manufacturer: Analog Devices

Table 13. R
D (DEC)
63
32
1
0
Note that in the zero-scale condition, a 75 Ω finite wiper
resistance is present. Care should be taken to limit the current
conduction between W and B in this state to no more than
±5 mA continuous for a total resistance of 1 kΩ, or a ±20 mA
pulse, to avoid degradation or possible destruction of the
internal switch contact.
Similar to the mechanical potentiometer, the resistance of the
RDAC between Wiper W and Terminal A also produces a
digitally controlled complementary resistance, R
terminals are used, the B terminal can be opened. Setting the
resistance value for R
and decreases as the data loaded in the latch increases in value
(see Figure 40). The general equation for this operation is
Table 14. R
Open
D (DEC)
63
32
1
0
The typical distribution of R
matches about ±0.15% within a given device. On the other
hand, device-to-device matching is process-lot dependent with
±20% tolerance.
100
AD5251: R
AD5252: R
75
50
25
0
0
Figure 42. AD5251 R
WB
WA
R
9918
5075
231
75
vs. Codes; R
vs. Codes; AD5251, R
WB
WA
WA
R
(Ω)
(D) = [(64 – D)/64] × R
(D) = [(256 – D)/256] × R
16
R
231
5075
9918
10075
WA
WA
WA
(Ω)
D (Code in Decimal)
starts at a maximum value of resistance
WA
Output State
Full scale
Midscale
1 LSB
Zero scale (wiper resistance)
AB
(D) and R
AB
= 10 kΩ, A Terminal = Open
32
from channel-to-channel
WB
AB
(D) vs. Decimal Code
Output State
Full scale
Midscale
1 LSB
Zero scale
=10 kΩ, B Terminal
AB
R
48
WB
+ 75 Ω
AB
+ 75 Ω
WA
. When these
63
(3)
(4)
Rev. 0 | Page 23 of 28
PROGRAMMABLE POTENTIOMETER OPERATION
If all three terminals are used, the operation is called potenti-
ometer mode and the most common configuration is the
voltage divider operation (see Figure 43).
If the wiper resistance is ignored, the transfer function is simply
A more accurate calculation, which includes the wiper
resistance effect, yields
Where 2
operation where the tolerance is high, potentiometer mode
operation yields an almost ratiometric function of D/2
relatively small error contributed by the R
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
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 |V
exceed V
AD5251:
AD5252:
V
W
N
(
DD
D
is the number of steps. Unlike in rheostat mode
)
to V
=
Figure 43. Potentiometer Mode Configuration
V
V
2
D
W
W
SS
R
N
V
.
AB
I
=
=
R
AB
64
256
+
D
D
2
+
×
R
×
V
R
W
V
W
AB
A
B
AB
V
+
+
A
V
V
B
B
W
A
AD5251/AD5252
|, |V
V
W
C
terms. Therefore,
W
|, and |V
B
N
W
| do not
with a
(5)
(6)
(7)