# AD5235 View Datasheet(PDF) - Analog Devices

 Part Name Description Manufacturer AD5235 Nonvolatile Memory, Dual 1024-Position Digital Potentiometer Analog Devices
AD5235 Datasheet PDF : 32 Pages
 First Prev 21 22 23 24 25 26 27 28 29 30 Next Last
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between Terminal A
and Terminal B, RAB, is available with 25 kΩ and 250 kΩ with
1024 positions (10-bit resolution). The final digits of the part
number determine the nominal resistance value, for example,
25 kΩ = 24.4 Ω; 250 kΩ = 244 Ω.
The 10-bit data-word in the RDAC latch is decoded to select one
of the 1024 possible settings. The following description provides
the calculation of resistance, RWB, at different codes of a 25 kΩ
part. The first connection of the wiper starts at Terminal B for
Data 0x000. RWB(0) is 30 Ω because of the wiper resistance, and
it is independent of the nominal resistance. The second connection
is the first tap point where RWB(1) becomes 24.4 Ω + 30 Ω = 54.4 Ω
for Data 0x001. The third connection is the next tap point
representing RWB(2) = 48.8 Ω + 30 Ω = 78.8 Ω for Data 0x002,
and so on. Each LSB data value increase moves the wiper up the
resistor ladder until the last tap point is reached at RWB(1023) =
25006 Ω. See Figure 45 for a simplified diagram of the equivalent
RDAC circuit. When RWB is used, Terminal A can be left
floating or tied to the wiper.
100
RWA
RWB
75
50
25
0
0
256
512
768
1023
CODE (Decimal)
Figure 46. RWA(D) and RWB(D) vs. Decimal Code
The general equation that determines the programmed output
resistance between Terminal Bx and Terminal Wx is
RWB
(D)
=
D
1024
× R AB
+
RW
(1)
where:
D is the decimal equivalent of the data contained in the RDAC
register.
RAB is the nominal resistance between Terminal A and Terminal B.
RW is the wiper resistance.
For example, the output resistance values in Table 12 are set for
the given RDAC latch codes (applies to RAB = 25 kΩ digital
potentiometers).
Table 12. RWB (D) at Selected Codes for RAB = 25 kΩ
D (Dec) RWB(D) (Ω) Output State
1023
25,006
Full scale
512
12,530
Midscale
1
54.4
1 LSB
0
30
Zero scale (wiper contact resistor)
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 20 mA to
avoid degradation or possible destruction of the internal switches.
Like the mechanical potentiometer that the RDAC replaces, the
AD5235 part is symmetrical. The resistance between Wiper W
and Terminal A also produces a digitally controlled complementary
resistance, RWA. Figure 46 shows the symmetrical
programmability of the various terminal connections. When RWA
is used, Terminal B can be left floating or 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 transfer equation for this operation is
RWA (D)
=
1024 D
1024
× RAB
+
RW
(2)
For example, the output resistance values in Table 13 are set for
the given RDAC latch codes (applies to RAB = 25 kΩ digital
potentiometers).
Table 13. RWA(D) at Selected Codes for RAB = 25 kΩ
D (Dec) RWA(D) (Ω) Output State
1023
54.4
Full scale
512
12,530
Midscale
1
25,006
1 LSB
0
25,030
Zero scale (wiper contact resistance)
The typical distribution of RAB from channel to channel is
±0.2% within the same package. Device-to-device matching is
process lot dependent upon the worst case of ±30% variation.
However, the change in RAB with temperature has a 35 ppm/°C
temperature coefficient.
PROGRAMMING THE POTENTIOMETER DIVIDER
Voltage Output Operation
The digital potentiometer can be configured to generate an output
voltage at the wiper terminal that is proportional to the input
voltages applied to Terminal A and Terminal B. For example,
connecting Terminal A to 5 V and Terminal B to ground
produces an output voltage at the wiper that can be any value
from 0 V to 5 V. Each LSB of voltage is equal to the voltage
applied across Terminal A to Terminal B divided by the 2N
position resolution of the potentiometer divider.
Rev. E | Page 22 of 32