Datasheet

AD5235 Data Sheet
Rev. F | Page 22 of 32
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between Terminal A
and Terminal B, R
AB
, 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, R
WB
, at different codes of a 25 kΩ
part. The first connection of the wiper starts at Terminal B for
Data 0x000. R
WB
(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 R
WB
(1) becomes 24.4 Ω + 30 Ω = 54.4 Ω
for Data 0x001. The third connection is the next tap point
representing R
WB
(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 R
WB
(1023) =
25006 Ω. See Figure 45 for a simplified diagram of the equivalent
RDAC circuit. When R
WB
is used, Terminal A can be left
floating or tied to the wiper.
CODE (Decimal)
100
75
0
0 1023256
R
WA
(D), R
WB
(D) (% R
WF
)
512
768
50
25
R
WA
R
WB
02816-045
Figure 46. R
WA
(D) and R
WB
(D) vs. Decimal Code
The general equation that determines the programmed output
resistance between Terminal Bx and Terminal Wx is
W
AB
WB
RR
D
DR +×=
1024
)(
(1)
where:
D is the decimal equivalent of the data contained in the RDAC
register.
R
AB
is the nominal resistance between Terminal A and Terminal B.
R
W
is the wiper resistance.
For example, the output resistance values in Table 12 are set for
the given RDAC latch codes (applies to R
AB
= 25 kΩ digital
potentiometers).
Table 12. R
WB
(D) at Selected Codes for R
AB
= 25 kΩ
D (Dec) R
WB
(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, R
WA
. Figure 46 shows the symmetrical
programmability of the various terminal connections. When R
WA
is used, Terminal B can be left floating or tied to the wiper.
Setting the resistance value for R
WA
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
W
ABWA
RR
D
DR +×
−
=
1024
1024
)(
(2)
For example, the output resistance values in Table 13 are set for
the given RDAC latch codes (applies to R
AB
= 25 kΩ digital
potentiometers).
Table 13. R
WA
(D) at Selected Codes for R
AB
= 25 kΩ
D (Dec) R
WA
(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 R
AB
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 R
AB
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 2
N
position resolution of the potentiometer divider.