Datasheet

AD5231 Data Sheet
Rev. D | Page 20 of 28
CODE (Decimal)
100
75
0
0 1023256
R
WA
(D), R
WB
(D); (% of Nominal R
AB
)
512 768
50
25
R
WB
R
WA
02739-043
Figure 44. R
WA
(D) and R
WB
(D) vs. Decimal Code
The general equation that determines the programmed output
resistance between W and B 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 11 are set
for the given RDAC latch codes with V
DD
= 5 V (applies to
R
AB
= 10 kΩ digital potentiometers).
Table 11. R
WB
(D) at Selected Codes for R
AB
= 10 kΩ
D (DEC) R
WB
(D) (Ω) Output State
1023 10,005 Full scale
512 50,015 Midscale
1 24.7 1 LSB
0 15 Zero scale (wiper contact resistor)
Note that, in the zero-scale condition, a finite wiper resistance
of 15 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
AD5231 part is totally symmetrical. The resistance between
Wiper W and Terminal A also produces a digitally controlled
complementary resistance, R
WA
. Figure 44 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
AB
WB
RR
D
DR +×
−
=
1024
1024
)(
(2)
For example, the output resistance values in Table 12 are set for
the RDAC latch codes with V
DD
= 5 V (applies to R
AB
= 10 kΩ
digital potentiometers).
Table 12. R
WA
(D) at Selected Codes for R
AB
= 10 kΩ
D (DEC) R
WA
(D) (Ω) Output State
1023 24.7 Full scale
512
5015
Midscale
1 10005 1 LSB
0 10,015 Zero scale
The typical distribution of R
AB
from device to device matches
tightly when they are processed in the same batch. When
devices are processed at a different time, device-to-device
matching becomes process-lot dependent and exhibits a −40%
to +20% variation. The change in R
AB
with temperature has a
600 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 Terminals A–B divided by the 2
N
position
resolution of the potentiometer divider.
Because AD5231 can also be supplied by dual supplies, the
general equation defining the output voltage at V
W
with respect
to ground for any given input voltages applied to Terminal A
and Terminal B is
B
AB
W
VV
D
DV +×
=
1024
)(
(3)
Equation 3 assumes that V
W
is buffered so that the effect of
wiper resistance is minimized. Operation of the digital
potentiometer in divider mode results in more accurate
operation over temperature. Here, the output voltage is
dependent on the ratio of the internal resistors and not the
absolute value; therefore, the drift improves to 15 ppm/°C.
There is no voltage polarity restriction between Terminal A,
Terminal B, and Terminal W as long as the terminal voltage
(V
TERM
) stays within V
SS
< V
TERM
< V
DD
.