Datasheet

AD5111/AD5113/AD5115 Data Sheet
Rev. | Page 20 of 24
Similar to the mechanical potentiometer, the resistance of
the RDAC between the W terminal and the A terminal also
produces a digitally controlled complementary resistance, R
WA
.
R
WA
starts at the maximum resistance value and decreases as the
data loaded into the latch increases. The general equations for
this operation are
AD5111:
W
ABAW
RRR +=
Bottom scale (7)
W
ABAW
RR
D
DR +×
−
=
128
128
)(
From 0 to 127 (8)
TS
AW
RR =
Top scale (9)
AD5113:
W
ABAW
RRR +=
Bottom scale (10)
W
ABAW
RR
D
DR +×
−
=
64
64
)(
From 0 to 63 (11)
TS
AW
RR =
Top scale (12)
AD5115:
W
ABAW
RRR +=
Bottom scale (13)
W
ABAW
RR
D
DR +×
−
=
32
32
)(
From 0 to 31 (14)
TS
AW
RR =
Top scale (15)
where:
D is the decimal equivalent of the binary code in the 5-/6-/7-bit
RDAC register; 128, 64, and 32 refer to top scale step.
R
AB
is the end-to-end resistance.
R
W
is the wiper resistance.
R
TS
is the wiper resistance at top scale.
Regardless of which setting the part is operating in, take care
to limit the current between A to B, W to A, and W to B, to
the maximum continuous current of ±6 mA (5 kΩ and 10 kΩ)
or ±1.5 mA (80 kΩ), or pulse current specified in Table 6.
Otherwise, degradation or possible destruction of the internal
switch contact can occur.
PROGRAMMING THE POTENTIOMETER DIVIDER
Voltage Output Operation
The digital potentiometer easily generates a voltage divider at
W-to-B and W-to-A that is proportional to the input voltage at
A-to-B, as shown in Figure 44. Unlike the polarity of V
DD
to
GND, which must be positive, current across A-to-B, W-to-A,
and W-to-B can be in either direction.
A
V
I
W
B
V
O
09654-045
Figure 44. Potentiometer Mode Configuration
If ignoring the effect of the wiper resistance for simplicity,
connecting Terminal A to 5 V and Terminal B to ground
produces an output voltage at W to B ranging from 0 V to 5 V.
The general equation defining the output voltage at V
W
with
respect to ground for any valid input voltage applied to
Terminal A and Terminal B, is
B
AB
AW
A
AB
WB
W
V
R
DR
V
R
DR
DV ×+×=
)(
)(
)(
(16)
where:
R
WB
(D) can be obtained from Equation 1 to Equation 6.
R
AW
(D) can be obtained from Equation 7 to Equation 14.
Operation of the digital potentiometer in the divider mode
results in a more accurate operation over temperature. Unlike
the rheostat mode, the output voltage is dependent mainly
on the ratio of the internal resistors, R
WA
and R
WB
, and not the
absolute values. Therefore, the temperature drift reduces to
5 ppm/°C.
TERMINAL VOLTAGE OPERATING RANGE
The AD5111/AD5113/AD5115 are designed with internal
ESD diodes for protection. These diodes also set the voltage
boundary of the terminal operating voltages. Positive signals
present on the A, B, or W terminals that exceed V
DD
are
clamped by the forward-biased diode. There is no polarity
constraint between V
A
, V
W
, and V
B
, but they cannot be higher
than V
DD
or lower than GND.
A