AD5291/AD5292
Rev. D | Page 26 of 32
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation—1% Resistor Tolerance
The AD5291 and AD5292 operate in rheostat mode when only
two terminals are used as a variable resistor. The unused
terminal can be left floating or tied to the W terminal, as shown in
W
A
B
W
A
B
W
A
B
0
7
67
4-
05
2
Figure 66. Rheostat Mode Configuration
The nominal resistance between Terminal A and Terminal B,
RAB, is available in 20 kΩ, 50 kΩ, and 100 kΩ, and 256 or 1024
tap points accessed by the wiper terminal. The 8-/10-bit data in
the RDAC latch is decoded to select one of the 256/1024
possible wiper settings. The AD5291 and AD5292 contain an
internal ±1% resistor performance mode that can be disabled or
enabled (this is enabled by default), by programming Bit C2 of
programmed output resistance between the W terminal and the
A terminal, RWA, and between the W terminal and B terminal,
RWB, is internally calibrated to give a maximum of ±1% absolute
resistance error across a wide code range. As a result, the
general equations for determining the digitally programmed
output resistance between the W terminal and B terminal are
AD5291:
AB
WB
R
D
R
×
=
256
)
(
(1)
AD5292:
AB
WB
R
D
R
×
=
1024
)
(
(2)
where:
D is the decimal equivalent of the binary code loaded in the
8-/10-bit RDAC register.
RAB is the end-to-end resistance.
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, RWA. RWA is also
calibrated to give a maximum of 1% absolute resistance error.
RWA starts at the maximum resistance value and decreases as the
data loaded into the latch increases. The general equations for
this operation are
AD5291:
AB
WA
R
D
R
×
=
256
)
(
(3)
AD5292:
AB
WA
R
D
R
×
=
1024
)
(
(4)
where:
D is the decimal equivalent of the binary code loaded in the
8-/10-bit RDAC register.
RAB is the end-to-end resistance.
In the zero-scale condition, a finite total wiper resistance of 120 Ω
is present. Regardless of which setting the part is operating in,
take care to limit the current between Terminal A and Terminal B,
between Terminal W and Terminal A, and between Terminal W
and Terminal B, to the maximum continuous current of ±3 mA or
to the pulse current specified in
Table 8. Otherwise, degradation
or possible destruction of the internal resistors may occur.
PROGRAMMING THE POTENTIOMETER DIVIDER
Voltage Output Operation
The digital potentiometer easily generates a voltage divider at
the wiper to B and at the wiper to A that is proportional to the
input voltage at A to B, as shown in
Figure 67. Unlike the polarity
of VDD to GND, which must be positive, voltage across A to B,
W to A, and W to B can be at either polarity.
W
A
B
VIN
VOUT
07
67
4-
05
3
Figure 67. Potentiometer Mode Configuration
If ignoring the effect of the wiper resistance for simplicity, con-
necting the A terminal to 30 V and the B terminal to ground
produces an output voltage at the Wiper W to Terminal B
ranging from 0 V to 1 LSB less than 30 V. Each LSB of voltage is
equal to the voltage applied across Terminal A and Terminal B,
divided by the 256/1024 positions of the potentiometer divider.
The general equations defining the output voltage at VW with
respect to ground for any valid input voltage applied to Terminal A
and Terminal B are
AD5291:
B
A
W
V
D
V
D
V
×
+
×
=
256
)
(
(5)
AD5292:
B
A
W
V
D
V
D
V
×
+
×
=
1024
)
(
(6)
If using the AD5291 and AD5292 in voltage divider mode as
shown in
Figure 67, then the ±1% resistor tolerance calibration
feature reduces the error when matching with discrete resistors.
However, it is recommended to disable the internal ±1% resistor
tolerance calibration feature by programming Bit C2 of the
position update rate. In this configuration, the RDAC is ratiome-
tric and resistor tolerance error does not affect performance.