
Preliminary Technical Data
AD5290
OPERATION
The AD5290 is a 256-position digitally controlled variable
resistor device that can be controlled digitally through SPI
interface.
Rev. Pr C | Page 8 of 11
An internal power-on preset places the wiper at midscale
during power-on, which simplifies the fault condition recovery
at power-up.
DETERMINING THE VARIABLE RESISTANCE
AND VOLTAGE
Rheostat Mode Operation
If only the W-to-B or W-to-A terminals are used as variable
resistors, the unused terminal can be opened or shorted with W.
This operation is called rheostat mode (Figure 3).
A
W
B
A
W
B
A
W
B
0
Figure 3. Rheostat Mode Configuration
The nominal resistance (R
AB
) of the RDAC has 256 contact
points accessed by the wiper terminal, plus the B terminal
contact if R
WB
is considered. The 8-bit data in the RDAC latch is
decoded to select one of the 256 settings. Assuming that a 10
k part is used, the wiper’s first connection starts at the B
terminal for data 0x00. Such connection yields a minimum of
60 resistance between terminals W and B because of the 60
wiper contact resistance. The second connection is the first tap
point, which corresponds to 99 (
R
WB
= (1) ×
R
AB
/256 +
R
W
)
for data 0x01, and so on. Each LSB data value increase moves
the wiper up the resistor ladder until the last tap point is reached
at 10020 ((255) ×
R
AB
/256 +
R
W
). Figure 6 shows a simplified
diagram of the equivalent RDAC circuit. The general equation
determining
R
WB
is
W
AB
WB
R
R
256
D
)
D
(
R
+
×
=
(1)
where:
D
is the decimal equivalent of the 8-bit binary code.
R
AB
is the end-to-end resistance.
R
W
is the wiper resistance contributed by the on-resistance of
the internal switch.
Table 1. R
WB
vs. Codes; R
AB
= 10 k and
the A Terminal Is Opened
D (Dec)
R
WB
()
Output State
255
10020
Full-Scale (R
AB
+ R
W
)
128
5060
Midscale
1
99
1 LSB
0
60
Zero-Scale (Wiper Contact Resistance)
Since a finite wiper resistance of 60 is present in the zero-
scale condition, care should be taken to limit the current flow
between W and B in this state to a maximum pulse current of
no more than 20 mA. Otherwise, degradation or possible
destruction of the internal switch contact can occur.
Similar to the mechanical potentiometer, the resistance of the
RDAC between the wiper W and terminal A also produces a
complementary resistance R
WA
. When these terminals are used,
the B terminal can be opened or shorted to W. Setting the
resistance value for R
WA
starts at a maximum value of resistance
and decreases as the data loaded in the latch increases in value.
The general equation for this operation is
W
AB
WA
R
R
256
D
256
)
D
(
R
+
×
=
(2)
Table 2. R
WA
vs. Codes; R
AB
=10 k and
B Terminal Is Opened
D (Dec)
R
WA
()
255
60
128
5060
1
10020
0
10060
Output State
Full-Scale
Midscale
1 LSB
Zero-Scale
The typical distribution of the resistance tolerance from device
to device is process lot dependent, and it is possible to have
±30% tolerance.
Figure 6. AD5290 Equivalent RDAC Circuit
Potentiometer Mode Operation
If all three terminals are used, the operation is called the
potentiometer mode. The most common configuration is the
voltage divider operation (Figure 7).
B
RDAC
LATCH
AND
DECODER
W
A
R
S
R
S
R
S
R
S
D7
D6
D5
D4
D3
D2
D1
D0
R
W