AD5165BUJZ100 Datasheet, PDF (13/16 Pages) Analog Devices – 256-Position, Ultralow Power 1.8 V Logic-Level Digital Potentiometer

AD5165
THEORY OF OPERATION
The AD5165 is a 256-position digitally controlled variable
resistor (VR) device.
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between terminals A and
B is available in 100 kΩ. The nominal resistance (RAB) of the VR
has 256 contact points accessed by the wiper terminal, plus the
B terminal contact. The 8-bit data in the RDAC latch is decoded
to select one of the 256 possible settings.
A
W
A
W
A
W
B
B
B
Figure 36. Rheostat Mode Configuration
Assuming that a 100 kΩ part is used, the wiper’s first connec-
tion starts at the B terminal for data 0x00. Because there is a
50 Ω wiper contact resistance, such a connection yields a mini-
mum of 100 Ω (2 × 50 Ω) resistance between terminals W and
B. The second connection is the first tap point, which corres-
ponds to 490 Ω (RWB = RAB/256 + 2 × RW = 390 Ω + 2 × 50 Ω)
for data 0x01. The third connection is the next tap point,
representing 880 Ω (2 × 390 Ω + 2 × 50 Ω) for data 0x02, and
so on. Each LSB data value increase moves the wiper up the
resistor ladder until the last tap point is reached at 100,100 Ω
(RAB + 2 × RW).
A
R_S
D7
R_S
D6
D5
D4
D3
R_S
D2
D1
D0
W
RDAC
R_S
LATCH
AND
B
DECODER
Figure 37. AD5165 Equivalent RDAC Circuit
The general equation determining the digitally programmed
output resistance between W and B is
R_WB (D)
=
D
256
× R_AB
+
2 × R_W
(1)
where:
D is the decimal equivalent of the binary code loaded in the
8-bit RDAC register.
RAB is the end-to-end resistance.
RW is the wiper resistance contributed by the on resistance of
the internal switch.
In summary, if RAB = 100 kΩ and the A terminal is open
circuited, the following output resistance RWB is set for the
indicated RDAC latch codes.
Table 6. Codes and Corresponding RWB Resistance
D (Dec.) RWB (Ω) Output State
255
99,710 Full scale (RAB – 1 LSB + RW)
128
50,100 Midscale
1
490
1 LSB
0
100
Zero scale (wiper contact resistance)
Note that, in the zero-scale condition, a finite wiper resistance
of 100 Ω is present. 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
digitally controlled complementary resistance, RWA. When these
terminals are used, the B terminal can be opened. Setting the
resistance value for RWA 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
R_WA (D)
=
256 − D
256
×
R_AB
+
2×
R_W
(2)
For RAB = 100 kΩ with the B terminal open circuited, the
following output resistance RWA is set for the indicated RDAC
latch codes.
Table 7. Codes and Corresponding RWA Resistance
D (Dec.)
RWA (Ω)
Output State
255
490
Full scale
128
50,100
Midscale
1
99, 710
1 LSB
0
100,100
Zero scale
Typical device-to-device matching is process-lot dependent
and may vary by up to ±20%. Because the resistance element
is processed in thin film technology, the change in RAB with
temperature has a very low 35 ppm/°C temperature coefficient.
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