ad5160 Analog Devices, Inc., ad5160 Datasheet - Page 12

• Manufacturer Part Number: ad5160
• Description: 256-position Spi Compatible Digital Potentiometer
• Manufacturer: Analog Devices, Inc.

AD5160
OPERATION
The AD5160 is a 256-position digitally controlled variable
resistor (VR) device.
An internal power-on preset places the wiper at midscale
during power-on, which simplifies the fault condition recovery
at power-up.
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between terminals A and
B is available in 5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ. The final two
or three digits of the part number determine the nominal
resistance value, e.g., 10 kΩ = 10; 50 kΩ = 50. The nominal
resistance (R
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. Assume a 10 kΩ part is used, the wiper’s first
connection starts at the B terminal for data 0x00. Since there is a
60 Ω wiper contact resistance, such connection yields a
minimum of 60 Ω resistance between terminals W and B. The
second connection is the first tap point, which corresponds to
99 Ω (R
The third connection is the next tap point, representing 177 Ω
(2 × 39 Ω + 60 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 9961 Ω (R
a simplified diagram of the equivalent RDAC circuit where the
last resistor string will not be accessed; therefore, there is 1 LSB
less of the nominal resistance at full scale in addition to the
wiper resistance.
WB
= R
AB
Figure 38. AD5160 Equivalent RDAC Circuit
AB
) of the VR has 256 contact points accessed by
/256 + R
D7
D6
D5
D4
D3
D2
D1
D0
DECODER
LATCH
RDAC
AND
W
= 39 Ω + 60 Ω) for data 0x01.
R
R
R
R
S
S
S
S
AB
– 1 LSB + R
W
W
A
B
). Figure 38 shows
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The general equation determining the digitally programmed
output resistance between W and B is
where
the 8-bit RDAC register,
R
the internal switch.
In summary, if R
circuited, the following output resistance R
indicated RDAC latch codes.
Table 6. Codes and Corresponding R
D (Dec.)
255
128
1
0
Note that in the zero-scale condition a finite wiper resistance of
60 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 R
terminals are used, the B terminal can be opened. Setting the
resistance value for R
and decreases as the data loaded in the latch increases in value.
The general equation for this operation is
For R
following output resistance R
RDAC latch codes.
Table 7. Codes and Corresponding R
D (Dec.)
255
128
1
0
Typical device to device matching is process lot dependent and
may vary by up to ±30%. Since the resistance element is
processed in thin film technology, the change in R
temperature has a very low 45 ppm/°C temperature coefficient.
W
is the wiper resistance contributed by the on resistance of
R
R
AB
WB
WA
D
= 10 kΩ and the B terminal open circuited, the
(
is the decimal equivalent of the binary code loaded in
(
D
D
)
)
R
9,961
5,060
99
60
=
=
WB
256
256
(Ω)
D
AB
256
R
99
5,060
9,961
10,060
= 10 kΩ and the A terminal is open
WA
×
−
R
WA
D
(Ω)
AB
Output State
Full Scale (R
Midscale
1 LSB
Zero Scale (Wiper Contact Resistance)
×
starts at a maximum value of resistance
R
R
AB
+
AB
R
is the end-to-end resistance, and
WA
W
+
will be set for the indicated
R
W
AB
Output State
Full Scale
Midscale
1 LSB
Zero Scale
– 1 LSB + R
WB
WA
Resistance
Resistance
WB
will be set for the
WA
W
)
AB
. When these
with
(1)
(2)