AD5243 AD [Analog Devices], AD5243 Datasheet - Page 13

• Manufacturer Part Number: AD5243
• Description: Dual 256-Position I2C Compatible Digital Potentiometer
• Manufacturer: AD [Analog Devices]

THEORY OF OPERATION
The AD5243/AD5248 are 256-position digitally controlled
variable resistor (VR) devices.
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 AND
VOLTAGE
Rheostat Operation
The nominal resistance of the RDAC between Terminals A and
B is available in 2.5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ. 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.
Assuming that a 10 kΩ part is used, the wiper’s first connection
starts at the B terminal for data 0x00. Because there is a 50 Ω
wiper contact resistance, such a connection yields a minimum
of 100 Ω (2 × 50 Ω) resistance between Terminals W and B. The
second connection is the first tap point, which corresponds to
139 Ω (R
0x01. The third connection is the next tap point, representing
178 Ω (2 × 39 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 10,100 Ω (R
A
B
WB
= R
AB
Figure 37. AD5243 Equivalent RDAC Circuit
) of the VR has 256 contact points accessed by
W
Figure 36. Rheostat Mode Configuration
AB
D7
D6
D5
D4
D3
D2
D1
D0
DECODER
LATCH
/256 + 2 × R
RDAC
AND
A
B
R
R
R
R
S
S
S
S
W
W
= 39 Ω + 2 × 50 Ω) for data
AB
A
B
+ 2 × R
A
W
B
W
W
).
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The general equation determining the digitally programmed
output resistance between W and B is
where:
D is the decimal equivalent of the binary code loaded in the
8-bit RDAC register.
R
R
the internal switch.
In summary, if R
circuited, the following output resistance R
indicated RDAC latch codes.
Table 7. Codes and Corresponding R
D (Dec)
255
128
1
0
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, 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
latch codes.
Table 8. Codes and Corresponding R
D (Dec)
255
128
1
0
AB
W
is the wiper resistance contributed by the on resistance of
is the end-to-end resistance.
R
R
AB
WB
WA
= 10 kΩ and the B terminal open circuited, the
(
(
D
D
R
9,961
5,060
139
100
)
)
R
139
5,060
9,961
10,060
WB
=
=
WA
256
(Ω)
256
D
AB
(Ω)
256
= 10 kΩ and the A terminal is open
×
−
WA
R
D
Output State
Full scale (R
Midscale
1 LSB
Zero scale (wiper contact resistance)
AB
starts at a maximum value of resistance
×
Output State
Full scale
Midscale
1 LSB
Zero scale
+
R
AB
2
WA
×
+
R
2
is set for the indicated RDAC
W
×
AB
R
− 1 LSB + R
W
WA
WB
AD5243/AD5248
Resistance
Resistance
WB
is set for the
W
WA
)
. When these
(1)
(2)