AD5282 Analog Devices, AD5282 Datasheet - Page 6

• Manufacturer Part Number: AD5282
• Description: +15V/ I2C Compatible Digital Potentiometers
• Manufacturer: Analog Devices

AD5280/AD5282
Figure 3. Reading Data from a Previously Selected RDAC Register
OPERATION
The AD5280/AD5282 provides a single/dual channel, 256-
position digitally-controlled variable resistor (VR) device. The
terms VR and RDAC are used interchangeably throughout this
documentation. To program the VR settings, refer to the Digital
Interface section. Both parts have an internal power ON preset
that places the wiper in mid scale during power on, which
simplifies the fault condition recovery at power up. In addition,
the shutdown SHDN pin of AD5280/AD5282 places the RDAC
in a zero power consumption state where terminal A is open
circuited and the wiper W is connected to terminal B, resulting
in only leakage currents being consumed in the VR structure. In
shutdown mode the VR latch settings are maintained, so that,
returning to operational mode from power shutdown, the VR
settings return to their previous resistance values.
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between terminals A and B
are available in 20K , 50K , and 200K . The final three
digits of the part number determine the nominal resistance
value, e.g. 20K = 20; 50K = 50; 200K = 200. The
nominal resistance (R
accessed by the wiper terminal, plus the B terminal contact. The
eight bit data in the RDAC latch is decoded to select one of the
256 possible settings. Assume a 20K part is used, the wiper's
first connection starts at the B terminal for data 00
is a 60 wiper contact resistance, such connection yields a
minimum of 60 resistance between terminals W and B. The
second connection is the first tap point corresponds to 138
(R
connection is the next tap point representing 216 (78x2+60)
Information contained in this Product Concept Data Sheet describes a product in the early definition stage. There is no guarantee that the information contained here will become a final
product in its present form. For latest information contact Walt Heinzer/Analog Devices, Santa Clara, CA. TEL [phone]; FAX [phone]; email; [e-mail]
START BY
MASTER
WB
= R
Figure 4. AD5280/AD5282 Equivalent RDAC Circuit
SCL
SDA
AB
/256 + R
1
0
1
SHDN
W
DECODER
D7
D6
D5
D4
D3
D2
D1
D0
LATCH
AB
RDAC
= 78 +60 ) for data 01
0
&
) of the VR has 256 contact points
Slave Address Byte
1
R S
R S
R S
R S
FRAME 1
PRELIMINARY TECHNICAL DATA
1
AD1
AD0
Wx
Ax
Bx
R/ W
H
. The third
ACK. BY
AD5280
H
. Since there
9
D7
1
D6
D5
6
D4
Data From Select ed
RDAC Regis ter
FRAME 2
for data 02
wiper up the resistor ladder until the last tap point is reached at
19982 [R
to the B terminal. See Figure 4 for a simplified diagram of the
equivalent RDAC circuit.
The general equation determining the digitally programmed
output resistance between W and B is:
where D is the decimal equivalent of the binary code which is
loaded in the 8-bit RDAC register, and R
to-end resistance.
For example, R
circuit, the following output resistance values R
the following RDAC latch codes. Result will be the same if
terminal A is tied to W:
D
(DEC)
256
128
1
0
Note that in the zero-scale condition a finite wiper resistance of
60 is present. Care should be taken to limit the current flow
between W and B in this state to a maximum current of no more
than 5mA. 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 resistance R
used the B–terminal should be let open or tied to the wiper
terminal. Setting the resistance value for R
maximum value of resistance and decreases as the data loaded in
the latch is increased in value. The general equation for this
operation is:
R
R
D3
WA
WB
(
(
D2
D
D
)
)
R
(
19982
10060
138
60
D1
H
AB
WB
256
256
and so on. Each LSB data value increase moves the
D
–1LSB+R
256
D0
AB
BY MASTER
R
=20K , when V
NO ACK.
D
AB
Output State
Full-Scale (R
Mid-Scale
1 LSB
Zero-Scale (Wiper contact resistance)
9
R
W
AB
R
STO P BY
MASTER
]. The wiper does not directly connect
W
R
W
WA
AB
B
. When these terminals are
= 0V and A–terminal is open
- 1LSB + R
AB
WA
eqn.
is the nominal end-
starts at a
W
REV PrE 12 MAR 02
WB
)
2
will be set for
eqn.
1