AD5241(RevB) 查看數據表（PDF） - Analog Devices
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AD5241 Datasheet PDF : 16 Pages
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AD5241/AD5242
D
(DEC)
255
128
1
0
RWB
(⍀)
10021
5060
99
60
Output State
Full-Scale (RWB – 1 LSB + RW)
Midscale
1 LSB
Zero-Scale (Wiper Contact Resistance)
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 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 Wiper W and Terminal A also produces a digi-
tally controlled resistance, RWA. When these terminals are used,
the B Terminal can be opened or tied to the Wiper Terminal.
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:
( ) RWA
D
=
256 – D
256
×
R AB
+
RW
(2)
For RAB = 10 kΩ, and the B Terminal can be either open circuit
or tied to W. The following output resistance RWA will be set for
the following RDAC latch codes.
D
(DEC)
255
128
1
0
RWA
(⍀)
99
5060
10021
10060
Output State
Full-Scale
Midscale
1 LSB
Zero-Scale
The typical distribution of the nominal resistance RAB from
channel to channel matches within ± 1% for AD5242. Device-
to-device matching is process lot dependent and it is possible to
have ± 30% variation. Since the resistance element is processed
in thin film technology, the change in RAB with temperature has
no more than a 30 ppm/°C temperature coefficient.
PROGRAMMING THE POTENTIOMETER DIVIDER
Voltage Output Operation
The digital potentiometer easily generates output voltages at
wiper-to-B and wiper-to-A to be proportional to the input volt-
age at A-to-B. Unlike the polarity of VDD – VSS, which must be
positive, voltage across A–B, W–A, and W–B can be at either
polarity provided that VSS is powered by a negative supply.
If ignoring the effect of the wiper resistance for approximation,
connecting the A Terminal to 5 V and the B Terminal to ground
produces an output voltage at the wiper-to-B starting at 0 V up
to 1 LSB less than 5 V. Each LSB of voltage is equal to the voltage
applied across Terminal AB divided by the 256 positions of the
potentiometer divider. Since AD5241/AD5242 can be sup-
plied by dual supplies, the general equation defining the output
voltage at VW with respect to ground for any valid input voltage
applied to Terminals A and B is:
( ) VW
D
=
D
256 VA
+
256 − D
256 VB
(3)
which can be simplified to
( ) VW
D
=
D
256 VAB
+ VB
(4)
where D is the decimal equivalent of the binary code between 0
to 255 that is loaded in the 8-bit RDAC register.
For more accurate calculation including the effects of wiper
resistance, VW can be found as:
( ) ( ) ( ) VW
D
=
RWB D
RAB
VA
+
RWA D
RAB
VB
(5)
where RWB(D) and RWA(D) can be obtained from Equations 1
and 2.
Operation of the digital potentiometer in the Divider Mode results
in a more accurate operation over temperature. Unlike the
Rheostat Mode, the output voltage is dependent on the ratio
of the internal resistors RWA and RWB, and not the absolute
values; therefore, the temperature drift reduces to 5 ppm/°C.
DIGITAL INTERFACE
2-Wire Serial Bus
The AD5241/AD5242 are controlled via an I2C compatible
serial bus. The RDACs are connected to this bus as slave devices.
Referring to Figures 2 and 3, the first byte of AD5241/AD5242
is a Slave Address Byte. It has a 7-bit slave address and an R/W
Bit. The 5 MSBs are 01011 and the following two bits are
determined by the state of the AD0 and AD1 Pins of the device.
AD0 and AD1 allow users to use up to four of these devices on
one bus.
The 2-wire I2C serial bus protocol operates as follows:
1. The master initiates data transfer by establishing a START
condition, which is when a high-to-low transition on the SDA
line occurs while SCL is high (Figure 2). The following byte
is the Slave Address Byte, Frame 1, which consists of the
7-bit slave address followed by an R/W Bit (this bit deter-
mines whether data will be read from or written to the
slave device).
The slave whose address corresponds to the transmitted
address will respond by pulling the SDA line low during the
ninth clock pulse (this is termed the Acknowledge Bit). At
this stage, all other devices on the bus remain idle while the
selected device waits for data to be written to or read from its
serial register. If the R/W Bit is high, the master will read
from the slave device. If the R/W Bit is low, the master will
write to the slave device.
2. A Write operation contains an extra Instruction Byte more
than the Read operation. This Instruction Byte, Frame 2,
in Write Mode follows the Slave Address Byte. The MSB of
the Instruction Byte labeled A/B is the RDAC subaddress
select. A “low” selects RDAC1 and a “high” selects RDAC2
for the dual-channel AD5242. Set A/B to low for AD5241.
The second MSB, RS, is the midscale reset. A logic high of
this bit moves the wiper of a selected RDAC to the center tap
where RWA = RWB. The third MSB, SD, is a shutdown bit. A
logic high on SD causes the RDAC open circuit at Terminal A
while shorting the wiper to Terminal B. This operation yields
almost a 0 Ω in Rheostat Mode or 0 V in Potentiometer
Mode. This SD Bit serves the same function as the SHDN
–10–
REV. B
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