參數(shù)資料
型號： EVAL-AD5282EBZ
廠商： Analog Devices Inc
文件頁數(shù)： 6/28頁
文件大?。?/td> 0K
描述： BOARD EVALUATION FOR AD5282
標(biāo)準(zhǔn)包裝： 1
主要目的： 數(shù)字電位器
嵌入式： 否
已用 IC / 零件： AD5282
主要屬性： 2 通道，256 位置
次要屬性： I²C 接口
已供物品： 板
AD5280/AD5282
Rev. C | Page 14 of
28
THEORY OF OPERATION
The AD5280/AD5282 are single-channel and dual-channel,
256-position, digitally controlled variable resistors (VRs). To
program the VR settings, see the Digital Interface section. Both
parts have an internal power-on preset that places the wiper at
midscale during power-on, which simplifies the fault condition
recovery at power-up. Operation of the power-on preset function
also depends on the state of the VL pin.
D7
D6
D5
D4
D3
D2
D1
D0
RDAC
LATCH
AND
DECODER
RS
AX
WX
RS
BX
SHDN
0xFF
0x01
SWB
SWA
0x00
5
02
92
9-
04
()
Figure 44. AD5280/AD5282 Equivalent RDAC Circuit
RHEOSTAT OPERATION
The nominal resistance of the RDAC between Terminal A and
Terminal B is available in 20 kΩ, 50 kΩ, and 200 kΩ. The final
two or three digits of the part number determine the nominal
resistance value, for example, 20 kΩ = 20, 50 kΩ = 50, and
200 kΩ = 200. The nominal resistance (RAB) of the VR has
256 contact points 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. Assuming
that a 20 kΩ part is used, the wiper’s first connection starts at
the B terminal for data 0x00. Because there is a 60 Ω wiper
contact resistance, such a connection yields a minimum of 60 Ω
resistance between Terminal W and Terminal B.
The second connection is the first tap point that corresponds to
138 Ω (RWB = RAB/256 + RW = 78 Ω + 60 Ω) for data 0x01. The
third connection is the next tap point representing 216 Ω (78 ×
2 + 60) 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 19,982 Ω (RAB – 1 LSB + RW). Figure 46 shows a
simplified diagram of the equivalent RDAC circuit where the
last resistor string is not accessed; therefore, there is 1 LSB less
of the nominal resistance at full scale in addition to the wiper
resistance.
The general equation determining the digitally programmed
output resistance between W and B is
W
AB
WB
R
D
R
+
×
=
256
()
(1)
where:
D
is the decimal equivalent of the binary code loaded in the 8-
bit RDAC register.
RAB
is the nominal end-to-end resistance.
RW
is the wiper resistance contributed by the on resistance of
the internal switch.
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.
As in the mechanical potentiometer, the resistance of the RDAC
between 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
W
AB
WA
R
D
R
+
×
=
256
(2)
The typical distribution of the nominal resistance, RAB, from
channel to channel matches within ±1%. Device-to-device
matching is process lot dependent, and it is possible to have a
±30% variation. Because the resistance element is processed in
thin film technology, the change in RAB with temperature is very
small (30 ppm/°C).
POTENTIOMETER OPERATION
The digital potentiometer easily generates a voltage divider at
wiper to B and wiper to A to be proportional to the input voltage
at A to B. Unlike the polarity of VDD – VSS, which must be
positive, voltage across A to B, W to A, and W to B can be at
either polarity, provided that VSS is powered by a negative supply.
If the effect of the wiper resistance for approximation is ignored,
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 A to B divided by the 256 positions of the
potentiometer divider. Because the AD5280/AD5282 can be
supplied by dual supplies, the general equation defining the
output voltage at VW with respect to ground for any valid
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