AD5255(Rev0) 查看數據表（PDF） - Analog Devices
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AD5255 Datasheet PDF : 20 Pages
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POWER-UP SEQUENCE
Since the ESD protection diodes limit the voltage compliance at
the A, B, and W terminals (Figure 30), it is important to power
VDD/VSS before applying any voltage to the A, B, and W
terminals. Otherwise, the diode is forward-biased such that
VDD/VSS are powered unintentionally, which affects the rest of
the circuit. The ideal power-up sequence is as follows: GND,
VDD, VSS, digital inputs, and VA/B/W. The order of powering VA,
VB, VW, and the digital inputs is not important as long as they
are powered after VDD/VSS.
LAYOUT AND POWER SUPPLY BIASING
It is always a good practice to use compact, minimum lead
length layout design. Make the leads to the input as direct as
possible with a minimum conductor length. Make sure that
ground paths have low resistance and low inductance.
Similarly, it is also good practice to bypass the power supplies
with quality capacitors. Use low equivalent series resistance
(ESR) 1 µF to 10 µF tantalum or electrolytic capacitors at the
supplies to minimize any transient disturbance and filter low
frequency ripple. Figure 31 illustrates the basic supply-
bypassing configuration for the AD5255.
VDD
+
C3
C1
10µF
0.1µF
+
C4
10µF
C2
0.1µF
VSS
AD5255
VDD
GND
VSS
Figure 31. Power Supply Bypassing
RDAC STRUCTURE
The patent pending RDAC contains a string of equal resistor
segments, with an array of analog switches. The switches act as
the wiper connection.
The AD5255 has two RDACs with 512 connection points
allowing it to provide better than 0.2% set-ability resolution.
The AD5255 also contains a third RDAC with 128-step
resolution.
Figure 32 shows an equivalent structure of the connections
between the two terminals that make up one channel of an
RDAC. The SWB switch is always on, while one of the switches,
SW(0) to SW(2N − 1), may or may not be on at any given time
depending on the resistance position decoded from the data bits
in the RDAC register.
AD5255
Since the switches are nonideal, there is a 100 Ω wiper
resistance, RW. Wiper resistance is a function of supply voltage
and temperature; lower supply voltages and higher temperatures
result in higher wiper resistances. Consideration of wiper
resistance dynamics is important in applications in which
accurate prediction of output resistance is required.
SWA
AX
SW(2N–1)
RDAC
WIPER
REGISTER
RS
SW(2N–2)
WX
AND
DECODER
RS
SW(1)
RS
RS = RAB/2N
SW(0)
DIGITAL
CIRCUITRY
OMITTED FOR
CLARITY
SWB
BX
Figure 32. Equivalent RDAC Structure
CALCULATING THE PROGRAMMABLE RESISTANCE
The nominal resistance of the RDAC between the A and B
terminals is available in 25 kΩ or 250 kΩ. The final two or three
digits of the part number determine the nominal resistance
value, for example, 25 kΩ = 25 and 250 kΩ = 250.
The following discussion describes the calculation of resistance
RWB(d) at different codes of a 25 kΩ part for RDAC0. The 9-bit
data word in the RDAC latch is decoded to select one of the 512
possible settings.
The first wiper connection starts at the B terminal for data 0x000.
RWB(0) is 100 Ω of the wiper resistance and it is independent of
the full-scale resistance. The second connection is the first tap
point where RWB(1) becomes 48.8 Ω + 100 = 148.8 Ω for data
0x001. The third connection is the next tap point representing
RWB(2) = 97.6 + 100 = 197.6 Ω for data 0x002, and so on. Each
LSB data-value increase moves the wiper up the resistor ladder
until the last tap point is reached at RWB(511) = 25051 Ω. See
Figure 32 for a simplified diagram of the equivalent RDAC
circuit.
These general equations determine the programmed output
resistance between W and B.
Rev. 0 | Page 17 of 20
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