AD5255BRU250-RL7 Analog Devices Inc, AD5255BRU250-RL7 Datasheet - Page 17

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AD5255BRU250-RL7

Manufacturer Part Number
AD5255BRU250-RL7
Description
Manufacturer
Analog Devices Inc
Datasheet

Specifications of AD5255BRU250-RL7

Number Of Elements
3
Resistance (max)
250KOhm
Power Supply Requirement
Single/Dual
Interface Type
Serial (2-Wire/I2C)
Single Supply Voltage (typ)
5V
Dual Supply Voltage (typ)
±2.5V
Single Supply Voltage (min)
2.7V
Single Supply Voltage (max)
5.5V
Dual Supply Voltage (min)
±2.2V
Dual Supply Voltage (max)
±2.7V
Operating Temp Range
-40C to 85C
Operating Temperature Classification
Industrial
Mounting
Surface Mount
Pin Count
24
Lead Free Status / RoHS Status
Not Compliant
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 V
terminals. Otherwise, the diode is forward-biased such that
V
the circuit. The ideal power-up sequence is as follows: GND,
V
V
they are powered after V
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 the
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.
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 the
RDAC. The SW
SW(0) to SW(2
depending on the resistance position decoded from the data bits
in the RDAC register.
DD
DD
A
, V
/V
, V
B
SS
, V
SS
, digital inputs, and V
DD
are powered unintentionally, which affects the rest of
V
W
V
DD
SS
/V
, and the digital inputs is not important as long as
SS
before applying any voltage to the A, B, and W
N
B
+
10μF
+
10μF
− 1), may or may not be on at any given time
switch is always on, while one of the switches,
C3
C4
Figure 31. Power Supply Bypassing
DD
0.1μF
0.1μF
C1
C2
/V
SS
A
/V
.
B
/V
GND
V
V
DD
SS
W
. The order of powering
AD5255
Rev. A | Page 17 of 20
Since the switches are nonideal, there is a 100 Ω wiper resis-
tance, R
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.
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
R
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.
R
the full-scale resistance. The second connection is the first tap
point where R
0x001. The third connection is the next tap point representing
R
LSB data-value increase moves the wiper up the resistor ladder
until the last tap point is reached at R
See Figure 32 for a simplified diagram of the equivalent
RDAC circuit.
These general equations determine the programmed output
resistance between W and B.
WB
WB
WB
(D) at different codes of a 25 kΩ part for RDAC0. The 9-bit
(0) is 100 Ω of the wiper resistance and it is independent of
(2) = 97.6 + 100 = 197.6 Ω for data 0x002, and so on. Each
W
. Wiper resistance is a function of supply voltage and
WB
R
DIGITAL
CIRCUITRY
OMITTED FOR
CLARITY
REGISTER
DECODER
S
WIPER
(1) becomes 48.8 Ω + 100 = 148.8 Ω for data
RDAC
Figure 32. Equivalent RDAC Structure
AND
= R
AB
/2
N
R
R
R
S
S
S
SW(2
SW(2
SW(1)
SW(0)
SW
SW
N
N
A
B
–1)
–2)
WB
(511) = 25051 Ω.
A
W
B
X
X
X
AD5255

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