参数资料
型号: AD5251BRU100-RL7
厂商: ANALOG DEVICES INC
元件分类: 数字电位计
英文描述: 32-Tap. Nonvolatile. Linear-Taper Digital Potentiometers in SOT23
中文描述: DUAL 100K DIGITAL POTENTIOMETER, 2-WIRE SERIAL CONTROL INTERFACE, 64 POSITIONS, PDSO14
封装: MO-153AB-1, TSSOP-14
文件页数: 22/28页
文件大小: 859K
代理商: AD5251BRU100-RL7
AD5251/AD5252
LAYOUT AND POWER SUPPLY BIASING
It is always a good practice to employ a compact, minimum
lead-length layout design. The leads to the input should be as
direct as possible, with a minimum conductor length. Ground
paths should have low resistance and low inductance.
Rev. 0 | Page 22 of 28
Similarly, it is also good practice to bypass the power supplies
with quality capacitors. Low equivalent series resistance (ESR)
1 μF to 10 μF tantalum or electrolytic capacitors should be
applied at the supplies to minimize any transient disturbance
and filter low frequency ripple. Figure 39 illustrates the basic
supply bypassing configuration for the AD5251/AD5252.
V
DD
V
DD
V
SS
V
SS
GND
C3
AD5251/AD5252
C4
C1
+
+
10
μ
F
C2
10
μ
F
0.1
μ
F
0.1
μ
F
0
Figure 39. Power Supply Bypassing
The ground pin of the AD5251/AD5252 is used primarily as a
digital ground reference. To minimize the digital ground
bounce, the AD5251/AD5252 ground terminal should be joined
remotely to the common ground (see Figure 39).
DIGITAL POTENTIOMETER OPERATION
The structure of the RDAC is designed to emulate the
performance of a mechanical potentiometer. The RDAC
contains a string of resistor segments, with an array of analog
switches acting as the wiper connection to the resistor array.
The number of points is the resolution of the device. For
example, the AD5251/AD5252 emulates 64 or 256 connection
points with 64 or 256 equal resistance, R
S
, allowing it to provide
better than 1.5%/0.4% settability resolution.
Figure 40 provides an equivalent diagram of the connections
between the three terminals that make up one channel of the
RDAC. Switches SW
A
and SW
B
are always ON, while one of
switches SW(0) to SW(2
N–1
) is ON one at a time, depending on
the setting decoded from the data bit. Because the switches are
nonideal, there is a 75 wiper resistance, R
W
. 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 where accurate prediction of output
resistance is required.
SW
B
SW(1)
SW(0)
B
X
R
S
R
S
SW
A
SW(2
N
–1)
A
X
W
X
SW(2
N
–2)
RDAC
WIPER
REGISTER
AND
DECODER
R
S
= R
AB
/2
N
R
S
DIGITAL
CIRCUITRY
OMITTED FOR
CLARITY
0
Figure 40. Equivalent RDAC Structure
PROGRAMMABLE RHEOSTAT OPERATION
If either the W-to-B or W-to-A terminal is used as a variable
resistor, the unused terminal can be opened or shorted with W;
such operation is called rheostat mode (see Figure 41). The
resistance tolerance can range ± 20%.
A
W
B
0
A
W
B
A
W
B
Figure 41. Rheostat Mode Configuration
The nominal resistance of the AD5251/AD5252 has 64 or 256
contact points accessed by the wiper terminal, plus the
B terminal contact. The 6-or 8-bit data-word in the RDAC
register is decoded to select one of the 64 or 256 settings. The
wiper’s first connection starts at the B terminal for Data 0x00.
This B-terminal connection has a wiper contact resistance, R
W
,
of 75 , regardless of the nominal resistance. The second
connection (the AD5251 10 k part) is the first tap point where
R
WB
= 231 (R
WB
= R
AB
/64 + R
W
= 156 + 75 ) for Data
0x01, and so on. Each LSB data value increase moves the wiper
up the resistor ladder until the last tap point is reached at
R
WB
= 9893 . See Figure 40 for a simplified diagram of the
equivalent RDAC circuit.
The general equation that determines the digitally programmed
output resistance between W and B, is
AD5251:
R
WB
(
D
) = (
D
/64) ×
R
AB
+ 75
(1)
AD5252:
R
WB
(
D
) = (
D
/256) ×
R
AB
+ 75
(2)
Where
D
is the decimal equivalent data contained in the RDAC
latch and
R
AB
is the nominal end-to-end resistance.
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