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參數(shù)資料
| 型號： | ADADC85Z-12 |
| 元件分類： | ADC |
| 英文描述： | Analog-to-Digital Converter??? 12-Bit |
| 中文描述： | 12位模數(shù)轉(zhuǎn)換器 |
| 文件頁數(shù)： | 9/16頁 |
| 文件大?。?/td> | 364K |
| 代理商： | ADADC85Z-12 |
REV. A
–9–
AD5241/AD5242
OPERATION
The AD5241/AD5242 provides a single-/dual-channel, 256-
position digitally controlled variable resistor (VR) device. The
terms VR, RDAC, and programmable resistor are commonly
used interchangeably to refer to digital potentiometer.
To program the VR settings, refer to the Digital Interface sec-
tion. Both parts have an internal power ON preset that places
the wiper in midscale during power-on, which simplifies the
fault condition recovery at power-up. In addition, the shutdown
SHDN
pin of AD5241/AD5242 places the RDAC in an almost
zero power consumption state where Terminal A is open cir-
cuited and the Wiper W is connected to Terminal B, resulting
in only leakage current being consumed in the VR structure.
During shutdown, the VR latch contents are maintained when
the RDAC is inactive. When the part is returned from shut-
down, the stored VR setting will be applied to the RDAC.
SW
SHDN
SW
N
2
–
1
R
R
SW
N
2
–
2
RDAC
LATCH &
DECODER
R
R
AB
/2
N
B
W
DIGITAL CIRCUITRY
OMITTED FOR CLARITY
A
SW
1
SW
0
R
R
D7
D6
D5
D4
D3
D2
D1
D0
SHDN
Figure 4. AD5241/AD5242 Equivalent RDAC Circuit
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between Terminals A and
B are available in 10 k
, 100 k
, and 1 M
. The final two or
three digits of the part number determine the nominal resistance
value, e.g. 10 k
= 10; 100 k
= 100; 1 M
= 1 M. The
nominal resistance (R
AB
) of the VR has 256 contact points
accessed by the wiper terminal, plus the B terminal contact.
The 8-bit data in the RDAC latch is decoded to select one of
the 256 possible settings. Assume a 10 k
part is used; the
wiper’s first connection starts at the B terminal for data 00
H
.
Since there is a 60
wiper contact resistance, such connection
yields a minimum of 60
resistance between terminals W and
B. The second connection is the first tap point corresponds to
99
(R
WB
= R
AB
/256 + R
W
= 39 + 60) for data 01
H
. The third
connection is the next tap point representing 138
(39
×
2 + 60)
for data 02
H
and so on. Each LSB data value increase moves the
wiper up the resistor ladder until the last tap point is reached at
10021
[R
AB
– 1 LSB + R
W
]. Figure 4 shows a simplified dia-
gram of the equivalent RDAC circuit where the last resistor
string will not be 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
resistance between W and B is:
R
D
( )
=
D
R
R
WB
AB
W
×
+
256
(1)
where:
D
is the decimal equivalent of the binary code between 0
and 255 which is loaded in the 8-bit RDAC register.
R
AB
is the nominal end-to-end resistance.
R
W
is the wiper resistance contributed by the on-resistance
of the internal switch.
Again, if
R
AB
= 10 k
and A terminal can be either open circuit
or tied to W, the following output resistance at
R
WB
will be set
for the following RDAC latch codes.
FREQUENCY
–
Hz
6
–
36
–
42
–
48
–
54
G
–
100k
10k
1k
100
–
30
–
24
–
18
–
12
–
6
0
FF
H
80
H
40
H
20
H
10
H
08
H
04
H
02
H
01
H
TPC 13. AD5242 100 k
Gain vs. Frequency vs. Code
FREQUENCY
–
Hz
6
–
36
–
42
–
48
–
54
G
–
100k
10k
1k
100
–
30
–
24
–
18
–
12
–
6
0
FF
H
80
H
40
H
20
H
10
H
08
H
04
H
02
H
01
H
TPC 14. AD5242 1 M
Gain vs. Frequency vs. Code
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