AD5247 Просмотр технического описания (PDF) - Analog Devices
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AD5247 Datasheet PDF : 20 Pages
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AD5247
OPERATION
The AD5247 is a 128-position, digitally controlled variable
resistor (VR) device. An internal power-on preset places the
wiper at midscale during power-on, which simplifies the default
condition recovery at power-up.
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between terminals A and
B is available in 5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ. The final two
or three digits of the part number determine the nominal
resistance value, e.g., 10 kΩ = 10, 50 kΩ = 50. The nominal
resistance (RAB) of the VR has 128 contact points accessed by
the wiper terminal, plus the B terminal contact. The 7-bit data
in the RDAC latch is decoded to select one of the 128 possible
settings.
Assuming a 10 kΩ part is used, the wiper’s first connection
starts at the B terminal for data 0x00. Since there is a 50 Ω wiper
contact resistance, such a connection yields a minimum of
100 Ω (2 × 50 Ω) resistance between terminals W and B. The
second connection is the first tap point, which corresponds to
178 Ω (RWB = RAB/128+ RW = 78 Ω + 2 × 50 Ω) for data 0x01.
The third connection is the next tap point, representing 256 Ω
(2 × 78 Ω + 2 × 50 Ω) 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 10,100 Ω (RAB + 2 × RW).
Figure 34 shows a simplified diagram of the equivalent RDAC
circuit where the last resistor string will not be accessed.
Ax
D6
D5
RS
D4
D3
D2
RS
D1
D0
Wx
RDAC
LATCH
AND RS
DECODER
Bx
Figure 34. AD5247 Equivalent RDAC Circuit
The general equation determining the digitally programmed
output resistance between W and B is
RWB(D) = D × RAB + 2× RW
(1)
128
where D is the decimal equivalent of the binary code loaded in
the 7-bit RDAC register, RAB is the end-to-end resistance, and
RW is the wiper resistance contributed by the on resistance of
the internal switch. In summary, if RAB = 10 kΩ and the A
terminal is open-circuited, the output resistance RWB shown in
Table 7 will be set for the indicated RDAC latch codes.
Table 7. Codes and Corresponding RWB Resistance
D (Dec.) RWB (Ω) Output State
127
10,100 Full Scale (RAB + 2 × RW)
64
5,100 Midscale
1
178
1 LSB
0
100
Zero Scale (Wiper Contact Resistance)
Note that in the zero-scale condition, a finite resistance of
100 Ω between terminals W and B 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.
Similar to the mechanical potentiometer, the resistance of the
RDAC between the 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
RWA(D) = 128 – D × RAB + 2 × RW
(2)
128
For RAB = 10 kΩ and the B terminal open circuited, the output
resistance RWA shown in Table 8 will be set for the indicated
RDAC latch codes.
Table 8. Codes and Corresponding RWA Resistance
D (Dec.)
RWA (Ω)
Output State
127
178
Full Scale
64
5,100
Midscale
1
9,961
1 LSB
0
10,100
Zero Scale
Typical device-to-device matching is process lot dependent and
may vary by up to ±30%. Since the resistance element is
processed in thin film technology, the change in RAB with
temperature has a very low 45 ppm/°C temperature coefficient.
Rev. 0 | Page 13 of 20
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