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| 型號： | EVAL-AD5204SDZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 5/20頁 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR AD5204 |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 系列： | * |
AD5204/AD5206
Rev. C | Page 13 of 20
PROGRAMMING THE VARIABLE RESISTOR
RHEOSTAT OPERATION
The nominal resistance of the RDAC between Terminal A and
Terminal B is available with values of 10 kΩ, 50 kΩ, and 100 kΩ.
The last digits of the part number determine the nominal
resistance value; for example, 10 kΩ = 10 and 100 kΩ = 100.
The nominal resistance (RAB) of the VR has 256 contact points
accessed by the wiper terminal, plus Terminal B contact. The
8-bit data-word in the RDAC latch is decoded to select one of
the 256 possible settings. The first connection of the wiper starts
at Terminal B for the 0x00 data. This Terminal B connection has a
wiper contact resistance of 45 Ω. The second connection (for a
10 kΩ part) is the first tap point, located at 84 Ω [= RAB (nominal
resistance)/256 + RW = 84 Ω + 45 Ω] for the 0x01 data. The
third connection is the next tap point, representing 78 + 45 =
123 Ω for the 0x02 data. Each LSB data value increase moves
the wiper up the resistor ladder until the last tap point is
reached at 10,006 Ω. The wiper does not directly connect to
Terminal A. See Figure 21 for a simplified diagram of the
equivalent RDAC circuit.
The general transfer equation determining the digitally
programmed output resistance between the Wx and Bx
terminals is
RWB (Dx) = (Dx)/256 × RAB + RW
(1)
where Dx is the data contained in the 8-bit RDACx latch, and
RAB is the nominal end-to-end resistance.
For example, when VB = 0 V and Terminal A is open circuited, the
output resistance values are set as outlined in Table 7 for the
RDAC latch codes (applies to the 10 kΩ potentiometer).
Table 7. Output Resistance Values for the RDAC Latch Codes—
VB = 0 V and Terminal A = Open Circuited
D (Dec)
RWB (Ω)
Output State
255
10006
Full scale
128
5045
Midscale (PR = 0 condition)
1
84
1 LSB
0
45
Zero scale (wiper contact resistance)
In the zero-scale condition, a finite total wiper resistance of 45 Ω
is present. Regardless of which setting the part is operating in,
care should be taken to limit the current between Terminal A to
Terminal B, Wiper W to Terminal A, and Wiper W to Terminal
B, to the maximum continuous current of ±5.65 mA(10 kΩ) or
±1.35 mA(50 kΩ and 100 kΩ) or pulse current of ±20 mA.
Otherwise, degradation or possible destruction of the internal
switch contact, can occur.
Like the mechanical potentiometer that the RDAC replaces,
the RDAC is completely symmetrical. The resistance between
Wiper W and Terminal A produces a digitally controlled
resistance, RWA. When these terminals are used, Terminal B
should be tied to the wiper. Setting the resistance value for RWA
starts at a maximum value of resistance and decreases as the
data loaded to the latch is increased in value. The general
transfer equation for this operation is
RWA (Dx) = (256 Dx)/256 × RAB + RW
(2)
where Dx is the data contained in the 8-bit RDACx latch, and
RAB is the nominal end-to-end resistance.
For example, when VA = 0 V and Terminal B is tied to Wiper W,
the output resistance values outlined in Table 8 are set for the
RDAC latch codes.
Table 8. Output Resistance Values for the RDAC Latch Codes—
VA = 0 V and Terminal B Tied to Wiper W
D (DEC)
RWA (Ω)
Output State
255
84
Full scale
128
5045
Midscale (PR = 0 condition)
1
10006
1 LSB
0
10045
Zero scale
The typical distribution of RAB from channel to channel matches
to within ±1%. However, device-to-device matching is process
lot dependent, having a ±30% variation. The change in RAB in
terms of temperature has a 700 ppm/°C temperature coefficient.
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