Datasheet

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 k, 50 k, and 100 k.
The last digits of the part number determine the nominal
resistance value; for example, 10 k = 10 and 100 k = 100.
The nominal resistance (R
AB
) 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 . The second connection (for a
10 k part) is the first tap point, located at 84  [= R
AB
(nominal
resistance)/256 + R
W
= 84  + 45 ] for the 0x01 data. The
third connection is the next tap point, representing 78 + 45 =
123  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 . 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
R
WB
(Dx) = (Dx)/256 × R
AB
+ R
W
(1)
where Dx is the data contained in the 8-bit RDACx latch, and
R
AB
is the nominal end-to-end resistance.
For example, when V
B
= 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—
V
B
= 0 V and Terminal A = Open Circuited
D (Dec) R
WB
(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 
is present. Regardless of which setting the part is operating in,
care should be taken to limit the current between Terminal A to
Ter mina l B, Wip er W to Terminal A, and Wip er W to Ter minal
B, to the maximum continuous current of ±5.65 mA(10 k) or
±1.35 mA(50 k and 100 k) 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, R
WA
. When these terminals are used, Terminal B
should be tied to the wiper. Setting the resistance value for R
WA
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
R
WA
(Dx) = (256 − Dx)/256 × R
AB
+ R
W
(2)
where Dx is the data contained in the 8-bit RDACx latch, and
R
AB
is the nominal end-to-end resistance.
For example, when V
A
= 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—
V
A
= 0 V and Terminal B Tied to Wiper W
D (DEC) R
WA
(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 R
AB
from channel to channel matches
to within ±1%. However, device-to-device matching is process
lot dependent, having a ±30% variation. The change in R
AB
in
terms of temperature has a 700 ppm/°C temperature coefficient.