AD5203
–9–REV. 0
OPERATION
The AD5203 provides a quad channel, 64-position digitally-
controlled variable resistor (VR) device. Changing the pro-
grammed VR settings is accomplished by clocking in an 8-bit
serial data word into the SDI (Serial Data Input) pin. The for-
mat of this data word is two address bits, MSB first, followed by
six data bits, MSB first. Table I provides the serial register data
word format. The AD5203 has the following address assign-
ments for the ADDR decode, which determines the location of
VR latch receiving the serial register data in Bits B5 through B0:
VR# = A1 × 2 + A0 + 1
VR outputs can be changed one at a time in random sequence.
The serial clock running at 10 MHz makes it possible to load all
four VRs in under 3.2 µs (8 × 4 × 100 ns) for the AD5203. The
exact timing requirements are shown in Figure 1.
The AD5203 resets to a midscale by asserting the RS pin, sim-
plifying initial conditions at power-up. Both parts have a power
shutdown SHDN pin that places the RDAC in a zero power
consumption state where terminals Ax are open-circuited and
the wiper Wx is connected to Bx, resulting in only leakage cur-
rents being consumed in the VR structure. In shutdown mode
the VR latch settings are maintained so that, returning to opera-
tional mode from power shutdown, the VR settings return to
their previous resistance values.
D5
D4
D3
D2
D1
D0
RDAC
LATCH
&
DECODER
Ax
Wx
Bx
R
S
= R
AB
/64
R
S
R
S
R
S
R
S
SHDN
Figure 34. Equivalent RDAC Circuit
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between Terminals A and
B are available with values of 10 kΩ, and 100 kΩ. The final
digits of the part number determine the nominal resistance
value, e.g., 10 kΩ = 10; 100 kΩ = 100. The nominal resistance
(R
AB
) of the VR has 64 contact points accessed by the wiper
terminal, plus the B terminal contact. The 6-bit data word in
the RDAC latch is decoded to select one of the 64 possible
settings. The wiper’s first connection starts at the B terminal for
data 00
H
. This B–terminal connection has a wiper contact resis-
tance of 45 Ω. The second connection (10 kΩ part) is the first
tap point located at 201 Ω [= R
BA
(nominal resistance)/64 + R
W
= 156 Ω + 45 Ω)] for data 01
H
. The third connection is the next
tap point representing 312 + 45 = 357 Ω for data 02
H
. Each
LSB data value increase moves the wiper up the resistor ladder
until the last tap point is reached at 9889 Ω. The wiper does not
directly connect to the B Terminal. See Figure 34 for a simpli-
fied diagram of the equivalent RDAC circuit.
The general transfer equation that determines the digitally pro-
grammed output resistance between Wx and Bx is:
R
WB
(Dx) = (Dx)/64 × R
BA
+ R
W
(1)
where Dx is the data contained in the 6-bit RDACx latch and
R
BA
is the nominal end-to-end resistance.
For example, when V
B
= 0 V and A–terminal is open circuit the
following output resistance values will be set for the following
RDAC latch codes (applies to the 10K potentiometer):
D (DEC) R
WB
(⍀) Output State
63 9889 Full-Scale
32 5045 Midscale (RS = 0 Condition)
1 201 1 LSB
0 45 Zero-Scale (Wiper Contact Resistance)
Note that in the zero-scale condition a finite wiper resistance of
45 Ω is present. Care should be taken to limit the current flow
between W and B in this state to a maximum value of 5 mA to
avoid degradation or possible destruction of the internal switch
contact.
Like the mechanical potentiometer the RDAC replaces, it is
totally symmetrical. The resistance between the wiper W and
terminal A also produces a digitally controlled resistance R
WA
.
When these terminals are used the B–terminal should be tied to
the wiper. Setting the resistance value for R
WA
starts at a maxi-
mum value of resistance and decreases as the data loaded in the
latch is increased in value. The general transfer equation for this
operation is:
R
WA
(Dx) = (64-Dx)/64 × R
BA
+ R
W
(2)
where Dx is the data contained in the 6-bit RDACx latch and
R
BA
is the nominal end-to-end resistance. For example, when
V
A
= 0 V and B–terminal is tied to the wiper W, the following
output resistance values will be set for the following RDAC
latch codes:
D (DEC) R
WA
(⍀) Output State
63 201 Full-Scale
32 5045 Midscale (RS = 0 Condition)
1 9889 1 LSB
0 10045 Zero-Scale
The typical distribution of R
BA
from channel to channel matches
within ±1%. However, device-to-device matching is process-lot-
dependent, having a ±30% variation. The change in R
BA
with
temperature has a 700 ppm/°C temperature coefficient.
