Datasheet

AD5263
Rev. A | Page 18 of 28
The typical distribution of the end-to-end resistance R
AB
from
channel to channel matches within ±1%. Device-to-device
matching is process-lot dependent, and it is possible to have
±30% variation. Because the resistance element is processed in
thin film technology, the change in R
AB
with temperature has a
very low temperature coefficient of 30 ppm/°C.
PROGRAMMING THE POTENTIOMETER DIVIDER
VOLTAGE OUTPUT OPERATION
The digital potentiometer easily generates a voltage divider at
wiper-to-B and wiper-to-A proportional to the input voltage
from Terminal A and Terminal B. Unlike the polarity from
V
DD
to V
SS
, which must be positive, the voltage across A to B,
W to A, and W to B can be at either polarity, if V
SS
is powered
by a negative supply.
If the effect of the wiper resistance for approximation is
ignored, connecting the A terminal to 5 V and the B terminal to
ground produces an output voltage from the wiper to B, starting
at 0 V up to 1 LSB below 5 V. Each LSB step of voltage is equal
to the voltage applied across Terminal A to Terminal B divided
by the 256 positions of the potentiometer divider. Because the
AD5263 can be powered by dual supplies, the general equation
defining the output voltage V
W
with respect to ground for any
valid input voltages applied to Terminal A and Terminal B is
B
A
W
V
D
V
D
DV
256
256
256
)(
−
+=
(3)
For a more accurate calculation, which includes the effect of
wiper resistance, V
W
can be found as
B
WA
A
WB
W
V
DR
V
DR
DV
256
)(
256
)(
)( +=
(4)
Operation of the digital potentiometer in the divider mode
results in a more accurate operation over temperature. Unlike
the rheostat mode, the output voltage is dependent mainly on
the ratio of the internal resistances R
WA
and R
WB
, and not their
absolute values; therefore, the temperature drift reduces to
5 ppm/°C.
PIN-SELECTABLE DIGITAL INTERFACE
The AD5263 provides the flexibility of a selectable interface.
When the digital interface select (DIS) pin is tied low, the SPI
mode is engaged. When the DIS pin is tied high to the V
L
supply, the I
2
C mode is engaged.
SPI-COMPATIBLE 3-WIRE SERIAL BUS (DIS = 0)
The AD5263 contains a 3-wire SPI-compatible digital interface
(SDI,
CS
, and CLK). The 10-bit serial word must be loaded with
address bits A1 and A0, followed by the data byte, MSB first.
The format of the word is shown in the
Serial Data-Word
Forma
t section and bit map.
The positive-edge sensitive CLK input requires clean transitions
to avoid clocking incorrect data into the serial input register.
Standard logic families work well. If mechanical switches are
used for product evaluation, they should be debounced by a
flip-flop or other suitable means. When
CS
is low, the clock
loads data into the serial register on each positive clock edge
(see
Figure 40).
Table 7. AD5263 Address Decode Table
A1 A0 Latch Loaded
0 0 RDAC 1
0 1 RDAC 2
1 0 RDAC 3
1 1 RDAC 4
The data setup and data hold times in the specification table
determine the valid timing requirements. The AD5263 uses a
10-bit serial input data register word that is transferred to the
internal RDAC register when the
CS
line returns to logic high.
Note that only the last 10 bits that are clocked into the register
are latched into the decoder. As
CS
goes high, it activates the
address decoder and updates the corresponding channel
according to
Table 7.
During shutdown (
SHDN
), the serial data output (SDO) pin is
forced to logic high in order to avoid power dissipation in the
external pull-up resistor. For an equivalent SDO output circuit
schematic, see
Figure 46.
03142-045
SERIAL
REGISTER
CS
SDI
CLK
S
HDN
RES
RS
CK
SDO
D
Q
Figure 46. Detailed SDO Output Schematic of the AD5263
During reset (
RES
), the wiper is set to midscale. Note that
unlike
SHDN
, when the part is taken out of reset, the wiper
remains at midscale and does not revert to its pre-reset setting.
Daisy-Chain Operation
The serial data output (SDO) pin contains an open-drain
N-channel FET. This output requires a pull-up resistor in order
to transfer data to the SDI pin of the next package. This allows
for daisy-chaining several RDACs from a single processor serial
data line. The pull-up resistor termination voltage can be
greater than the V
DD
supply voltage. It is recommended to
increase the clock period when using a pull-up resistor to the
SDI pin of the following device because capacitive loading at the
daisy-chain node (SDO to SDI) between devices may induce
time delay to subsequent devices. Users should be aware of this
potential problem to achieve data transfer successfully (see
Figure 47). If two AD5263s are daisy-chained, a total of 20 bits