Datasheet
AD8400/AD8402/AD8403
Rev. E | Page 21 of 32
Like a mechanical potentiometer, RDAC is symmetrical. The
resistance between the Wiper W and Terminal A also produces
a digitally controlled complementary resistance, R
WA
. When
these terminals are used, the B terminal can be tied to the wiper
or left floating. R
WA
starts at the maximum and decreases as the
data loaded into the RDAC latch increases. The general transfer
equation for this R
WA
is
()
WABWA
RR
D
DR +×
−
=
256
256
(3)
where D is the data loaded into the 8-bit RDAC# latch, and R
AB
is the nominal end-to-end resistance.
For example, when the B terminal is either open-circuited or
tied to the Wiper W, the following RDAC latch codes result in
the following R
WA
(for the 10 kΩ version):
Table 11.
D (Dec)
R
WA
(Ω)
Output State
255 89 Full-Scale
128 5,050
Midscale (
RS
= 0 Condition)
1 10,011 1 LSB
0 10,050 Zero-Scale
The typical distribution of RAB from channel to channel
matches within ±1%. However, device-to-device matching
is process lot dependent and has a ±20% variation. The tem-
perature coefficient, or the change in R
AB
with temperature,
is 500 ppm/°C.
The wiper-to-end-terminal resistance temperature coefficient
has the best performance over the 10% to 100% of adjustment
range where the internal wiper contact switches do not con-
tribute any significant temperature related errors. The graph in
Figure 18 shows the performance of R
WB
tempco vs. code. Using
the potentiometer with codes below 32 results in the larger
temperature coefficients plotted.
PROGRAMMING THE POTENTIOMETER DIVIDER
Voltage Output Operation
The digital potentiometer easily generates an output voltage
proportional to the input voltage applied to a given terminal.
For example, connecting the A terminal to 5 V and the B termi-
nal to ground produces an output voltage at the wiper starting
at 0 V up to 1 LSB less than 5 V. Each LSB is equal to the voltage
applied across the A to B terminals divided by the 256-position
resolution of the potentiometer divider. The general equation
defining the output voltage with respect to ground for any given
input voltage applied to the A to B terminals is
BABW
VV
D
V +×=
256
(4)
Operation of the digital potentiometer in the voltage divider
mode results in more accurate operation over temperature.
Here the output voltage is dependent on the ratio of the internal
resistors, not the absolute value; therefore, the temperature drift
improves to 15 ppm/°C.
At the lower wiper position settings, the potentiometer divider
temperature coefficient increases because the contribution of
the CMOS switch wiper resistance becomes an appreciable
portion of the total resistance from the B terminal to the
Wiper W. See Figure 17 for a plot of potentiometer tempco
performance vs. code setting.
DIGITAL INTERFACING
The AD8400/AD8402/AD8403 contain a standard SPI-
compatible, 3-wire, serial input control interface. The three
inputs are clock (CLK), chip select (
CS
), and serial data input
(SDI). The positive-edge sensitive CLK input requires clean
transitions to avoid clocking incorrect data into the serial input
register. For the best result, use logic transitions faster than
1 V/μs. 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. The block diagrams in
, , and show the internal digital
circuitry in more detail. When
Figure 46 Figure 47 Figure 48
CS
is taken active low, the clock
loads data into the 10-bit serial register on each positive clock
edge (see ). Table 12
RDAC
LATCH
NO. 1
GND
A1
W1
B1
V
DD
AD8400
CS
CLK
8
D7
D0
EN
ADDR
DEC
A1
A0
SDI
DI D0
D7
10-BIT
SER
REG
0
1092-045
Figure 46. AD8400 Block Diagram
RDAC
LATCH
NO. 1
R
AGND
RS
A1
W1
B1
V
DD
AD8402
CS
CLK
D7
D0
RDAC
LATCH
NO. 2
R
A4
W4
B4
D7
D0
EN
ADDR
DEC
A1
A0
SDI DI
10-BIT
SER
REG
D0
S
HDN
DGND
D7
8
01092-046
Figure 47. AD8402 Block Diagram