AD8400/AD8402/AD8403
OPERATION
The AD8400/AD8402/AD8403 provide a single, dual and quad
channel, 256 position digitally controlled variable resistor (VR)
device. Changing the programmed VR settings is accomplished
by clocking in a 10-bit serial data word into the SDI (Serial
Data Input) pin. The format of this data word is two address
bits, MSB first, followed by eight data bits, MSB first. Table I
provides the serial register data word format. The AD8400/
AD8402/AD8403 has the following address assignments for the
ADDR decode, which determines the location of VR latch re-
ceiving the serial register data in Bits B7 through B0:
VR#
=
A1
×
2 +
A0
+ 1
Equation 1
The single-channel AD8400 requires A1 = A0 = 0. The dual-
channel AD8402 requires A1 = 0. VR settings can be changed
one at a time in random sequence. The serial clock running at
10 MHz makes it possible to load all 4 VRs in under 4
µs
(10
×
4
×
100 ns) for the AD8403. The exact timing requirements are
shown in Figures 1a, 1b and 1c.
The AD8402/AD8403 resets to midscale by asserting the
RS
pin, simplifying initial conditions at power up. Both parts have a
power shutdown
SHDN
pin that places the VR 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. The digital interface is still ac-
tive in shutdown, except that SDO is deactivated. Code changes
in the registers can be made that will produce new wiper posi-
tions when the device is taken out of shutdown.
R
S
Ax
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the VR (RDAC) between terminals A
and B are available with values of 1 kΩ, 10 kΩ, 50 kΩ and 100 kΩ.
The final digits of the part number determine the nominal resis-
tance value, e.g., 10 kΩ = 10; 100 kΩ = 100. The nominal resis-
tance (R
AB
) of the VR has 256 contact points accessed by the
wiper terminal, plus the B terminal contact. The 8-bit data word
in the RDAC latch is decoded to select one of the 256 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 50
Ω.
The second connection (10 kΩ part) is the first
tap point located at 89
Ω
[= R
BA
(nominal resistance)/256 + R
W
= 39
Ω
+ 50
Ω]
for data 01
H
. The third connection is the next
tap point representing 78 + 50 = 128
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 10011
Ω.
The wiper does not di-
rectly connect to the B terminal. See Figure 37 for a simplified
diagram of the equivalent RDAC circuit.
The AD8400 contains one RDAC, the AD8402 contains two
independent RDACs and the AD8403 contains four independent
RDACs. The general transfer equation that determines the digi-
tally programmed output resistance between Wx and Bx is:
R
WB
(Dx) = (Dx)/256
×
R
BA
+
R
W
Equation 2
where Dx is the data contained in the 8-bit RDAC# 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 10 kΩ potentiometers):
D
(Dec)
255
128
1
0
R
WB
(Ω)
10011
5050
89
50
Output State
Full Scale
Midscale (RS = 0 Condition)
1 LSB
Zero-Scale (Wiper Contact Resistance)
SHDN
D7
D6
D5
D4
D3
D2
D1
D0
R
S
R
S
Wx
RDAC
LATCH
&
DECODER
R
S
R
S
= R
NOMINAL
/256
Bx
Note in the zero-scale condition a finite wiper resistance of 50
Ω
is present. Care should be taken to limit the current flow be-
tween 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 to-
tally symmetrical. The resistance between the wiper W and ter-
minal 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
RDAC latch is increased in value. The general transfer equation
for this operation is:
R
WA
(Dx) = (256–Dx)/256
×
R
BA
+
R
W
Equation 3
Figure 37. AD8402/AD8403 Equivalent VR (RDAC) Circuit
–12–
REV. B
AD [ ANALOG DEVICES ]
