AD5233
Rev. B | Page 20 of 32
Using Additional Internal Nonvolatile EEMEM
The AD5233 contains additional user EEMEM registers for
storing any 8-bit data.
Table 9 provides an address map of the
internal storage registers shown in the functional block diagram
as EEMEM1, EEMEM2, and 11 bytes of user EEMEM.
Table 9. EEMEM Address Map
EEMEM Number
Address
EEMEM Content
1
0000
2
0001
3
0010
4
0011
5
0100
6
0101
7
0110
USER2
…
15
1110
USER10
16
1111
USER11
1 RDAC data stored in the EEMEM location is transferred to the RDAC register
at power-on, or when Instruction 1, Instruction 8, and PR are executed.
2 Execution of Instruction 1 leaves the device in the read mode power
consumption state. After the last Instruction 1 is executed, the user
should perform a NOP, Instruction 0, to return the device to the low
power idling state.
3 O1 and O2 data stored in EEMEM locations is transferred to the corresponding
digital register at power-on, or when Instruction 1 and Instruction 8 are
executed.
4 USERx are internal nonvolatile EEMEM registers available to store and
retrieve constants and other 8-bit information using Instruction 3 and
Instruction 9, respectively.
RDAC STRUCTURE
The patent-pending RDAC contains multiple strings of equal
resistor segments, with an array of analog switches that act as
the wiper connection. The number of positions is the resolution
of the device. The AD5233 has 64 connection points, allowing it
to provide better than 1.5% set ability resolution.
Figure 43shows an equivalent structure of the connections between the
three terminals of the RDAC. The SWA and SWB are always on,
while the switches, SW(0) to SW(2N1), are on, one at a time,
depending on the resistance position decoded from the data
bits. Because the switch is not ideal, there is a 15 Ω wiper
resistance, RW. Wiper resistance is a function of supply voltage
and temperature. The lower the supply voltage or the higher the
temperature, the higher the resulting wiper resistance. Users
should be aware of the wiper resistance dynamics if an accurate
prediction of the output resistance is needed.
SW(1)
SW(0)
SWB
B
SWA
SW(2N – 1)
SW(2N – 2)
A
W
RDAC
WIPER
REGISTER
AND
DECODER
RS = RAB/2N
RS
DIGITAL
CIRCUITRY
OMITTED FOR
CLARITY
0
27
94
-04
4
Figure 43. Equivalent RDAC Structure
PROGRAMMING THE VARIABLE RESISTOR
Rheostat Operation
The nominal resistance of the RDAC between Terminal A
and Terminal B, RAB, is available with 10 kΩ, 50 kΩ, and 100 kΩ
with 64 positions (6-bit resolution). The final digit(s) of the part
number determine the nominal resistance value, for example,
10 = 10 kΩ; 50 = 50 kΩ; 100 = 100 kΩ.
The 6-bit data-word in the RDAC latch is decoded to select
one of the 64 possible settings. The following discussion
describes the calculation of resistance (RWB) at different codes
of a 10 kΩ part. For VDD = 5 V, the wiper’s first connection
starts at Terminal B for Data 0x00. RWB(0) is 15 Ω because of
the wiper resistance and because it is independent of the nominal
resistance. The second connection is the first tap point, where
RWB(1) becomes 156 Ω + 15 Ω = 171 Ω for Data 0x01. The third
connection is the next tap point, representing RWB(2) = 321 Ω +
15 Ω = 327 Ω for Data 0x02, and so on. Each LSB data value
increase moves the wiper up the resistor ladder until the last
tap point is reached at RWB(63) = 9858 Ω. See Figure 43 for a simplified diagram of the equivalent RDAC circuit. When RWB
is used, Terminal A can be left floating or tied to the wiper.
CODE (Decimal)
100
75
0
06
16
R
WA
(D
),
4
R
WB
(D
)(
%
o
fF
u
ll
-S
ca
le
R
AB
)
32
48
50
25
RWB
RWA
0
27
94
-04
5
Figure 44. RWA(D) and RWB(D) vs. Decimal Code