Rdac Structure - Analog Devices AD5235 Manual

Using CS to Re-Execute a Previous Command
Another subtle feature of the AD5235 is that a subsequent CS
strobe, without clock and data, repeats a previous command.
Using Additional Internal Nonvolatile EEMEM
The AD5235 contains additional user EEMEM registers for
storing any 16-bit data such as memory data for other compo-
nents, look-up tables, or system identification information.
Table 9 provides an address map of the internal storage registers
shown in the functional block diagram as EEMEM1, EEMEM2,
and 26 bytes (13 addresses × 2 bytes each) of USER EEMEM.
Table 9. EEMEM Address Map
EEMEM No. Address
1 0000
2 0001
3 0010
4 0011
... ...
15 1110
16 1111
1 RDAC data stored in EEMEM locations is transferred to the corresponding
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
USERx are internal nonvolatile EEMEM registers available to store and
retrieve constants and other 16-bit information using Instruction 3 and
Instruction 9, respectively.
4
Read only.
Calculating Actual End-to-End Terminal Resistance
The resistance tolerance is stored in the EEMEM register during
factory testing. The actual end-to-end resistance can, therefore,
be calculated, which is valuable for calibration, tolerance
matching, and precision applications. Note that this value is
matches with R
read only and the R
AB2
The resistance tolerance in percentage is contained in the last
16 bits of data in EEMEM Register 15. The format is the sign
magnitude binary format with the MSB designate for sign (0 =
negative and 1 = positive), the next 7 MSB designate the integer
number, and the 8 LSB designate the decimal number (see
Table 11).
For example, if R = 250 kΩ and the data in the SDO
AB_RATED
shows XXXX XXXX 1001 1100 0000 1111, R
Table 11. Calculating End-to-End Terminal Resistance
Bit D15 D14 D13
Sign
6 5
Mag Sign 2 2
EEMEM Content for ...
1, 2
RDAC1
RDAC2
3
USER1
USER2
...
USER13
4
R Tolerance
AB1
, typically 0.1%.
AB1
can be
AB_ACTUAL
D12 D11 D10 D9
4 3 2 1
2 2 2 2
7 Bits for Integer Number
calculated as follows:
MSB: 1 = Positive
Next 7 LSB: 001 1100 = 28
8 LSB: 0000 1111 = 15 × 2
% Tolerance = 28.06%
Therefore, R

RDAC STRUCTURE

The patent-pending RDAC contains multiple strings of equal
resistor segments with an array of analog switches that acts as
the wiper connection. The number of positions is the resolution
of the device. The AD5235 has 1024 connection points, allowing
it to provide better than 0.1% settability resolution. Figure 43
shows an equivalent structure of the connections among the
three terminals of the RDAC. The SW
while the switches SW(0) to SW(2
depending on the resistance position decoded from the data
bits. Because the switch is not ideal, there is a 50 Ω wiper
resistance, R . Wiper resistance is a function of supply voltage
W
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 accurate
prediction of the output resistance is needed.
Table 10. Nominal Individual Segment Resistor Values
Device Resolution
1024-Step
D8 D7 D6
.
0 −1 −2
2 2 2
Decimal
Point
Rev. B | Page 19 of 28
−8
= 0.06
= 320.15 kΩ
AB_ACTUAL
and SW
A
N
−1) are on one at a time,
SW
A
N –
SW(2 1)
W
R
S
RDAC
N –
SW(2 2)
WIPER
REGISTER
AND
DECODER
SW
R
(1)
S
R
S SW
(0)
N
R = R /2
S AB
DIGITAL
SW
B
CIRCUITRY
OMITTED FOR
CLARITY
Figure 43. Equivalent RDAC Structure
25 kΩ
24.4
D5 D4 D3
−3 −4 −5
2 2 2
8 Bits for Decimal Number
AD5235
are always on,
B
A
B
250 kΩ
244
D2 D1 D0
−6 −7 −8
2 2 2