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 components,
look-up tables, or system identification information. Table 9 pro-
vides an address map of the internal storage registers shown in the
functional block diagram (see Figure 1) as EEMEM1, EEMEM2,
and 26 bytes (13 addresses × 2 bytes each) of User EEMEM.
Table 9. EEMEM Address Map
EEMEM Content for …
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
2 USERx are internal nonvolatile EEMEM registers available to store and
retrieve constants and other 16-bit information using Instruction 3 and
Instruction 9, respectively.
3 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 read only and
the RAB2 matches with RAB1, typically 0.1%.
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 RAB_RATED = 250 kΩ and the data in the SDO
shows XXXX XXXX 1001 1100 0000 1111, RAB_ACTUAL can be
calculated as follows:
MSB: 1 = positive
Next 7 LSB: 001 1100 = 28
8 LSB: 0000 1111 = 15 × 2−8 = 0.06
% tolerance = 28.06%
Therefore, RAB_ACTUAL = 320.15 kΩ
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% setability resolution. Figure 45 shows
an equivalent structure of the connections among the three
terminals of the RDAC. The SWA and SWB are always on, while
the switches, SW(0) to SW(2N − 1), 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 50 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 accurate prediction of the
output resistance is needed.
RS = RAB/2N
Figure 45. Equivalent RDAC Structure
Table 10. Nominal Individual Segment Resistor Values
Table 11. Calculating End-to-End Terminal Resistance
Bit D15 D14 D13 D12 D11 D10 D9 D8
Mag Sign 26
7 Bits for Integer Number
D7 D6 D5 D4 D3 D2 D1 D0
8 Bits for Decimal Number
Rev. E | Page 21 of 32