Datasheet

ADN2850 Data Sheet
Rev. E | Page 18 of 28
Using
CS
to Re-Execute a Previous Command
Another subtle feature of the ADN2850 is that a subsequent AA
CS
EE
AA
strobe, without clock and data, repeats a previous command.
Using Additional Internal Nonvolatile EEMEM
The ADN2850 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 10 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 10. EEMEM Address Map
EEMEM No. Address EEMEM Content for …
1
0000 RDAC1
1
2
0001 RDAC2
3 0010 USER1
2
4
0011 USER2
… … …
15
1110 USER13
16 1111 R
WB1
tolerance
3
1
RDAC data stored in EEMEM locations is transferred to the corresponding
RDAC register at power-on, or when Instruction 1, Instruction 8, and
AA
PR
EE
AA are
executed.
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 R
WB2
at full scale matches with R
WB1
at full scale, 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 12).
For example, if R
WB_RATED
= 250 kΩ and the data in the SDO
shows XXXX XXXX 1001 1100 0000 1111, R
WB
at full scale 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, R
WB
at full scale = 320.15 kΩ
RDAC STRUCTURE
The 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 ADN2850 has 1024 connection points, allowing it to provide
better than 0.1% setability resolution. Figure 32 shows an
equivalent structure of the connections among the three
terminals of the RDAC. The SW
B
is always on, while the
switches, SW(0) to SW(2
N
− 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 30 Ω wiper resistance, R
W
. 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.
SW
(1)
SW
(0)
SW
B
B
R
S
R
S
SW(2
N
–
1)
W
SW(2
N
–
2)
RDAC
WIPER
REGISTER
AND
DECODER
R
S
= R
WB_NOMINAL
/2
N
R
S
DIGITAL
CIRCUITRY
OMITTED FOR
CLARITY
02660-044
Figure 32. Equivalent RDAC Structure
Table 11. Nominal Individual Segment Resistor Values
Device Resolution 25 kΩ 250 kΩ
1024-Step
24.4Ω 244Ω
Table 12. Calculating End-to-End Terminal Resistance
Bit D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
Sign
Mag
Sign 2
6
2
5
2
4
2
3
2
2
2
1
2
0
.
2
−1
2
−2
2
−3
2
−4
2
−5
2
−6
2
−7
2
−8
7 Bits for Integer Number
Decimal
Point
8 Bits for Decimal Number