Datasheet

AD5258
Rev. 0 | Page 18 of 24
TOLERANCE READBACK MODES
Table 15. Traditional Readback of Tolerance (Individually)
S
7-Bit Device Address
(See Table 5)
0 SA 0 0 1 1 1 1 1 0 SA S
7-Bit Device Address
(See Table 5)
1SAD7D6D5D4D3D2D1D0NA P
Slave Address Byte Instruction Byte Slave Address Byte Sign + Integer Byte
↑
Repeat start
S
7-Bit Device Address
(See Table 5)
0 SA 0 0 1 1 1 1 1 1 SA S
7-Bit Device Address
(See Table 5)
1SAD7D6D5D4D3D2D1D0NA P
Slave Address Byte Instruction Byte Slave Address Byte Decimal Byte
↑
Repeat start
Table 16.Traditional Readback of Tolerance (Consecutively)
S
7-Bit Device Address
(See Table 5) 0 SA 0 0 1 1 1 1 1 0 SA S
7-Bit Device Address
(See Table 5) 1 SA D7 D6 D5 D4 D3 D2 D1 D0 MA D7 D6 D5 D4 D3 D2 D1 D0 NA P
Slave Address Byte Instruction Byte Slave Address Byte Sign + Integer Byte Decimal Byte
↑
Repeat start
Calculating R
AB
Tolerance Stored in Read-Only Memory
05029-005
A
AA
D7 D6 D5 D4 D3 D2 D1 D0
SIGN
SIGN
7 BITS FOR INTEGER NUMBER
2
6
2
5
2
4
2
3
2
2
2
1
2
0
D7 D6 D5 D4 D3 D2 D1 D0
8 BITS FOR DECIMAL NUMBER
2
–8
2
–1
2
–2
2
–3
2
–4
2
–5
2
–6
2
–7
Figure 40. Format of Stored Tolerance in Sign Magnitude Format with Bit Position Descriptions.
(Unit is Percent. Only Data Bytes are Shown.)
The AD5258 features a patented R
AB
tolerance storage in the
nonvolatile memory. The tolerance is stored in the memory
during factory production and can be read by users at any time.
The knowledge of stored tolerance allows users to accurately
calculate R
AB
. This feature is valuable for precision, rheostat
mode, and open-loop applications where knowledge of absolute
resistance is critical.
The stored tolerance resides in the read-only memory and is
expressed as a percentage. The tolerance is stored in two memory
location bytes in sign magnitude binary form (see Figure 40).
The two EEPROM address bytes are 11110 (sign + integer) and
11111 (decimal number). The two bytes can be individually
accessed with two separate commands (see Table 15). Alternatively,
readback of the first byte followed by the second byte can be
done in one command (see Table 16). In the latter case, the
memory pointer will automatically increment from the first to
the second EEPROM location (increments from 11110 to
11111) if read consecutively.
In the first memory location, the MSB is designated for the sign
(0 = + and 1= –) and the seven LSBs are designated for the integer
portion of the tolerance. In the second memory location, all eight
data bits are designated for the decimal portion of tolerance. Note
that the decimal portion has a limited accuracy of only 0.1%. For
example, if the rated R
AB
= 10 kΩ and the data readback from
Address 11110 shows 0001 1100, and Address 11111 shows
0000 1111, then the tolerance can be calculated as
MSB: 0 = +
Next 7 MSB: 001 1100 = 28
8 LSB: 0000 1111 = 15 × 2
–8
= 0.06
Tolerance = +28.06%
Rounded Tolerance = +28.1% and therefore,
R
AB_ACTUAL
= 12.810 kΩ