AD7856
–13–REV. A
CALIBRATION REGISTERS
The AD7856 has ten calibration registers in all, eight for the
DAC, one for the offset and one for gain. Data can be written
to or read from all ten calibration registers. In self- and system
calibration the part automatically modifies the calibration regis-
ters; only if the user needs to modify the calibration registers
should an attempt be made to read from and write to the cali-
bration registers.
Addressing the Calibration Registers
The calibration selection bits in the control register CALSLT1
and CALSLT0 determine which of the calibration registers are
addressed (see Table V). The addressing applies to both the
read and write operations for the calibration registers. The user
should not attempt to read from and write to the calibration
registers at the same time.
Table V. Calibration Register Addressing
CALSLT1 CALSLT0 Comment
0 0 This combination addresses the
Gain (1), Offset (1) and DAC Reg-
isters (8). Ten registers in total.
0 1 This combination addresses the
Gain (1) and Offset (1) Registers.
Two registers in total.
1 0 This combination addresses the
Offset Register. One register in
total.
1 1 This combination addresses the
Gain Register. One register in total.
Writing to/Reading from the Calibration Registers
For writing to the calibration registers a write to the control
register is required to set the CALSLT0 and CALSLT1 bits.
For reading from the calibration registers a write to the control
register is required to set the CALSLT0 and CALSLT1 bits,
but also to set the RDSLT1 and RDSLT0 bits to 10 (this ad-
dresses the calibration registers for reading). The calibration
register pointer is reset upon writing to the control register
setting the CALSLT1 and CALSLT0 bits, or upon completion
of all the calibration register write/read operations. When reset,
it points to the first calibration register in the selected write/
read sequence. The calibration register pointer will point to the
gain calibration register upon reset in all but one case, this case
being where the offset calibration register is selected on its own
(CALSLT1 = 1, CALSLT0 = 0). Where more than one cali-
bration register is being accessed the calibration register pointer
will be automatically incremented after each calibration register
write/read operation. The order in which the ten calibration
registers are arranged is shown in Figure 7. The user may abort
at any time before all the calibration register write/read opera-
tions are completed, and the next control register write opera-
tion will reset the calibration register pointer. The flowchart in
Figure 8 shows the sequence for writing to the calibration regis-
ters and Figure 9 for reading.
CAL REGISTER
ADDRESS POINTER
GAIN REGISTER
OFFSET REGISTER
DAC 1ST MSB REGISTER
DAC 8TH MSB REGISTER
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(1)
(2)
(3)
(10)
CALIBRATION REGISTERS
CALIBRATION REGISTER
ADDRESS POINTER
POSITION IS DETERMINED
BY THE NUMBER OF
CALIBRATION REGISTERS
ADDRESSED AND THE
NUMBER OF READ/WRITE
OPERATIONS
Figure 7. Calibration Register Arrangements
When reading from the calibration registers there will always be
two leading zeros for each of the registers. When operating in
Serial Interface Mode 1 the read operations to the calibration
registers cannot be aborted. The full number of read operations
must be completed (see section on Serial Interface Mode 1
Timing for more detail).
START
WRITE TO CONTROL REGISTER SETTING STCAL = 0
AND CALSLT1, CALSLT0 = 00, 01, 10, 11
CAL REGISTER POINTER IS
AUTOMATICALLY RESET
WRITE TO CAL REGISTER
(ADDR1 = 1, ADDR0 = 0)
CAL REGISTER POINTER IS
AUTOMATICALLY INCREMENTED
LAST
REGISTER
WRITE
OPERATION
OR
ABORT
?
FINISHED
YES
NO
Figure 8. Flowchart for Writing to the Calibration
Registers
