–13–

REV. B

AD7851

CALIBRATION REGISTERS

The AD7851 has 10 calibration registers in all, 8 for the DAC, 1 for the offset, and 1 for gain. Data can be written to or read from all

10 calibration registers. In self- and system calibration, the part automatically modifies the calibration registers; only if the user needs

to modify the calibration registers should an attempt be made to read from and write to the calibration 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 IV). 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 IV. Calibration Register Addressing

CALSLT1

CALSLT0

This Combination Addresses the

0

0

Gain (1), Offset (1) and DAC Registers (8). Ten registers in total.

0

1

Gain (1) and Offset (1) Registers. Two registers in total.

1

0

Offset Register. One register in total.

1

1

Gain Register. One register in total.

Writing to/Reading from the Calibration Registers

For writing to the calibration registers, a write to the control reg-

ister is required to set the CALSLT0 and CALSLT1 bits. For

reading from the calibration registers, a write to the control regis-

ter is required to set the CALSLT0 and CALSLT1 bits, but also

to set the RDSLT1 and RDSLT0 bits to 10 (this addresses the

calibration registers for reading). The calibration register pointer

is reset on 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 cali-

bration 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 calibration register is being

accessed, the calibration register pointer will be automatically

incremented after each calibration register write/read operation.

The order in which the 10 calibration registers are arranged is

shown in Figure 7. The user may abort at any time before all

the calibration register write/read operations are completed,

and the next control register write operation will reset the cali-

bration register pointer. The flowchart in Figure 8 shows the

sequence for writing to the calibration registers and Figure 9

shows the sequence for reading.

CAL REGISTER

ADDRESS POINTER

CALIBRATION REGISTERS

GAIN REGISTER

OFFSET REGISTER

DAC 1st MSB REGISTER

DAC 8th MSB REGISTER

(1)

(2)

(3)

(10)

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 Arrangement

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 the Serial Interface Summary section

for more detail).

START

WRITE TO CAL REGISTER

(ADDR1 = 1, ADDR0 = 0)

FINISHED

LAST

REGISTER

WRITE

OPERATION

OR

ABORT

?

YES

NO

CAL REGISTER POINTER IS

AUTOMATICALLY RESET

WRITE TO CONTROL REGISTER SETTING STCAL = 0

AND CALSLT1, CALSLT0 = 00, 01, 10, 11

CAL REGISTER POINTER IS

AUTOMATICALLY INCREMENTED

Figure 8. Flowchart for Writing to the Calibration Registers