AD7705/AD7706
Rev. C | Page 20 of 44
DATA REGISTER (RS2, RS1, RS0 = 0, 1, 1)
The data register is a 16-bit, read-only register that contains the
most up-to-date conversion result from the AD7705/AD7706. If
the communication register sets up the part for a write
operation to this register, a write operation must take place to
return the part to its default state. However, the 16 bits of data
written to the part will be ignored by the AD7705/AD7706.
TEST REGISTER (RS2, RS1, RS0 = 1, 0, 0);
POWER-ON/RESET STATUS: 00 HEXADECIMAL
The part contains a test register that is used when testing the
device. The user is advised not to change the status of any of the
bits in this register from the default (power-on or reset) status
of all 0s, because the part will be placed in one of its test modes
and will not operate correctly.
ZERO-SCALE CALIBRATION REGISTER
(RS2, RS1, RS0 = 1, 1, 0);
POWER-ON/RESET STATUS: 1F4000 HEXADECIMAL
The AD7705/AD7706 contain independent sets of zero-scale
registers, one for each of the input channels. Each register is a
24-bit read/write register; therefore, 24 bits of data must be
written, or no data is transferred to the register. This register is
used in conjunction with its associated full-scale register to
form a register pair. These register pairs are associated with
input channel pairs, as outlined in
Table 12 and Table 13.
While the part is set up to allow access to these registers over
the digital interface, the parts themselves can no longer access
the register coefficients to scale the output data correctly. As a
result, the first output data read from the part after accessing
the calibration registers (for either a read or write operation)
might contain incorrect data. In addition, a write to the
calibration register should not be attempted while a calibration
is in progress. These eventualities can be avoided by taking the
FSYNC bit in the mode register high before the calibration
register operation, and taking it low after the operation is
complete.
FULL-SCALE CALIBRATION REGISTER
(RS2, RS1, RS0 = 1, 1, 1);
POWER-ON/RESET STATUS: 5761AB HEXADECIMAL
The AD7705/AD7706 contain independent sets of full-scale
registers, one for each of the input channels. Each register is a
24-bit read/write register; therefore, 24 bits of data must be
written, or no data is transferred to the register. This register is
used in conjunction with its associated zero-scale register to
form a register pair. These register pairs are associated with
input channel pairs, as outlined in
Table 12 and Table 13.
While the part is set up to allow access to these registers over
the digital interface, the part itself can no longer access the
register coefficients to scale the output data correctly. As a
result, the first output data read from the part after accessing
the calibration registers (for either a read or write operation)
might contain incorrect data. In addition, a write to the
calibration register should not be attempted while a calibration
is in progress. These eventualities can be avoided by taking
FSYNC bit in the mode register high before the calibration
register operation, and taking it low after the operation is
complete.
Calibration Sequences
The AD7705/AD7706 contain a number of calibration options,
as previously outlined.
Table 21 summarizes the calibration
types, the operations involved, and the duration of the
operations. There are two methods for determining the end of a
calibration. The first is to monitor when
DRDY
returns low at
the end of the sequence. This technique not only indicates when
the sequence is complete, but also when the part has a valid new
sample in its data register. This valid new sample is the result of
a normal conversion that follows the calibration sequence. The
second method for determining when calibration is complete is
to monitor the MD1 and MD0 bits of the setup register. When
these bits return to 0 following a calibration command, the
calibration sequence is complete. This technique can indicate
the completion of a calibration earlier than the first method
can, but it cannot indicate when there is a valid new result in
the data register. The time that it takes the mode bits, MD1 and
MD0, to return to 0 represents the duration of the calibration.
The sequence when
DRDY
goes low includes a normal
conversion and a pipeline delay, t
P
, to scale the results of this
first conversion correctly. Note that t
P
never exceeds 2000 ×
t
CLKIN
. The time for both methods is shown in Table 21.
Table 21. Calibration Sequences
Calibration Type MD1, MD0 Calibration Sequence Duration of Mode Bits Duration of
DRDY
Self-Calibration 0, 1
Internal ZS calibration @ selected gain
+ internal FS calibration @ selected
gain
6 × 1/output rate 9 × 1/output rate + t
P
ZS System Calibration 1, 0 ZS calibration on AIN @ selected gain 3 × 1/output rate 4 × 1/output rate + t
P
FS System Calibration 1, 1 FS calibration on AIN @ selected gain 3 × 1/output rate 4 × 1/output rate + t
P
