AD7715
“full-scale” points. These points are derived by performing a
conversion on the different input voltages provided to the input
of the modulator during calibration. As a result, the accuracy of
the calibration can only be as good as the noise level that it
provides in normal mode. The result of the “zero-scale” calibra-
tion conversion is stored in the Zero-Scale Calibration Register
while the result of the “full-scale” calibration conversion is
stored in the Full-Scale Calibration Register. With these read-
ings, the on-chip microcontroller can calculate the offset and the
gain slope for the input to output transfer function of the con-
verter. Internally, the part works with a resolution of 33 bits to
determine its conversion result of 16 bits.
step is complete. Once the system zero scale voltage has been set
up, a ZS System Calibration is then initiated by writing the ap-
propriate values (1, 0) to the MD1 and MD0 bits of the Setup
Register. The zero-scale system calibration is performed at the
selected gain. The duration of the calibration is 3 × 1/Output
Rate. At this time the MD1 and MD0 bits in the Setup Register
return to 0, 0. This gives the earliest indication that the calibration
sequence is complete. The DRDY line goes high when calibration
is initiated and does not return low until there is a valid new
word in the data register. The duration time from the calibra-
tion command being issued to DRDY going low is 4 × 1/Output
Rate as the part performs a normal conversion on the AIN volt-
age before DRDY goes low. If DRDY is low before (or goes low
during) the calibration command write to the Setup Register, it
may take up to one modulator cycle (MCLK IN/128) before
DRDY goes high to indicate that calibration is in progress.
Therefore, DRDY should be ignored for up to one modulator
cycle after the last bit is written to the Setup Register in the
calibration command.
Self-Calibration
A self-calibration is initiated on the AD7715 by writing the
appropriate values (0, 1) to the MD1 and MD0 bits of the
Setup Register. In the self-calibration mode with a unipolar
input range, the zero-scale point used in determining the cali-
bration coefficients is with the inputs of the differential pair
internally shorted on the part (i.e., AIN(+) = AIN(–) = Internal
Bias Voltage). The PGA is set for the selected gain (as per G1
and G0 bits in the Communications Register) for this zero-scale
calibration conversion. The full-scale calibration conversion is
performed at the selected gain on an internally generated voltage
of VREF/Selected Gain.
After the zero-scale point is calibrated, the full-scale point is
applied to AIN and the second step of the calibration process is
initiated by again writing the appropriate values (1, 1) to MD1
and MD0. Again the full-scale voltage must be set up before
the calibration is initiated and it must remain stable throughout
the calibration step. The full-scale system calibration is per-
formed at the selected gain. The duration of the calibration is
3 × 1/Output Rate. At this time the MD1 and MD0 bits in the
Setup Register return to 0, 0. This gives the earliest indication
that the calibration sequence is complete. The DRDY line goes
high when calibration is initiated and does not return low until
there is a valid new word in the data register. The duration time
from the calibration command being issued to DRDY going low
is 4 × 1/Output Rate as the part performs a normal conversion
on the AIN voltage before DRDY goes low. If DRDY is low
before (or goes low during) the calibration command, write to
the Setup Register, it may take up to one modulator cycle
(MCLK IN/128) before DRDY goes high to indicate that cali-
bration is in progress. Therefore, DRDY should be ignored for
up to one modulator cycle after the last bit is written to the
Setup Register in the calibration command.
The duration time for the calibration is 6 × 1/Output Rate. This
is made up of 3 × 1/Output Rate for the zero-scale calibration
and 3 × 1/Output Rate for the full-scale calibration. At this time
the MD1 and MD0 bits in the Setup Register return to 0, 0.
This gives the earliest indication that the calibration sequence is
complete. The DRDY line goes high when calibration is initi-
ated and does not return low until there is a valid new word in
the data register. The duration time from the calibration com-
mand being issued to DRDY going low is 9 × 1/Output Rate.
This is made up of 3 × 1/Output Rate for the zero-scale calibra-
tion, 3 × 1/Output Rate for the full-scale calibration, 3 × 1/
Output Rate for a conversion on the analog input and some
overhead to set up the coefficients correctly. If DRDY is low
before (or goes low during) the calibration command write to
the Setup Register, it may take up to one modulator cycle
(MCLK IN/128) before DRDY goes high to indicate that cali-
bration is in progress. Therefore, DRDY should be ignored for
up to one modulator cycle after the last bit is written to the
Setup Register in the calibration command.
In the unipolar mode, the system calibration is performed be-
tween the two endpoints of the transfer function; in the bipolar
mode, it is performed between midscale (zero differential volt-
age) and positive full scale.
For bipolar input ranges in the self-calibrating mode, the se-
quence is very similar to that just outlined. In this case, the two
points are exactly the same as above, but since the part is config-
ured for bipolar operation, the shorted inputs point is actually
midscale of the transfer function.
The fact that the system calibration is a two-step calibration
offers another feature. After the sequence of a full system cali-
bration has been completed, additional offset or gain calibra-
tions can be performed by themselves to adjust the system zero
reference point or the system gain. Calibrating one of the pa-
rameters, either system offset or system gain, will not affect the
other parameter.
System Calibration
System calibration allows the AD7715 to compensate for system
gain and offset errors as well as its own internal errors. System
calibration performs the same slope factor calculations as self-
calibration but uses voltage values presented by the system to
the AIN inputs for the zero- and full-scale points. Full System
calibration requires a two step process, a ZS System Calibration
followed by a FS System Calibration.
System calibration can also be used to remove any errors from
source impedances on the analog input when the part is used in
unbuffered mode. A simple R, C antialiasing filter on the front
end may introduce a gain error on the analog input voltage but
the system calibration can be used to remove this error.
For a full system calibration, the zero-scale point must be pre-
sented to the converter first. It must be applied to the converter
before the calibration step is initiated and remain stable until the
Span and Offset Limits
Whenever a system calibration mode is used, there are limits on
the amount of offset and span which can be accommodated.
The overriding requirement in determining the amount of offset
REV. C
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ADI [ ADI ]
