AD7714YN datasheet(24/40 Pages) AD

24 / 40 page

AD7714

REV. C

–24–

The part also offers ZS Self-Calibration and FS Self-Calibration

options. In these cases, the part performs just a zero-scale or

full-scale calibration respectively and not a full calibration of the

part. A full-scale calibration should not be carried out unless

the part contains valid zero-scale coefficients. These calibrations

are initiated on the AD7714 by writing the appropriate values

(1, 1, 0 for ZS Self-Calibration and 1, 1, 1 for FS Self Calibra-

tion) to the MD2, MD1 and MD0 bits of the Mode Register.

The zero-scale or full-scale calibration is exactly the same as

that described for the full self-calibration. In these cases, the

duration of the calibration is 3

× 1/Output Rate. At this time the

MD2, MD1 and MD0 bits in the Mode Register return to

0, 0, 0. This gives the earliest indication that the calibration

sequence is complete. The

DRDY line goes high when calibra-

tion is initiated and does not return low until there is a valid

new word in the data register. The time from the calibration

command being issued to

DRDY going low is 6

× 1/Output

Rate. This is made up of 3

× 1/Output Rate for the zero-scale or

full-scale calibration and 3

× 1/Output Rate for a conversion on

the analog input. If

DRDY is low before (or goes low during)

the calibration command write to the Mode 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 of the calibration command is written to the Mode

The fact that the self-calibration can be performed as a two step

calibration offers another feature. After the sequence of a full

self calibration has been completed, additional offset or gain

calibrations can be performed by themselves to adjust the part’s

zero point or gain. Calibrating one of the parameters, either

offset or gain, will not affect the other parameter.

System Calibration

System calibration allows the AD7714 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.

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

step is complete. Once the system zero scale has been set up at

the analog input, a ZS System Calibration is then initiated by

writing the appropriate values (0, 1, 0) to the MD2, MD1 and

MD0 bits of the Mode Register. The zero-scale system calibra-

tion is performed at the selected gain. The duration of the cali-

bration is 3

× 1/Output Rate. At this time, the MD2, MD1 and

MD0 bits in the Mode Register return to 0, 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 time from the calibration command being issued to

DRDY

going low is 4

× 1/Output Rate. This is made up of 3 × 1/Output

Rate for the zero-scale system calibration and 1/Output Rate for

a conversion on the analog input. This conversion on the analog

input is on the same voltage as the zero-scale system calibration

and, therefore, the resultant word in the data register from this

conversion should be a zero-scale reading. If

DRDY is low

before (or goes low during) the calibration command write to

the Mode 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 of the calibration

command is written to the Mode Register.

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 (0, 1, 1) to

MD2, 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 calibra-

tion is performed at the selected gain. The duration of the cali-

bration is 3

× 1/Output Rate. At this time, the MD2, MD1 and

MD0 bits in the Mode Register return to 0, 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 time from the calibration command being issued to

DRDY

going low is 4

× 1/Output Rate. This is made up of 3 × 1/Out-

put Rate for the full-scale system calibration and 1/Output Rate

for a conversion on the analog input. This conversion on the

analog input is on the same voltage as the full-scale system

calibration and, therefore, the resultant word in the data register

from this conversion should be a full-scale reading. If

DRDY is

low before (or goes low during) the calibration command write

to the Mode 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 of the calibration

command is written to the Mode Register.

In the unipolar mode, the system calibration is performed

between the two endpoints of the transfer function; in the bipo-

lar mode, it is performed between midscale (zero differential

voltage) and positive full scale.

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

parameters, either system offset or system gain, will not affect

the other parameter. A full-scale calibration should not be car-

ried out unless the part contains valid zero-scale coefficients.

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.

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