AD7714YN datasheet(25/40 Pages) AD

25 / 40 page

AD7714

REV. C

–25–

System-Offset Calibration

System-offset calibration is a variation of both the system cali-

bration and self-calibration. In this case, the zero-scale point is

determined in exactly the same way as a ZS System Calibration.

The system zero-scale point is presented to the AIN inputs of

the converter. This 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, a

System-Offset Calibration is then initiated by writing the appro-

priate values (1, 0, 0) to the MD2, MD1 and MD0 bits of the

Mode Register. The zero-scale system calibration is performed

at the selected gain.

The full-scale calibration is performed in exactly the same way

as an FS Self Calibration. The full-scale calibration conversion

is performed at the selected gain on an internally generated

voltage of VREF/Selected Gain. This is a one step calibration

sequence and the time for calibration is 6

× 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 cali-

bration 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 9

× 1/

Output Rate. This is made up of 3

× 1/Output Rate for the zero-

scale system calibration, 3

× 1/Output Rate for the full-scale

self-calibration and 3

× 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 Regis-

ter, 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 Register.

In the unipolar mode, the system-offset calibration is performed

between the two endpoints of the transfer function; in the bipolar

mode, it is performed between midscale and positive full scale.

Background Calibration

The AD7714 also offers a background calibration mode where

the part interleaves its calibration procedure with its normal

conversion sequence. In the background calibration mode, the

part provides continuous zero-scale self-calibrations; it does not

provide any full-scale calibrations. The zero-scale point used in

determining the calibration coefficients in this mode is exactly

the same as for a ZS Self-Calibration. The background calibra-

tion mode is invoked by writing 1, 0, 1 to the MD2, MD1,

MD0 bits of the Mode Register. When invoked, the back-

ground calibration mode performs a zero-scale self calibration

after every output update and this reduces the output data rate

of the AD7714 by a factor of six. Its advantage is that the part

is continually performing offset calibrations and automatically

updating its zero-scale calibration coefficients. As a result, the

effects of temperature drift, supply sensitivity and time drift on

zero-scale errors are automatically removed. When the back-

ground calibration mode is turned on, the part will remain in

this mode until bits MD2, MD1 and MD0 of the Mode Regis-

ter are changed.

Because the background calibration does not perform full-scale

calibrations, a self-calibration should be performed before plac-

ing the part in background calibration mode. Removal of the

offset drift in this mode leaves gain drift as the only source of

error not removed from the part. The typical gain drift of the

AD7714 with temperature is 0.2 ppm/

°C. The SYNC input or

FSYNC bit should not be exercised when the part is in back-

ground calibration mode.

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

and gain which can be accommodated by the part is the require-

ment that the positive full-scale calibration limit is

≤ 1.05 ×

VREF/GAIN. This allows the input range to go 5% above the

nominal range. The built-in headroom in the AD7714’s analog

modulator ensures that the part will still operate correctly with a

positive full-scale voltage which is 5% beyond the nominal.

The range of input span in both the unipolar and bipolar modes

has a minimum value of 0.8

× V

REF/GAIN and a maximum

value of 2.1

× V

REF/GAIN. However, the span (which is the

difference between the bottom of the AD7714’s input range and

the top of its input range) has to take into account the limitation

on the positive full-scale voltage. The amount of offset which

can be accommodated depends on whether the unipolar or

bipolar mode is being used. Once again, the offset has to take

into account the limitation on the positive full-scale voltage. In

unipolar mode, there is considerable flexibility in handling nega-

tive (with respect to AIN(–)) offsets. In both unipolar and bipo-

lar modes, the range of positive offsets which can be handled by

the part depends on the selected span. Therefore, in determin-

ing the limits for system zero-scale and full-scale calibrations,

the user has to ensure that the offset range plus the span range

does exceed 1.05

× V

REF/GAIN. This is best illustrated by

looking at a few examples.

If the part is used in unipolar mode with a required span of

0.8

× V

REF/GAIN, the offset range the system calibration can

handle is from –1.05

× V

REF/GAIN to +0.25

× V

REF/GAIN. If

the part is used in unipolar mode with a required span of VREF/

GAIN, the offset range the system calibration can handle is

from –1.05

× V

REF/GAIN to +0.05

× V

REF/GAIN. Similarly, if

the part is used in unipolar mode and required to remove an

offset of 0.2

× V

REF/GAIN, the span range the system calibra-

tion can handle is 0.85

× V

REF/GAIN.

If the part is used in bipolar mode with a required span of

±0.4 × V

REF/GAIN, then the offset range which the system cali-

bration can handle is from –0.65

× V

REF/GAIN to +0.65

VREF/GAIN. If the part is used in bipolar mode with a required

span of

±V

REF/GAIN, the offset range the system calibration can

handle is from –0.05

× V

REF/GAIN to +0.05

× V

REF/GAIN.

Similarly, if the part is used in bipolar mode and required to

remove an offset of

±0.2 × V

REF/GAIN, the span range the sys-

tem calibration can handle is

±0.85 × V

REF/GAIN.

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