AD7713 datasheet(17/28 Pages) AD

17 / 28 page

–17–

REV. C

AD7713

The AD7713 also provides the facility to write to the on-chip

calibration registers, and in this manner the span and offset for

the part can be adjusted by the user. The offset calibration regis-

ter contains a value which is subtracted from all conversion

results, while the full-scale calibration register contains a value

which is multiplied by all conversion results. The offset calibra-

tion coefficient is subtracted from the result prior to the multi-

plication by the full-scale coefficient. In the first three modes

outlined here, the DRDY line indicates that calibration is com-

plete by going low. If DRDY is low before (or goes low during)

the calibration command, it may take up to one modulator cycle

before DRDY goes high to indicate that calibration is in

progress. Therefore, the DRDY line should be ignored for up

to one modulator cycle after the last bit of the calibration com-

mand is written to the control register.

Self-Calibration

In the self-calibration mode with a unipolar input range, the

zero-scale point used in determining the calibration coefficients

is with both inputs shorted (i.e., AIN1(+) = AIN1(–) =

VBIAS for AIN1 and AIN2 and AIN3 = VBIAS for AIN3 ) and the

full-scale point is VREF. The zero-scale coefficient is determined

by converting an internal shorted inputs node. The full-scale co-

efficient is determined from the span between this shorted in-

puts conversion and a conversion on an internal VREF node. The

self-calibration mode is invoked by writing the appropriate val-

ues (0, 0, 1) to the MD2, MD1 and MD0 bits of the control

switched in to the modulator first and a conversion is performed;

the VREF node is then switched in, and another conversion is per-

formed. When the calibration sequence is complete, the calibration

coefficients updated and the filter resettled to the analog input

voltage, the DRDY output goes low. The self-calibration proce-

dure takes into account the selected gain on the PGA.

For bipolar input ranges in the self-calibrating mode, the

sequence is very similar to that just outlined. In this case, the

two points that the AD7713 calibrates are midscale (bipolar

zero) and positive full scale.

System Calibration

System calibration allows the AD7713 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. System cali-

bration is a two-step process. 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. System calibration is initiated by writing the

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

the control register. The DRDY output from the device will sig-

nal when the step is complete by going low. After the zero-scale

point is calibrated, the full-scale point is applied 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 initi-

ated, and it must remain stable throughout the calibration step.

DRDY

goes low at the end of this second step to indicate that

the system calibration is complete. In the unipolar mode, the

system calibration is performed between the two endpoints of

the transfer function; in the bipolar mode, it is performed be-

tween midscale and positive full scale.

This two-step system calibration mode offers another feature.

After the sequence has been completed, additional offset or gain

calibrations can be performed by themselves to adjust the zero

reference point or the system gain. This is achieved by perform-

ing the first step of the system calibration sequence (by writing

0, 1, 0 to MD2, MD1, MD0). This will adjust the zero-scale or

offset point but will not change the slope factor from what was

set during a full system calibration sequence.

System calibration can also be used to remove any errors from

an antialiasing filter on the analog input. A simple R, C anti-

aliasing 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.

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

for the system is presented to the AIN input of the converter.

System offset calibration is initiated by writing 1, 0, 0 to MD2,

MD1, MD0. The system zero-scale coefficient is determined by

converting the voltage applied to the AIN input, while the full-

scale coefficient is determined from the span between this AIN

conversion and a conversion on VREF. The zero-scale point

should be applied to the AIN input for the duration of the cali-

bration sequence. This is a one-step calibration sequence with

DRDY

going low when the sequence is completed. In the uni-

polar 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 AD7713 also offers a background calibration mode where

the part interleaves its calibration procedure with its normal

conversion sequence. In the background calibration mode, the

same voltages are used as the calibration points as are used in

the self-calibration mode, i.e., shorted inputs and VREF. The

background calibration mode is invoked by writing 1, 0, 1 to

MD2, MD1, MD0 of the control register. When invoked, the

background calibration mode reduces the output data rate of the

AD7713 by a factor of six while the –3 dB bandwidth remains

unchanged. Its advantage is that the part is continually perform-

ing calibration and automatically updating its calibration coeffi-

cients. As a result, the effects of temperature drift, supply sensitivity

and time drift on zero- and full-scale errors are automatically

removed. When the background calibration mode is turned on,

the part will remain in this mode until bits MD2, MD1 and

MD0 of the control register are changed. With background cali-

bration mode on, the first result from the AD7713 will be incor-

rect as the full-scale calibration will not have been performed.

For a step change on the input, the second output update will

have settled to 100% of the final value.

Table IV summarizes the calibration modes and the calibration

points associated with them. It also gives the duration from

when the calibration is invoked to when valid data is available to

the user.

AD7713 Datasheet(PDF) 17 Page - Analog Devices

AD7713 Datasheet(HTML) 17 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL