AD7713 datasheet(17/28 Pages) AD

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REV. D

AD7713

–17–

is invoked by writing the appropriate values (0, 0, 1) to the

MD2, MD1, and MD0 bits of the control register. In this cali-

bration mode, the shorted inputs node is switched in to the

modulator first and a conversion is performed; the VREF node is

then switched in, and another conversion is performed. 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 procedure 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-scale and full-scale points. System

calibration is a 2-step process. The zero-scale point must be

presented 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

signal 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 writ-

ing the appropriate values (0, 1, 1) to MD2, MD1, and MD0.

Again, the full-scale voltage must be set up before the calibra-

tion is initiated, 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 unipo-

lar mode, the system calibration is performed between the two

endpoints of the transfer function; in the bipolar mode, it is

performed between midscale and positive full scale.

This 2-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, and 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, and MD0. The system zero-scale coefficient is deter-

mined 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

calibration sequence. This is a 1-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, and MD0 of the control register. When invoked,

the background calibration mode reduces the output data rate of

the AD7713 by a factor of 6 while the –3 dB bandwidth remains

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

forming calibration and automatically updating its calibration

coefficients. As a result, the effects of temperature drift, supply

sensitivity and time drift on zero- and full-scale errors are auto-

matically 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 back-

ground calibration mode on, the first result from the AD7713

will be incorrect 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.

Span and Offset Limits

Whenever a system calibration mode is used, there are limits on

the amount of offset and span that can be accommodated. The

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

AIN1 and AIN2 has a minimum value of 0.8

VREF/GAIN and

a maximum value of 2.1

VREF/GAIN. For AIN3, the mini-

mum value is 3.2

VREF/GAIN, while the maximum value is

4.2

VREF/GAIN.

Table IV. Calibration Truth Table

Zero-Scale

Full-Scale

Calibration Type

MD2, MD1, MD0

Calibration

Sequence

Duration

Self-Calibration

0, 0, 1

Shorted Inputs

VREF

1-Step

1/Output Rate

System Calibration

0, 1, 0

AIN

2-Step

1/Output Rate

System Calibration

0, 1, 1

AIN

2-Step

1/Output Rate

System Offset Calibration

1, 0, 0

AIN

VREF

1-Step

1/Output Rate

Background Calibration

1, 0, 1

Shorted Inputs

VREF

1-Step

1/Output Rate

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