AD7731 datasheet(29/44 Pages) AD

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AD7731

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CALIBRATION

The AD7731 provides a number of calibration options that can

be programmed via the MD2, MD1 and MD0 bits of the Mode

Mode Register and Calibration Operations sections. A calibra-

tion cycle may be initiated at any time by writing to these bits of

the Mode Register. Calibration on the AD7731 removes offset

and gain errors from the device.

The AD7731 gives the user access to the on-chip calibration

registers allowing the microprocessor to read the device’s cali-

bration coefficients and also to write its own calibration coeffi-

cients to the part from prestored values in E

2PROM. This gives

the microprocessor much greater control over the AD7731’s

calibration procedure. It also means that by comparing the

coefficients after calibration with prestored values in E

2PROM,

the user can verify that the device has correctly performed its

calibration. The values in these calibration registers are 24 bits

wide. In addition, the span and offset for the part can be ad-

justed by the user.

Internally in the AD7731, the coefficients are normalized before

being used to scale the words coming out of the digital filter.

The offset calibration register contains a value which, when

normalized, is subtracted from all conversion results. The gain

calibration register contains a value which, when normalized, is

multiplied by all conversion results. The offset calibration coeffi-

cient is subtracted from the result prior to the multiplication by

the gain coefficient.

The AD7731 offers self-calibration or system calibration facili-

ties. For full calibration to occur on the selected channel, the

on-chip microcontroller must record the modulator output for

two different input conditions. These are “zero-scale” and “full-

scale” points. These points are derived by performing a conver-

sion on the different input voltages provided to the input of the

modulator during calibration. The result of the “zero-scale”

calibration conversion is stored in the Offset Calibration Regis-

ter for the appropriate channel. The result of the “full-scale”

calibration conversion is stored in the Gain Calibration Register

for the appropriate channel. With these readings, the micro-

controller can calculate the offset and the gain slope for the

input-to-output transfer function of the converter. Internally,

the part works with 33 bits of resolution to determine its conver-

sion result of either 16 bits or 24 bits.

The sequence in which the zero-scale and full-scale calibration

occurs depends upon the type of full-scale calibration being

performed. The internal full-scale calibration is a two-step cali-

bration that alters the value of the Offset Calibration Register.

Thus, the user must perform a zero-scale calibration (either

internal or system) after an internal full-scale calibration to

correct the Offset Calibration Register contents. When using

system full-scale calibration, it is recommended that the zero-

scale calibration (either internal or system) is performed first.

Calibration time is the same regardless of whether the SKIP

mode is enabled or not. This is because the SKIP bit is ignored

and the second stage filter is included in the calibration cycle.

This is done to derive more accurate calibration coefficients. If

the subsequent operating mode is with CHP = 0, the calibration

should be performed with CHP = 0 so the offset calibration

coefficient and the subsequent conversion offsets are consistent.

Since the calibration coefficients are derived by performing a

conversion on the input voltage provided, the accuracy of the

calibration can only be as good as the noise level which the part

provides in normal mode. To optimize the calibration accuracy,

it is recommended to calibrate the part at its lowest output rate

where the noise level is lowest. The coefficients generated at any

output update rate will be valid for all selected output update

rates. This scheme of calibrating at the lowest output update

rate does mean that the duration of calibration is longer.

Internal Zero-Scale Calibration

An internal zero-scale calibration is initiated on the AD7731 by

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

MD0 bits of the Mode Register. In this calibration mode with a

unipolar input range, the zero-scale point used in determining

the calibration coefficients is with the inputs of the differential

pair internally shorted on the part (i.e., AIN[+] = AIN[–] =

Externally-Applied AIN[–] voltage). The PGA is set for the

selected gain (as per the RN2, RN1, RN0 bits in the Mode

The duration time of the calibration depends upon the CHP bit

of the Filter Register. With CHP = 1, the duration is 22

× 1/

Output Rate; with CHP = 0, the duration is 24

× 1/Output

Rate. At this time the MD2, MD1 and MD0 bits in the Mode

The

RDY line goes high when calibration is initiated and re-

turns low when calibration is complete. Note, the part has not

performed a conversion at this time; it has simply performed a

zero-scale calibration and updated the Offset Calibration

0, 0, 1 or 0, 1 ,0 to the MD2, MD1, MD0 bits of the Mode

RDY 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/16) be-

fore

RDY goes high to indicate that calibration is in progress.

Therefore,

RDY should be ignored for up to one modulator

cycle after the last bit of the calibration command is written to

the Mode Register.

For bipolar input ranges in the internal zero-scale calibrating

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

case, the zero-scale point is exactly the same as above but since

the part is configured for bipolar operation, the output code for

zero differential input is 800000 Hex in 24-bit mode.

The internal zero-scale calibration needs to be performed as one

part of a two-step full calibration. However, once a full cali-

bration has been performed, additional internal zero-scale

calibrations can be performed by themselves to adjust the

part’s zero-scale point only. When performing a two-step full

calibration, care should be taken as to the sequence in which the

two steps are performed. If the internal zero-scale calibration is

one part of a full self-calibration, then it should take place after

an internal full-scale calibration. If it takes place in association

with a system full-scale calibration, then this internal zero-scale

calibration should be performed first.

AD7731 Datasheet(PDF) 29 Page - Analog Devices

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