AD7798 datasheet(24/28 Pages) AD

24 / 28 page

AD7798/AD7799

Data Sheet

Rev. B | Page 24 of 28

AVDD MONITOR

Along with converting external voltages, the ADC can be used

to monitor the voltage on the AVDD pin. When Bits CH2 to CH0

equal 1, the voltage on the AVDD pin is internally attenuated by 6,

and the resulting voltage is applied to the ∑-∆ modulator using

an internal 1.17 V reference for analog-to-digital conversion.

This is useful because variations in the power supply voltage

can be monitored.

CALIBRATION

The AD7798/AD7799 provide four calibration modes that can

be programmed via the mode bits in the mode register. These

are internal zero-scale calibration, internal full-scale calibration,

system zero-scale calibration, and system full-scale calibration,

which effectively reduce the offset error and full-scale error to

the order of the noise. After each conversion, the ADC con-

version result is scaled using the ADC calibration registers

before being written to the data register. The offset calibration

coefficient is subtracted from the result prior to multiplication

by the full-scale coefficient.

To start a calibration, write the relevant value to the MD2 to

MD0 bits in the mode register. After the calibration is complete,

the contents of the corresponding calibration registers are

updated, the RDY bit in the status register is set, the DOUT/

RDY pin goes low (if CS is low), and the AD7798/AD7799

revert to idle mode.

During an internal zero-scale or full-scale calibration, the

respective zero-scale and full-scale input are automatically

connected internally to the ADC input pins. A system calibration,

however, expects the system zero-scale and system full-scale

voltages to be applied to the ADC pins before the calibration

mode is initiated. In this way, external ADC errors are removed.

From an operational point of view, a calibration should be

treated like an ADC conversion. A zero-scale calibration (if

required) should always be performed before a full-scale

calibration. System software should monitor the RDY bit in the

status register or the DOUT/RDY pin to determine the end of

calibration via a polling sequence or an interrupt-driven routine.

Both an internal offset calibration and system offset calibration

take two conversion cycles. An internal offset calibration is not

needed because the ADC itself removes the offset continuously.

To perform an internal full-scale calibration, a full-scale input

voltage is automatically connected to the selected analog input

for this calibration. When the gain equals 1, a calibration takes

two conversion cycles to complete. For higher gains, four

conversion cycles are required to perform the full-scale

calibration. DOUT/RDY goes high when the calibration is

initiated and returns low when the calibration is complete. The

ADC is placed in idle mode following a calibration. The measured

full-scale coefficient is placed in the full-scale register of the

selected channel. Internal full-scale calibrations cannot be

performed when the gain equals 128. A factory calibration

is performed at this gain setting, and the factory value is

automatically loaded into the full-scale register when the gain is

set to 128. With this gain setting, a system full-scale calibration

can be performed. A full-scale calibration is required each time

the gain of a channel is changed to minimize the full-scale error.

An internal full-scale calibration can only be performed at

specified update rates. For gains of 1, 2, and 4, an internal full-

scale calibration can be performed at any update rate. However,

for higher gains, internal full-scale calibrations must be performed

when the update rate is less than or equal to 16.7 Hz, 33.2 Hz,

or 50 Hz. Because the full-scale error does not vary with the

update rate, a calibration at one update rate is valid for all update

rates (assuming the gain or reference source is not changed).

A system full-scale calibration takes two conversion cycles to

complete, irrespective of the gain setting. A system full-scale

calibration can be performed at all gains and update rates. If

system offset calibrations are performed along with system full-

scale calibrations, the offset calibration should be performed

before the system full-scale calibration is initiated.

AD7798 Datasheet(PDF) 24 Page - Analog Devices

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