24/7 Data Acquisition Wildcard Users Guide

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Background Calibration

A Background Calibration interleaves the calibration procedure with the normal conversion sequence. In background calibration mode, the analog to digital converter provides continuous zero-scale self-calibrations; it does not provide any full-scale calibrations. When invoked, the background calibration mode performs a Zero Scale Self Calibration after every sample, reducing the sampling frequency by a factor of six. Its advantage is that the analog to digital converter 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. Because the background calibration does not perform full-scale calibrations, a Full Self Calibration should be performed before starting a background calibration. Removal of the offset drift in this mode leaves gain drift as the only source of error not removed. Typical gain drift with temperature for the AD7714 is 0.2 ppm/degree C. The synchronization option discussed below should not be used when a Background Calibration is operating.

System Calibration

A system calibration performs the same calculations as a self calibration but uses voltages at the analog field inputs for the zero and full scale points. Full system calibration requires a two-step process, a Zero Scale System Calibration followed by a Full Scale System Calibration. A Full Scale System Calibration should not be performed unless the analog to digital converter has valid zero-scale coefficients. The zero-scale coefficients are loaded by calling Start_Conv_With_Values or by performing a Zero Scale System Calibration. The input voltages used for both system calibrations must be applied to the 24/7 Data Acquisition Wildcard before the calibration is initiated and remain stable until the calibration is complete. In unipolar mode, the system calibration is performed between the two endpoints of the transfer function; in the bipolar mode, it is preformed between midscale (zero differential voltage) and positive full scale. You can perform a system-calibration in separate steps for additional offset or gain calibrations. Calibrating one of the parameters, either offset or gain, does not affect the other parameter.

System Offset Calibration

The System Offset Calibration is a variation of both the system calibration and self-calibration. In this case, the zero-scale point is determined with a Zero Scale System Calibration and the full-scale calibration is performed with a Full Scale Self Calibration. The zero-scale point must be applied to the analog field inputs before the calibration is initiated and remain stable until the calibration is complete.

Whenever you use a system calibration mode, there are limits on how much the offset and span can be adjusted. The positive full-scale calibration limit is > 1.05 x VREF/GAIN. This allows the input range to go 5% above the nominal range so that the digital to analog converter will still operate correctly with a positive full-scale voltage beyond the nominal. For more information on span and offset limits, see page 25 of the data sheet.

Choosing the Sample Rate

The 24/7 Data Acquisition Wildcard supports a wide range of sample rates from 4.8 to over 1010 samples per second. The sample frequency is specified as an integer (frequency integer in this document) from 19 to 4000 and provided as an input parameter to Start_Conversion or Start_Conv_With_Values. Equation 2 shows the relationship between the frequency integer, n, and the sample frequency and Table 5 lists some of the available sample frequencies.

where n is the frequency integer whose value ranges from 19 - 4000

Equation 2: Calculation of the Sample Frequency from the Frequency Integer

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