How is self-calibration implemented on the AD779x ?
For calibration to occur, regardless of which calibration mode is used on the selected channel, the ADC’s on-chip microcontroller must record the modulator output for two different analog input conditions. These are the zero-scale and full-scale points. With these conversions, the microcontroller can calculate the gain slope for the input-to-output transfer function of the converter. Internally, the part works with a resolution of 33 bits to determine its conversion result of either 16 bits or 24 bits.
In self-calibration mode, the ADC determines the calibration points internal to the ADC. The zero-scale point used to determine the calibration coefficients is with both inputs shorted, (that is, AIN(+) = AIN(–) = AVDD/gain, internally within the ADC. Signals connected to the analog input pins of the device will not affect the calibration procedure as long as they are within the common range of the input. The full-scale coefficient is determined by applying a voltage of AVDD/gain to the modulator inputs.
Self-calibration mode is invoked in the AD779x by writing to the mode bits (MD2, MD1, and MD0 bits) in the mode register.
In this calibration mode, the shorted inputs’ node is switched into the modulator first, and a conversion is performed. The AVDD/gain node is then switched in, and another conversion is performed. DRDY can be used to determine when the calibration is complete and new data pertaining to the analog input is available at the output. DRDY goes high on the initiation of the calibration and will not return low until a conversion result on the external analog input is available.
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