110407763AN663_0| Application Note
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AN-663 APPLICATION NOTE
AD7732/AD7734/AD7738/AD7739 Calibration Registers
By Tom Meany
This application note concentrates on the AD7739 but is also applicable to the AD7732, AD7734, and AD7738.The purpose of this application note is to explore the calibration registers in more detail than is found on the data sheets. INTRODUCTION The AD7739 features two sets of calibration registers. The main function of these registers is to store the offset and gain coefficients for the various calibration modes. However, since all nine sets of registers can be read to and written from over the serial interface, these registers are available to manipulate raw ADC conversion results. The first set of registers is a 24-bit offset (ADC offset) register and 24-bit gain (ADC gain) register, which affect all channels. The second set includes a per channel 24-bit offset register and a 24-bit per channel gain register, which affect a specific channel only.
FROM FILTER
The effect of the calibration registers is independent of conversion time, but does depend on the range. All calibration registers on the AD7732, AD7734, AD7738, and AD7739 can be read to or written from over the serial interface. However, a write to the registers will be ignored if the ADC is converting or calibrating when the write takes place. This avoids corrupting the conversion result. Therefore, you must set the ADC into its sync mode by writing 000 to the mode bits before writing to any of the calibration registers.
CONVERSION DATA AVAILABLE
SUBTRACT ADC OFFSET REGISTER
MULTIPLY BY ADC GAIN REGISTER
FOR UNIPOLAR RANGES: DATA = ((ADC RESULT R ADC ZS CAL. REG.) XADC FS REG./ 200000h R CH. ZS CAL. REG.) XCH. FS CAL. REG./200000h FOR BIPOLAR RANGES: DATA = ((ADC RESULT R ADC ZS CAL. REG.) XADC FS REG./ 400000h + 800000h R CH. ZS CAL. REG.) XCH. FS CAL. REG./200000h WHERE THE ADC RESULT IS IN THE RANGE OF 0 TO FFFFFh. AND R = 1 ON 2.5 V AND 1.25 V RANGES AND R = 2 ON THE 0.625 V.
SUBTRACT PER CHANNEL OFFSET REGISTER
MULTIPLY BY PER CHANNEL GAIN REGISTER
APPLY CLAMP IF SELECTED
WRITE TO DATA REGISTER
Figure 1. Flow Chart for Calibration Register Contents
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