JAJSMJ1B March 2023 – June 2024 ADC12DJ5200SE
PRODUCTION DATA
Foreground calibration and background calibration modes inherently calibrate the offsets of the ADC cores; however, the input buffers sit outside of the calibration loop and therefore their offsets are not calibrated by the standard calibration process. In both dual-channel mode and single-channel mode, uncalibrated input buffer offsets result in a shift in the mid-code output (DC offset). Further, in single-channel mode uncalibrated input buffer offsets can result in a fixed spur at fS / 2. A separate calibration is provided to correct the input buffer offsets.
There must be no signals at or near DC or aliased signals that fall at or near DC to properly calibration the offsets. Requiring the system to be sure of the condition during normal operation, or have the ability to mute the input signal during calibration. The lower bandwidth of the balun will signficantly suppress signals near DC, but care must the taken to avoid AC signals near the sample rate from aliasing near DC. Foreground offset calibration is enabled via CAL_OS and only performs the calibration one time as part of the foreground calibration procedure. Background offset calibration is enabled via CAL_BGOS and continues to correct the offset as part of the background calibration routine to account for operating condition changes. When CAL_BGOS is set, the system must be sure there are no DC or near DC signals or aliased signals that fall at or near DC during normal operation. When background offset calibration is used the analog to digital conversion is disturbed by a bandwidth difference. The calibration time is relatively long becuase the offset calibration engine requires a lot of averaging. A preferred method for offset calibration is to use foreground calibration as a one-time operation so the timing of the disturbing glitch can be controlled. A one time foreground calibration can be performed by setting CAL_OS to 1 before setting CAL_EN. However, this will not correct for variations as operating conditions change.
The offset calibration correction uses the input offset voltage trim registers (see Table 6-66) to correct the offset; therefore, must not be written by the user when offset calibration is used. The user can read the calibrated values by reading the OADJ_x_VINy registers, where x is the ADC core and y is the input (INA or INB), after calibration is completed. Only read the values when FG_DONE is read as 1 when using foreground offset calibration (CAL_OS = 1) and do not read the values when using background offset calibration (CAL_BGOS = 1).