Low-power background calibration (LPBG) mode reduces the power-overhead of enabling additional ADC cores while still allowing background calibration of the ADC cores to maintain optimal performance as operating conditions change. LPBG calibration modifies the background calibration procedure by powering down the spare ADC cores until they are ready to be calibrated. Set LP_EN = 1 to enable the low-power background calibration feature. Calibration and swapping of ADC cores can be controlled either automatically by the device or manually by the system by setting LP_TRIG appropriately. Manual control (LP_TRIG=1) allows the system to trigger calibration in order to limit the number of calibration cycles that occur to avoid unnecessary core swaps or to keep power consumption at a minimum. For instance, the user may decide to run calibration only when the system temperature changes by some fixed temperature. If manual control is not necessary the automatic calibration control can be enabled (LP_TRIG=0) to calibrate at fixed time intervals.
In automatic calibration mode (LP_TRIG=0) the spare ADC core sleep time can be controlled by the LP_SLEEP_DLY register setting. LP_SLEEP_DLY is used to adjust the amount of time an ADC sleeps before waking up for calibration (when LP_EN=1 and LP_TRIG = 0). LP_WAKE_DLY sets how long the core is allowed to stabilize after being awoken before calibration begins. In automatic calibration control mode the freshly calibrated core is swapped in for an active core as soon as calibration finishes and the new spare core is powered down for the sleep duration before waking up and calibrating.
Manual calibration control is enabled by setting LP_TRIG high in order to use the calibration trigger (CAL_SOFT_TRIG or CALTRIG) to trigger calibrations and core swaps. When manual control is enabled (LP_TRIG=1) the spare ADC is held in sleep mode while the calibration trigger is high. Setting the calibration trigger low then wakes up the spare ADC core and starts the calibration routine after waiting for the specified wake delay (LP_WAKE_DLY). The spare ADC core is swapped in for an active core once calibration is complete and the calibration trigger is set high again. If the calibration trigger is held low, then the spare ADC core calibrates and powers until the calibration trigger goes high; therefore consuming power. ADC09QJ1300-Q1 can report when the spare ADC finishes calibration on the CALSTAT output pin by setting the CALSTAT pin to output the CAL_STOPPED signal (CAL_STATUS_SEL = 1). For lowest power consumption, set the calibration trigger high before calibration finishes to allow the spare ADC to swap in for an active ADC core as soon as calibration finishes. Otherwise, the ADC core swap can be timed manually by setting the calibration trigger high at the desired time to minimize system impact of potential glitches caused by the swapping procedure.
In LPBG mode there is an increase in power consumption during the ADC core calibration. The longer the spare ADC is held asleep the lower the average power consumption, however large shifts in operating conditions during the sleep cycle may cause degraded ADC performance due to non-optimized calibration data for the active ADC core. The power consumption roughly alternates between the power consumption in foreground calibration when the spare ADC core is sleeping to the power consumption in background calibration when the spare ADC is being calibrated. Design the power-supply network to handle the transient power requirements for this mode, including bulk capacitance after any power supply filtering network to help regulate the supply voltage during the supply transient.