This adjustment is similar to the FM Path Offset, but this adjustment generates a DAC value to null the voltage offset on the differential inverting amplifier on the synthesizer board.
The signal generator frequency is set for 525 MHz at 0 dBm. The EXT1 input is DC coupled and left open for this calibration. FM is enabled with an FM deviation of 100 kHz. The calibration loads the FM_OFFSET_DACs with the values calculated in the FM Path Offset Adjustment. The two FM multiplexers, on the reference board have their ground inputs selected. The FM_SCALE_DAC is set to 0 and an internal voltage measurement is made on the analog bus through the FM_ABUS node. If the FM1_OFFSET_DAC values are greater than 20 DAC values, the calibration fails. Otherwise the difference DAC value (which takes into account the FM Path Offset and the FM Inverting Amp Offset) is stored.