I am using the AD9643 IP core to implement IQ Correction for phase and amplitude in line with the reference material @ AXI_AD9643 IP core [Analog Devices Wiki] .
When I bypass the IQ correction block my ADC data has approximately 12dB more power than when the ADC data passes through the IQ correction block. The coefficients I use are based around A = 1 (hence alpha = 1), refer to Correcting I-Q Imbalance in Direct Conversion Receivers,S.W. Ellingson,February 10, 2003.
So when A is 1 there should be no amplitude scalling, only phase correction. Yet I clearly see changes in the signal level when the IQ correction block is inserted in the data path. The symptons are mysteriously like a 2 bit left shift and given that the data going in is 14 bit and the coefficients are 16 bit, I wonder if this might be the cause. 12dB is almost equivalent to a facgtor of 4 gain difference.
Can anyone else offer an explanation for this difference?