I have a single ended signals transmitted from an AC coupled DAC. The DAC has 4 outputs and I am using 2 of the outputs as I and Q (90 degrees offset). In order to drive the ADL5375-15 modulator with a DC biased differential input I am using 2 x DC1147 (LTC6404) differential amplifiers. I trimmed the DC bias as follows:
- QBBP - 1.499 V
- QBBN - 1.501 V
- IBBN - 1.502 V
- IBBP - 1.495 V
DC1147: I have also replaced all of the differential gain resistors with 0.1% resistors to attempt to balance the outputs without individual trimming on the DC1147. The baluns have been been removed from both the input and output and replaced with zero ohm resistors, along with the output resistors. The negative input is open circuit and positive input resistor R4 was replaced with a 33 ohm resistor.
While this has reduced the bleed-though by approximately 20-25 dB, I still get a substantial amount of bleed-through (>-20 dBm)
I am using the EV-ADF4355 to drive the LO (AC coupled @ -1 dBm).