I have a SPI bus with two HMC834 devices on it. The SPI signals are driven by an FPGA with a 1.8V output through a TI TXB0108 level shifter capable of 50mA output shifting the signals to 3.3V. I can't get proper signal level with both devices on the bus, i.e. SDI and SCK signals tied together. Steady state high level ends up being about 700mV. Independent signals like the SEN signals look fine. When i isolate one of the devices from the bus, signals on the remaining device look correct.
What is the input impedance on these signals or how much current is required to drive 2 devices (or 3 - another bus I have) on the same bus? How does DVDD affect the interface? If after input filtering is 3.0V DVDD sufficient?