ad9778

ad9778 is an high speed 14 bit dac,

Ireference of the dac is 12ma when i use 100 ohms at i120 pin of it, so in oreder to find a full scale current of the dac i need dac gain , gain of the dac is 10 bit that can be set by reg 0A,0C, for i phase dac , what is the range of gain in this ic?

using 100ohms at Pin I120 is not allowed this will run the tansistors out of head room. You need to use 10Kohms. Then you can program the output current from 8.66 mA to 31.66 mA. the nominal 20ma is programmed with 512 programmed in the 10 bit  gain word of I and Q  "the I DAC or Q DAC gain is a 10-bit word in the 3-wire interface port register (Register 0x0B, Register 0x0C, Register 0x0F, and Register 0x10). "

The range of the gain is shown in figure 77 page 43. The all zero code yields 8.66ma and all "1" code yields the 31.66ma. Code 512 yields 20ma.

synco(p/n) i made nc, but synci(p/n) is needed in order to latch the datainput, so i have connected a fpga bank which is operating at 2.5v, where as synco is at 3.3v level so how to interface it ? is it through level translator?

the synci(p/n) is an LVDS input signal used for syncrhonizing multiple AD9778 parts, it is not used for latching the input data, it has to run 1/32 of  the DACCLK which is lower the DATa inptu frequency. The data is latch for a signal derived from DATACLK or REFCLK. There are description of setup and hold timing for either DATACLK output or refclk input.