I require an adc with 40 channel ,16 bit and 300KSPS to 1msps speed and 40 channel can be from 2 or 3 adcs but with 16 bit and 300ksps to 1msps speed and should have synchronization pin to have synchronization operation for all adcs.
I can recommend to you the two ADCs. The AD7616 and AD7768. They have different architecture, the AD7616 is a SAR and AD7768 is a Sigma Delta. The AD7616 is a 16bit , 16 channel dual simultaneous sampling ADC and has a throughput rate of 1MSPS. The sampling can be synchronize using the CONVST pins. Another ADC that I would recommend is the AD7768, which I think very close to what you are looking for. The AD7768 is a 24bit , 256ksps output data rate. The AD7768 can sample 8 channels at the same time. The AD7768 has a synchronization pins to allow multiple AD7768 connected attune from a single master clock. You can refer to the links of the datasheet of these ADCs.