Your best bet is to toss the source follower into the dumpster, and use a proper op amp follower where there will be no gain loss.
Thank you for your suggestion, actually a closed loop op-amp with unit gain feedback is nice for the track and hold circuit, because its gain can be controled accurately. But the ADC we designed is a high speed ADC , the signal bandwidth is >800MHz, it is difficult to use closed loop op-amp.
I have to use open loop topology in the analog chain, but the precision of TH become hard to control.
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Thank you for your reply.
"What is the influence to ADC of the gain loss?"
I mean if I don`t compensate the gain loss of TH circuit, how the output of ADC wil be influenced ?
"as your source follower buffer's gain may be not constant over signal voltage and temperature."
The temperature wil influence the gain, of course, all the PVT variation will make the gain slightly deviating from the designed value; the gain should be constant over signal voltage, this is about distortion issure,the linearity of source follower I designed is enough for the resolution of TH circuit and the whole ADC.