need your help for a low noise analog buffer

[yinhexi]

i need one low noise buffer to driver a large cap(about 30PF), the spec. of noise is <100uV,the spec. of GBW is about 40MHz,
anayone would like to give me an advice？ thank you for reading this issue！

 

[mtwieg]

Okay, so you need an input referred noise spectral density of <15.8nV/rt(Hz). That shouldn't be too bad, but it depends on what the source impedance driving the amp is.

Do you really need to drive the capacitor directly with the buffer output? If you only need 40MHz of bandwidth, then you could just put 10-50 ohms of series resistance on the output, and that should make the buffer happy while not impacting the overall circuit bandwidth. That way you could use a generic low noise op amp. The LT6200 comes to mind.

 

[yinhexi]

thank you ! mtwieg, let me take a try!:-D