Ps.: The sine signal simulates a little noise in the output of the reference.
The 1 Volt source is representing the voltage reference to be buffered. After the reference is applied a low-pass filter.
The first OpAmp need to have a very low input offset, like a zero drift amplifier. After this the reference pass through another low-pass filter and then is applied to the last OpAmp. This need to be a high speed amplifier and with generous current capacity, overcoming the current transients generated by the load, like a SAR ADC.
Normally fast amplifiers have bigger input offset, but in this arrangment the zero-drift amplifier corrects the offset of the high speed amplifier.
Based in this document: http://www.ti.com/tool/TIPD173
