BitScope Gain Buffer

U15 is configured as a (x 4.583) non-inverting amp to boost small signals to feed the ADC. It is possible to use the MAXIM 477 device here, but for a gain of 5 it has only a bandwidth of 25 MHz. The better choice is the Analog devices AD8048 which is optimized for gains of +2 or more.

Gain Buffer.

At a gain of 5, the AD8048 has a bandwidth of better than 50 MHz. For even better performance you may be able to reduce the gain of U15 slightly. The bandwidth improvement will follow 1/[1+Rf/Rg]. The AD8048 has a unity gain mate - the AD8047 - which is virtually a drop in replacement for the MAXIM 477.

If you use the Analog Devices AD9057 ADC (the pick of the litter) which has a 1 V span, you may halve the gain of this stage, preserving the 100MHz bandwidth across all ranges. This is useful, even though the AD9057 samples at 40/60/80 MSPS it has an analog bandwidth of 120MHz! This may seem confusing, but will be explained below. There is a bit more to sampling than you might think. 

By the way, because this is a 8 bit sample engine, we don't care too much for whole numbers in the gains. The Host display software can sort this out with high precision arithmetic. All the Host needs to know are the constants for each range for a device. There lies a use for some of the 64 bytes of EEPROM in the PIC16F84!