Would have liked to see if your discrete design would have less noise.

The majority of noise comes from the op-amps and the single OPA653 circuit is less noisy than the two op-amp circuit I followed the discrete JFET buffer with.

Quick analysis:

On the x1 attenuator setting (20dB attenuator switched out). The input is terminated in 1M || 18pF. Consider that the input is disconnected/floating.

1M corners with 18pF at 8842Hz. This is a single pole, low-pass response. A single pole LPF has an equivalent noise bandwidth of pi/2*fo, so:

pi/2 * 8842 = 13890 Hz.

1M generates 129nV/rt Hz of thermal noise at room temperature. The OPA653 has 1.8fA/rtHz input current noise:

1M * 1.8fA = 1.8nV.

Additionally, as noise sources sum as the square root of the sum of the squares, the current noise contribution is totally negligible and can be neglected.

So, the net rms noise contribution from the passive input network is therefore:

SQRT(13890Hz) * 129nV = 15.2uV rms.

This drops even lower when the 20dB attenuator is switched in as the 10:1 attenuator an impedance reduction as presented to the op-amp input.

It's often blithely stated that the noise performance of a CRO/DSO with 1M inputs is limited by the impedance of said input. Until someone can design a 1M input with next to no shunt input capacitance this will remain total BS. It's the wideband amplifiers/ADCs that are the noise performance barrier.

The OPA653 is speced at 6.1nV. Its 500MHz (lets just assume first order for a close enough simplification) frequency response returns a noise bandwidth of:

500MHz * pi/2 = 785MHz, so...

SQRT(785M) * 6.1nV = 171uVrms

Lets sum this with the worst case (15.2uV) noise contributed by the passive input stage:

SQRT((15.2*15.2)+(171*171)) = 171.67uV.

So the 1M input network (worse case) contributes:

log20*(171.67/171)

= 0.034dB to the total rms noise!

171.67uV of rms noise in a 500MHz bandwidth is pretty negligible when viewed on an oscilloscope with 10mV/div vertical sensitivity, so this buffer amplifier won't be a performance barrier in terms of noise performance in almost all cases of use.

NOTE: for simplification I've neglected in the above the <100kHz 1/f noise of the OPA653, but if you do the sums you'll find the contribution to the total rms noise quite negligible.