I'm trying to get a clue what PSRR one could expect from the "FET IMPUT AMP" subcircuit in this schematic:
http://www.eevblog.com/files/Rigol-DS1054Z-Schematic-FrontEnd.pdf(particularly for the negative V- supply rail)
The buffer amp circuit in the frontend of my Hantek 2D72 looks almost identical (as far as I briefly investigated the PCB, there seem to be only minor differences and slightly different component values) and I'm facing significant noise on the two scope channels which originates in the output amplifier of the built-in AWG (when it drives a 50 Ohm load with a 500 Hz square wave @ 2V amplitude). This noise primarily is a (distorted) 500 square wave too, with some additional higher-frequency components, and it is obviously injected into the frontends' FET buffer amplifier via the power supply rails - I think in particular via the two current sources. It is not present at the input (gate of the FET), but appears first at the source of the FET. And it is present on the power supply rails, too.
EDIT:
A more detailed inspection reveals the my scope's FET buffer stage and its component values are closer to this one
https://www.mikrocontroller.net/attachment/175587/Hantek_Tekway_Voltcraft_DSO_hw1007.pdf, page 2.
(Still the basic circuit with source follower + emitter follower + two current sources is pretty similar)
Initial thoughts regarding the circuit:
The current sources are biased with a resistive divider R01_23 and R01_24 from GND to the (the noisy) V-, therefore I'd expect their "constant" current to carry the noise as well. The bypass capaciter C01_11 from the divider's tap to V- seems too small to help at 500 Hz. I'm wondering in which amount the noisy "constant" current affects the buffer's output voltage then - I guess this transimpedance dominated by the output impedance of the source follower, is this correct?
Besides the fast path via the FET, there is still the the low-frequency/DC path via the opamp, with feedback from the output, but this path seems to join-in only at even lower frequencies (50 Hz or even lower), so its feedback can't compensate the injected noise either at 500 Hz.