I know 7A22 exists, but I have at this moment the 5000 series mainframe. And I do have to say the 50uV range of the 5A21N with 5000 Series is dissapointing, because it doesn't work well unless the bandwidth is limited to 10kHz. And even then it is not very silent. So going to a 10uV system even, I am a bit reluctant to hunt for a 5030 or 5031, or change to the 7000 series just to find out the same.
The HP130A goes down to only 200uV, but at 250kHz without frequency loss, and really DEAD silent. If there is no hum or noise on the DUT, and at zero signal, the scope produces a hair sharp horizontal line. The 5A21N with 5000 Series at 50uV, without bandwidth limiting, you can not use it. With bandwidth limiting, there is still some small noise and hum. It's not much, but when measuring very small signal, very small hum and noise is not what you want. At zero signal there is still hum and noise visible, which comes from the 5A21 itself. So even when sensitivity is 4x lower, the HP130A works effectively much better. Because it is dead silent by itself, even when I use it with a normal multi meter probe cable, there is no visible hum at 200uV range. I need no probe, and there is no problem where to connect the shield clip to. Which is very convenient. If you ever shorted a high voltage circuit with the shield clip, you know what I mean. But it works also better. From this I know it is possible to measure like this, and I really like that method. It works actually with the 5A21 also, but not as good as with the HP130A.
I am afraid with R5030/5031 or 7A22, there will be no significant improvement over the 5A21. I quote this from Walter Shawlee's initial post about the 5030. "...operation down to 50uV/Div works very well, below that noise can be a real issue. Operation at 10uV would have to be into a very low impedance via a short shielded cable. They were maybe optimistic on that range".
Though I see the issue here perhaps just resulting from the way this was measured. I think with a normal probe and BNC cable, you can not use it at 10uV. Even when measuring against ground, at 10uV you have to connect the ground of the DUT to the negative input of the differential amplifier, and not to the scope ground. When I try that with the 5A21N, it fills the whole screen with hum. But when I connect ground to the negative input, the hum falls back to just a few uV. Perhaps Walter can say something about this.
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