MSO X4024A strange meas values

[klaus1311]

Hello,
my name is Klaus and this is my first time I write in the this forum. I have a Keysight MSO X4024A. While I was working with it, I stumbled over a strange display value when using the "meas" option and only DC or no signal is connected. Depending on the atternuator setting it shows different voltages:
Att 50 V = 5 - 7V
Att 20 V = 2.7V
and so on, it's roughly 1/2 of the previous value.
Can anybody confirm this behavior, or is this just a problem of my scope?

My current FW: 04.08.2016071800

Klaus

[Keysight DanielBogdanoff]

Hi Klaus,

The noise of the scope (and any scope, generally) is directly related to the attenuation settings of the front end. Typically, the noise on the scope is a combination of the signal offset and the screen scaling. For the 4000 X-Series, page 24 of the data sheet specs the "DC vertical gain accuracy" as +- 2% of the full scale (assuming 1:1 probing attenuation). Then you have +-.1 divisions of "DC vertical offset accuracy"
.
The reason it's like this is that the scope's front end attenuates the signal for the ADC in such a way that the ADC gets similar voltage ranges regardless of the front end's attenuation settings. This is why when you go from 50 to 20 your noise % is roughly the same.

This is a pretty common question, let me know if I didn't explain well enough!

-Daniel