Wow, this is a very interesting thread. My own results without auto zero below. I have a coupla questions:
@Dr. Frank
1) - presumably it was the upward tip of the graph which tipped you off to the need for auto zero?
2) - if we compare the non-azero vs the a-zeroed results, presumably that gives a measure for the instrument/ref flicker noise? Would you guess this is short term temperature fluctuation or something else?
3) - did you get your noise spec from reading values off the graph in the datasheet or from somewhere else? (I read mine off the graph)
4) - the split between the spec and the non-auto-zeroed results seems to occur earlier for 100mV and 1/100V ranges. Any thoughts?
5) - (also) does it matter that you're not triggering off the PLC? i.e. you can trigger anywhere in the cycle?
6) - (also) does it matter (re APER and NPLC) that the line frequency isn't an integer at any particular moment or is APER time effectively defined on a line frequency basis (I guess I could RTFM for this one)?
Alan
Hi Alan,
1) Answer:
I used AZERO OFF only because I wanted to use DIGITIZE for small aperture times, i.e. getting data at the fastest possible rate, w/o dead time.
My measurements never showed an upward tendency for NPLC > 1, and my results are in good accordance with the 3458A specification, i.e. that the noise is going down for longer integration times.
I also did not get extraordinary better noise figures than the specification.
In the very beginning of this thread, I already made a comment, to avoid acquiring too many data especially for longer integration times. This also causes measurement periods which are also much too long, causing gain drifts of the 3458A.
In the region of NPLC >1 you already reach 1ppm noise level, and 0.001ppm for NPLC 1000!
The instrument has a gain drift of about 0.5ppm/K for the 10V range, and 1.2ppm/K for 1V and 100mV.
ACAL does NOT apply here!
Ambient temperature changes of a few tenths of °C therefore can easily cause drifts of many tenths of ppm.
And that's exactly, what you may see in your measurements.
Therefore I limited my measurements to a few seconds per run, where possible.
Also, the ambient temperature in my basement may be stable to 1/10°C over hours.
To extract drift from noise, you may use the modified Allan distribution.
2) I made no measurements with AZERO ON, only for NPLC 1000, where the offset drift already affected my noise measurements, equivalently to the explanation above.
You may identify different noise sources by the different time scales.
3) the standard 3458A noise graph originates from the specification
4) which split? can you describe more precisely what you suppose?
5) For NPLC 1, 10, 100, 1000 all measurements are per definition in phase with the line frequency, or in other words, always over multiples of 20msec.
For NPLC < 1, sampling is of course over fractions of one line cycle, and there is no synchronization to the line frequency, so you may of course pick up additional noise.
It doesn't matter, because the pick up noise seems to be minor compared to the other noise sources.
6) I do not understand that question. The situation is as explained in 5)
Frank