http://cds.linear.com/docs/en/application-note/an124f.pdf
775 Nanovolt Noise Measurement for A Low Noise Voltage Reference
by Jim Williams
Happy reading!
Hello,
as we have already discussed in this forum:
the bandwith of the AN124 is lesser than 0.1 .. 10 Hz
lower bandwidth -> lower measured noise.
E.g. for the high passes:
1) 1300uF * 1200R gives 0.1Hz lower bandwith.
2) 165uF * 10K = 0.1 Hz lower bandwidth.
3) root sum square correction = 0.1 Hz lower bandwidth
All series gives around 0.17 Hz lower bandwidth (or -9 dB instead of -3 dB at 0.1 Hz) and so on.
Sorry thats not a good example for a well designed cirquit.
with best regards
Andreas
Yes it is a direct answer. The DUT is the LTC6655
I think it is intentional:
" Figure 6, taken at the circuit’s oscilloscope output, shows 160nV 0.1Hz to 10Hz noise in a 10 second
ample window. Because noise adds in rootsum-square fashion, this represents about a 2% error in the LTC 6655’s expected 775nV noise fi gure. This term is accounted for by placing Figure 3’s “root-sum-square correction” switch in the appropriate position during reference testing. The resultant 2% gain attenuation fi rst order corrects LTC6655 output noise reading for the circuit’s 160nV noise fl oor
contribution."
He is mentioning that the caps need 24h because the dielectric absorption. So I guess it took him long time to do all this. But in fact, re-reading it, I've noticed this:
"Figure 8 is LTC6655 noise after the indicated 24-hour
dielectric absorption soak time. Noise is within 775nV
peak-to-peak in this 10 second sample window with
the root-sum-square correction enabled. The verifi ed,
extremely low circuit noise floor makes it highly likely
this data is valid."
So he is saying that in a single 10s sample the noise is less than the datasheet value, with root sum square correction?
I'm seeing some 700nV peak to peak noise on figure 8. And he did strip chart recording to verify that the circuit is not noisy, but there are no strip charts for the LTC6655???
Maybe he is telling us that the datasheet values are wrong? I mean most of the application note is "hey look, my circuit works!".
OK, I looked up the qoute at the end.
"It ceased; yet still the sails made on
A pleasant noise till noon,
A noise like of a hidden brook
In the leafy month of June,
That to the sleeping woods all night
Singeth a quiet tune."
This is too amusing. Maybe we should forget about that 0.25ppm noise.