- This work is related to klog#20658, klog#20667, and klog#20682.
- Low frequency noise was able to be removed by using a dedicated power supply unit for AA.
- GND of DC18V seems to be dirty.
- Next step is to find that a cause exists on the power supply unit (upstream of 18V power strip) or on the vacuum tank side (downstream of 18V power strip).
Because I wasn't able to reproduce the noise problem on the test bench with the combination of ADC and AA which seemed to be bad ones,
I did some trials in the mine again.
At first, I checked changes in ADC noise level by connecting GND of DC18V and DC24V.
Before this work, there was ~500ohm between GND of DC18V and DC24V. Voltage between two GNDs was ~70mV.
Both GNDs (at the center pin of the power strip) were connected to the digital rack.
Though I measured and compared ADC noise before and after this change, there was no improvement as shown in Fig.1 ~ Fig.4.
(Reference plots represent spectra before connecting GNDs.)
As the next, a dedicated power supply unit for AA chassis were installed.
AA chassis was unmounted from digital rack at this time.
So AA chassis should be completely separated from the original power supply unit for DC18V.
In this case, noise level was drastically improved as shown in Fig.5 ~ Fig.8.
This noise level is roughly same one in unplugging the SCSI cable between AA chassis and ADC.
And also, I tried to connect a common GND of the dedicated power supply unit to the GND pin of the power strip.
In this case, ADC noise got louder even if AA chassis was driven by the dedicated power supply unit (See Fig.9 ~ Fig.12).
According to these tests, It seems to be better to not connect AA chassis to current GND of DC18V.
I'm not sure GND was contaminated at the power supply unit or by connecting a vacuum tank via many signal cables yet.
Because I had no enough time to check this fact, I'll do that in next week.