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Nanovoltmeter internal filtering

Posted: August 7th, 2015, 9:44 am
by jjanagnost
We have several 2182A's that we use for precision measurements. My question is about the internal filtering on the 2182A. The users guide states that when the analog filter is on then an 18 Hz low pass filter is used on the incoming data. Hence if I set the PLC to be 1 (60 Hz) then I'm sampling a 18 Hz bandlimited system at 60 Hz, so no aliasing which is great. Thus, if I collect the 60 Hz data and then do my own filtering down to 1 Hz, i.e., digitally filter the 60 Hz down to 1 Hz in Matlab, say, am I missing anything compared to having the 2182A do the filtering and output at 60 PLC?

Along these same lines, if I do not put the analog filter in the loop, I assume there is some anti-aliasing filter in the 2182A? For example, if I select 0.1 PLC say, then I'm sampling at 600 Hz, so I would like to think there is some sort of anti-aliasing filter at 200 Hz or so to ensure the instrument doesn't alias high frequency content into the passband.

Thanks a bunch. This is an extremely capable and highly recommended product, but I want to make 100% sure I understand exactly what the instrument is doing so I can interpret the data correctly.

Re: Nanovoltmeter internal filtering

Posted: August 7th, 2015, 12:52 pm
by Dale C
First, the 2182A does not produce data at 60 Hz when the PLC is set to 1. There are two 1 PLC measurements per reading, plus overhead and backgrounding. I forget the exact number (it's on the datasheet), but you end up with ~23 measurements per second if Front Autozero and Autozero are both on (default settings).

The analog filter is for systems expecting large amounts of >= 60 Hz noise that would otherwise swamp the gain of the amplifiers. It's not recommended to use the analog filter unless that is the case -- you're better off with a digital filter (either using the DFILT, or with post-processing).

Regarding anti-aliasing, it should be noted that the 2182A uses an integrating A/D, not a sampling A/D. Such a filter is not necessary in this case, as near-continuous integration is occurring within the unit.