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[linrad] OP amps and antialiasing was...a simple front end

Hi, John et al!

Thanks for the note.

The OPA 2227 is rated at 3 nV/root Hz.
Note that that is nano and not micro.

The AD797 is rated at 0.9 nV/Hz, or slightly less than 1/3 the noise of the OPA 2227.
The AD797 also has THD of -98 to -120 dB.

Other 'common'  op amp specs I am aware of include:

LM833 4.5 nV/ root Hz
OP275 6 nV/ root Hz
TL07x 18 nV/ root Hz in ascending order of noise.

So the OPA 2227 is good, but the AD797 is excellent.

I found the 797's very easy to use, and had no problems or instability.

I had no significant aliasing problems for two reasons:

1.  I used IF filters before going 'wideband' with the Delta44.

2.  I originally used ISA Soundblaster cards and then went to the Delta44, which has
a reasonable [not perfect] anti-aliasing filter built right in.

Here are Leif's comments on the Delta44 anti-aliasing filter, from:  

----start quote
The builtin filter of Delta44
The -3dB point of the builtin filter is at 46.3kHz and the attenuation is only 10dB
at the Nyquist frequency 48kHz. Adequate attenuation, -60dB is obtained at 52kHz
making the useful range 0 to 44kHz. 

Low pass filter with notch
Since the builtin filter is perfectly ok above 53 kHz the additional filter does not
have to attenuate above this frequency. To get a very steep filter around 48kHz
notches are used, see fig.1. Having the components at hand, the easiest way of
producing notches is to use LC series links. Filter design in the audio range is
conventional engineering and many other ways are possible to realise a filter with
notches slightly above 48kHz. The useful frequency range is increased by about 2kHz
by this filter and it is certainly questionable if it is worth the trouble of adding
it to increase the total bandwidth from 88 to 92kHz (4.5%) 
-----end quote

So you can do reasonably well with a Delta44 without additional filtering, but do get
a bit of extra bandwidth by adding external antialias filtering.

So, again, start simple and add on as you go.  This makes getting everything working
easier and less intimidating, and you get to see the benefits of the added complexity
as you create it.  And when a problem crops up, you have a pretty good idea of where
it is.  I find that fact particularly helpful!

For practical purposes, there were additional references to anti-aliasing filters on
Leif's site on my most recent previous listing, and the QRP site URL in that post
also had data that would enable one to start playing around with low-pass filters. 
These URL's were:



Below are some low-pass URL's that might [or might not] be of interest.




----- Original Message Follows -----
> Hi all,
>   Regards low noise. I have an I&Q with diode mixers running on 40/20m. I
> finally wound up with a TI part IIRC 2227 p/n or so. It has 1.3uv/root
> hertz noise. Was 10-20 db quieter than til082.
>   Regards Bandwidth into audio card. This is a MUCH more serious problem
> than it first appears.
>   Older ISA audio cards, no problem. They all seem to have good
> anti-aliasing filters. 
>   On many PCI (3 or 4) that I have tested, there is *NO* anti-aliasing
> filter on the board, *AT ALL*. Motherboard audio seems to be particularly
> bad this way. 
>   Given the difficulties of building discrete audio filters, finding a
> board with real filters in whatever chip is probably the easiest approach. 
>   That said, at the higher rates (96k and 192k) the chip filters have been
> designed with *MUCH* too high a cutoff frequency for use in I&Q designs. 
> This limits a 192k part to quite a bit less. 
>   So far I have not found an easy workaround, though an additional digital
> filter on the output of the ADC appears possible.
>   warm regards,
>   john, ni1b

Roger Rehr