3 dB bandwidth

Older please Mike, O L D E R. :-)

"Arguing with anonymous strangers on the Internet is a sucker's game because they almost always turn out to be -- or to be indistinguishable from -- self-righteous sixteen-year-olds possessing infinite amounts of free time."

-Neil Stephenson, _Cryptonomicon_

IMO, CMRR is an example of the use of dB which has no technical justification, merely used as a convenient way of printing large numbers in specs. If you use a diff amp and want to see the effect of CMRR the first thing you have to do is unconvert it anyway.

Here's what an Analog Devices publication (edited by Dan Sheingold no less) said about the accepted practice of using 20*Log(Variable1/Variable2) for convenience.

" .... The term has been widely corrupted to express log ratios of any two quantities (even engineers' salaries), by the definition dB = 20*Log(Q1/Q2). Though confusing (some would say deplorable), it is almost universally accepted. "

It may be universally accepted in some quarters, but it is still a loose and inaccurate way to spec something, that may lead to confusion and errors.

"Tony Williams" a écrit dans le message de news: snipped-for-privacy@ledelec.demon.co.uk...

For engineers' salaries it should be dB = 10 log(Q1/Q2) (money is power)

On Mon, 27 Jun 2005 11:14:27 +0100, Tony Williams wrote in Msg.

The most useful and unambiguous way of expressing large numbers is with mantissa and exponent. There really is no reason to use dB at all except a historic one. A twentieth of an order of magnitude may be a handy unit at times though.

You always have to.

robert

Forget about AoE and write an alternative to Pallas-Areny/Webster's Sensors and Signal Conditioning...

On Mon, 27 Jun 2005 07:25:33 -0700, Richard Henry wrote in Msg.

About 20 dB.

robert

Oops- looks like I invented my own fundamental unit of gain dBG's...=20Log(gain/gain).

I was going to make that comment earlier, but didn't feel quite that cheeky. ;-)

OMG !!!!!!

That's what dBs *are* you clown ! Merely a convenient way of representing large ( or small ) number ratios.

dBs are simply *ratios* ! Would you prefer the spec sheet to say

Good God !

Maybe it confuses the terminally brain dead ?

Graham

Nonsence.

If you have a system with several series sections containg gain ( or loss ) all you have to do is add the dBs together and you have the total gain. Easy.

Now try multiplying all those mantissas and adding the exponents. Less convenient.

Like I've said all along, dBs are there as a mathematical convenience. They are also readily 'graspable' wrt their meaning IME too. I don't see them going out of fashion and consumer equipment spouting 3.10^2 style panel markings !

Graham

Call it the dBlogg...... you cheeky bugger. :-)

I'm sorry, but I really don't know what that means.

That's an interesting approach to a scientific/engineering subject.

But you should save all that vehemence for when you will really need it: ... to convince an irate customer and the Trading Standards authorities that your product is fine because it meets your definition of electrical units, even though it obviously doesn't meet theirs.

I disagree completely. It far easier, in my book to go 80dB, or 120db then 10000000. There are less zeros to count and confuse one with.

I "think" in dBs for CMRR. I can convert dBs reasonable well in my head and do it all the time. Its the standard. Only those not really familiar with the use has an issue. Its like anything else. It takes time to think in French if your English, it doesn't mean that French is a harder language than English.

Just goes to show that in modern times, its easy to get well known, yet have no more valid opinion than any other unknown. dB's are truly an excellent way to express ratios of large numbers.

Its down to my other favourite pastime. To wit, evolution. In the past being well known was an indication of being a "fitter" individual, therefore valued. Today, fame means absolutely nothing about the value of ones opinions. Communication is all to easy now.

No it isn't confusing or inaccurate at all. Its clear and precise as what it means. It means what it says above. 20Log(Q1/Q2). Period. Not a lot can be simpler and clearer.

It can only lead to errors if one doesn't understand what a ratio means, to wit, non of this, it has an output of 23db stuff.

You need to move on mate.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

Bell Labs definition is *not* the standard. Trading Standards would laugh at an attempt to refer to it.

The standard is set by the AES. They even introduced the dBu ( voltage reference unit = 0.775 V ) to replace the dBm ( 600ohms ) so that modern voltage matched audio circuits could be correctly specified without having to refer to a trashy old power-impedance related unit.

Do you perhaps also think that all audio is still done with toobs ?

Now grow up and read some *new* books !

Graham

That's Keithley's call. Perhaps the susceptibility to binding atmospheric impurities ( = the bad stuff) is reduced. Perhaps you can research the subject and let us know.

Some manufacturers seem to be well aware of this...

Before you go any further Graham, mouthing-off with personal insults, (such as "clown" and "brain dead") you might find it instructive to recalculate exactly what 130dB is in linear terms.... you've just proved the exact point, that the unwarranted use of dB can lead to errors.

Then tell me exactly what is clowninsh and brain dead about specifying a CMRR as the straightforward linear value of (say) 1E6 instead of 120 dB.

Could you let me have the name or number (or URL) of the standard you are referring to.