Talk:Common-mode rejection ratio

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741 is a "good one"? — Omegatron 17:03, 21 February 2006 (UTC)

I think "good one" means "desirable characteristics" -- the CMRR of the LM741, found at http://www.national.com/ds/LM/LM741.pdf shows it to be around 90dB optimal. I've re-worded it, though. Mystic Pixel 08:17, 16 March 2006 (UTC)

[edit] Formula

where is Vo in the formula 1? And what is Ad and As? Please explain the formulas in more detail! —Preceding unsigned comment added by Rdenooij (talk • contribs) 26 Feb 2007

[edit] Equation

Originally the equation specified the meaning of Ad and As:

where A_d is the differential gain

and A_s is the common-mode gain

Why was this changed to a single equation?

This isn't as clear about what the terms mean or how they would be measured. And why does it have a 1/2 term? — Omegatron 15:12, 26 February 2007 (UTC)

This is what I understood from the -- somewhat obscure -- explanation that was originally on the page: As is the amplification factor for a voltage Vsin that is on both inputs. So if V+ = V- = Vs, then Vs = (V+ + V-)/2. It could be that the original definition was wrong, I didn't check that. I agree that the equation looks kind of asymmetric with the factor 1/2. It would make 6 dB difference (on typically 90 dB) if you leave out the factor 1/2. Han-Kwang 18:36, 26 February 2007 (UTC)

Oh, I see. It depends on the circuit model you are using. [1] [2] — Omegatron 00:42, 3 March 2007 (UTC)

Moreover, I don't think the original eqs are "as clear about what the terms mean or how they are measured". Vo has a different meaning in the 1st and 2nd and it is tempting (but utterly incorrect) to cancel out Vo when you calculate Ad/As. And if I look at your ref.2, the equation would be

V_o = A_d(V ₊ − V ₋ ) + A_s(V ₋ ),

which is mathematically equivalent to

Since 1-As/2Ad is approximately 0.99998 for a typical setup, these two definitions are equivalent for all practical purposes (the nonlinearity in Ad is probably much larger than the difference anyway). I am therefore removing the inaccuracy tag from the article. Han-Kwang 11:46, 7 April 2007 (UTC)

I don't think the lack of a schematic warrants the {{accuracy}} tag, Omegatron. I think you should actually explain what part is inaccurate if you are to use that tag. I have explained here why I believe the current description is correct. Moreover, adding an electric circuit diagram would restrict the CMRR description to a specific type of circuit (such as an ideal opamp wired to behave as a low-gain voltage substractor), while the concept applies to a much more general class of devices with bi-linear inputs. Han-Kwang 19:56, 11 May 2007 (UTC)

[edit] Definition (why 20-log?)

Given that a decibel is a tenth of a Bel, why is the defintion 20log.. and not 10 log..?
--83.105.33.91 09:46, 24 September 2007 (UTC)

See decibel. With voltages, it is a factor 20, because the dissipated power (which is what dB is about) is proportional to the voltage squared. Han-Kwang (t) 11:30, 24 September 2007 (UTC)