Wow, good article, but also very good first reply! In simulation and theory you need to take things into account explicitely. Whereas in reality "unexpected" things just happen. And indeed the proposed methods are not as good in the presence of impedance unsymmetries as what THAT has done. On the other hand the well-known 3-opamp solution is also good, and using 2 buffers instead is not that much a saving, but all in all THAT offers a real good practicable solution.
There's so much to comment on here! Perhaps the most glaring omission from your discussion is the importance of extremely high common-mode input impedances (why do you think transformer inputs work so well when diff-amps perform miserably at noise rejection?). They are the key to consistently high CMRR in the real-world where signal sources have significant impedance imbalances (I convinced the IEC to change its CMRR test because testing with a lab-generated zero imbalance source proves nothing). The InGenius IC manufactured by THAT (and patented by me) overcomes this very serious issue. You, like many practitioners, obsess over the non-match of single-ended input impedance for the ordinary diff-amp. It is a trivial issue - what matters is that the common-mode input impedances match - and they do (drive both legs to the same voltage and you'll see that the input currents in the two legs match). The asymmetry of input voltage with non-zero source impedances affects only headroom and crosstalk if cabling is unshielded. Concerning single-ended (unbalanced) sources: a shielded twisted-pair cable, with diff-amp "low" grounded ONLY AT THE SOURCE and the unbalanced signal to diff-amp "high" will eliminate common-impedance coupling in the unbalanced cable. The "benefit" of being able to drive only one leg of the diff-amp input and ground throws away at least 30 dB of noise rejection. There's lots more to criticize here, but you should sit in on one of my free tutorials at the next AES convention (or CEDIA or InfoComm) or read the chapter I wrote in Ballou's handbook or just visit the Jensen website and read the generic seminar handout there. It may be a real "eye opener". - Bill Whitlock, President & Chief Engineer, Jensen Transformers, www.jensen-transformers.com, AES Life Fellow, IEEE Life Senior
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