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Old 27th November 2009, 01:10 PM   #1
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Default the stupid question series: AC coupled balanced/unbalanced output, servo useful?

Continuing on my list of stupid questions regarding instrumentation applications...

When designing an AC-coupled output driver that needs to work in a balanced as well as unbalanced mode (ie, occasionally one output will be tied to ~ground or more likely the output impedances of the two legs will be unequal), is it sufficient to simply use two buffers with complementary inputs and thus complementary outputs or would that result in an output that's attenuated by 6dB when one output is tied to ground? Or would one use a servo output to compensate for the AC impedance of one half being taken to near zero ohms?

Thanks in advance!
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Old 29th November 2009, 10:30 AM   #2
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The answer is "yes. The servo is required to prevent attenuation or other weird effects on [severe] impedance mismatch" which is what I thought.

...so why is it usually called a DC servo when it appears that the real value is coupling AC mismatches into the feedback not DC?
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