input impedance of THAT 1512 IC?

Hey all;

I've been reviewing the THAT 1512 data sheet

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and am trying to figure out what the input impedance would be for the circuits they've given.In Fig. 4 on p. 6 for instance, would the input impedance be

1K, or 1K||1K = 500 ohms, or 2K, or something else altogether? What would it be for the T bias configuration in Fig.5 on p. 7? They also use the expressions 'differential' and 'common mode' impedance. When desgning a mic pre, which of these should be my primary concern?


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tempus fugit
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It's 'high' impedance, but you want to look at the current noise and voltage noise to determine what effect the source impedance has on your circuit performance. Too high source impedance causes the current noise to grow; too low source impedance will cause voltage noise to dominate as the signal voltage drops. Something in the vicinity of 4k ohm is what I'd expect (because I did the full calculation once on another op amp).

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