input impedance of THAT 1512 IC?

[tempus fugit]

Hey all;

I've been reviewing the THAT 1512 data sheet
(http://www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf) 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?

Thanks

 

[whit3rd]

On Jul 4, 1:37 pm, "tempus fugit"
<tocc...@quitspammingme.ciaccess.com> wrote:

I've been reviewing the THAT 1512 data sheet
(http://www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf) and am
trying to figure out what the input impedance would be for the circuits

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).