input impedance of THAT 1512 IC?

Hey all;

I've been reviewing the THAT 1512 data sheet

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