Buffer OPAMP feedback resistor
The LT1630 lists a bias current up to 1000 nA (1 microamp).  This is the amount of current coming into both input terminals.  For a DC signal, the upper circuit will compensate for this error while the lower circuit could have an error contribution as high as 1 ua x 50 K (the signal impedance) or 50 mv.

The maximum input offset current is 150 nA, so you are contributing a worst-case 7.5 mV (150 nA x 50K ohms) of error by using a 50K feedback resistor.