Transimpedance amplifier
When Vin+ = 0V then Vin- must also = 0V 

Vout = I*R  always if not saturated.

When SW1=closed I source drives the output = Vout/Rf so that Vin-input stays at 0V.

For this to work well with high R  values, Op Amp Input bias current Iib = ? must be low.  FET types are very low.  

Tp1 voltage = Iib * R   e.g. 

for Iib=10nA, R= 100k


10nA * 100k= 1mV output.