hi,

if you look at the circuit as the inputs of Op-amp have very high impedence (idealy infinity) no current flows in to the op-amp (-) input, all transducer current current flows thru the feedback resistance .

using ohms law
V=IR
for 20 ma to get 5 volts the feedback R would be


R=5/20x10E-3
=250 ohms


for bias current compensation same value is tied from the (+) input to the ground.

but be care full, the direction of the input current should be outside the inverting (-) input, other wise output will stuck at zero.


abhishek