Log transformation of both sides

Dear NMusers, I am working on a one compartment model with additive and proportional error model. Y = F*(1+EPROP*EPS(1))+EADDS*EPS(2) When I performed VPC, I found some 2.5% percentile of the concentration CI is negative. I guess it may be due to the concentration is very low after several t1/2, and the additive error on top of the low DV values causes the negative values. So I'd like to take "log transformation both sides" approach to avoid the issue. I wonder if any one can comment on the differences of the following methods: 1. Generate a ln(DV) column in the dataset, and use Y =LOG(F*(1+EPROP*EPS(1))+EADDS*EPS(2)) in the $ERROR block. 2. Use DV in the model, and code the $ERROR block Y1=LOG(F*(1+EPROP*EPS(1))+EADDS*EPS(2)) Y=EXP(Y1) Are these two methods equivalent to achieve log transformation of both sides, or they are different? Thanks for your help. Norman