I am using an AD8421 with a 100 ohm gain resistor. The datasheet for the AD8229 explains how to calculate the Johnson noise contributed by the gain resistor. I would also like to calculate the current noise contributed by the gain resistor, but I'm not sure how to do that. The underlying question is whether the extra cost of a metal foil would be justified on the basis of noise reduction, or whether a thin film resistor will provide comparable results. I need the maximum possible S:N ratio, but I don't want to use a $10 resistor unless it is clearly justified by lower noise.