Write a VBA code to calculate significant digit for an approximation. Specific requirements:

1. Use two input boxes to acquire the input for true value and approximation value, respectively.

2. Calculate relative error.

3. If relative error is equal or greater than 0.5, pop up a message "Error is too much! No significant digit!".

4. If relative error is less than 0.5, calculate the approximation significant digit and present it in a message box.