Difference between Phase calculated using Band Math option and Multilook operation

I am trying to create a DEM using two SLC images (ERS-1/2 tandem). My workflow in as follows.

1. Apply orbits
2. coregister
3. interferogram generation
4. phase filtering
5. multillook
6. snaphu export and then import after phase unwrapping
7. phase to elevation

At the 5th step, I am not selecting any source bands. My understanding is that SNAP will multilook i, q bands and then it calculates phase from them.
I am also doing it in another way by selecting i, q, coherence as source bands (when multilooking) and then calculating phase using band math option. This way I observe some visual difference in the phase band when compared to above step. Also, when I tried to export the product (obtained using band math option) using Snaphu Export, it is not working. It gives me an error something like “wrapped phase band required”.

So, how phase calculated using band math option is different from phase calculated using multilook operation?

How did you use band math?

I am writing “phase(i,q)” expression in band math editor. Unchecked Virtual option.

I’d like to draw your attention that the fifth step of Multilook is not needed and you could jump to the sixth step.

Why ML is not needed? I have seen some tutorials in which they multilooked the image before phase unwrapping.

Please take a look once more at these tutorials and the step sequences are followed in, So what is the step before ML?, and what is the step after?, or ML was mentioned as an individual case.

To create a ML is totally correct in the sequence of InSAR, but not using SNAP, I’m talking using GAMMA software to get the average and then corregister the images.

Do you mean you are multilooking master and slave images and then performing coregistration?

No, there in GAMMA the sequence processing a bit different to what we use in SNAP, That is why I don’t see any necessity of applying ML in InSAR Processing’s sequence using SNAP.

Take a look at this please

Thanks for the information @falahfakhri
I have unwrapped the filtered interferogram without ML. After importing to SNAP, my unwrapped phase does not seem to be correct. What might be the problem in this case?

Please take a look at this

and at this