Apple Hardware Engineer interview questions

Two phone interviews followed by the onsite interview. The onsite interview includes six rounds. The interview panel asked relevant questions. I traveled to CA from a different state to CA for interview. Apple does to cover taxi expense to airport. I had a nightmare to get reimbursed.

Was contacted by a recruiter via LinkedIn who thought my background at my previous employer was a great fit. Had a phone screen and he cut it short stating it went very well and invited me in for a "whiteboard session". The opening was for a hardware engineer position and I was told to "brush up on the fundamentals" which was very broad so I went back and reviewed some material relative to communication protocols like i2c, SPI, PCIe,USB etc and other signal integrity techniques as well as fundamentals like voltage division etc. The first thing they had me do was derive the gain formula for an inverting op-amp. Which caught me off guard as I would not be working in op-amps nor did I have any familiarity with them for over a year coming from a digital design background. Then we went into some signal integrity questions.