I was contacted by a recruiter via LinkedIn who thought my background at my previous employer was a great fit.

I had a phone screen, and the recruiter 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 reviewed material relative to communication protocols like I2C, SPI, PCIe, USB, etc., and other signal integrity techniques, as well as fundamentals like voltage division.

The first thing they had me do was derive the gain formula for an inverting op-amp. This caught me off guard, as I would not be working with 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.

Draw an op-amp circuit and derive the gain formula.