Reason for brief spike in op amp output when crossing 0V?
What you're seeing is a mismatch of the turn-on/turn-off time of the msb compared to the other bits.

Pretend for a moment that this is an 8-bit DAC, sitting at code 01111111. If the next code were 1000000, you'd get a well-behaved step. But what's happening is that internally the msb responds a bit quicker than the other bits. This means that the transition is actually 0111111 to 1111111 to 10000000. The 11111111 code only persists for a very short time, but while it's there the output tries to go to full-scale. Since the mismatch time (called the bit skew) only lasts for a very brief time you just get a relatively small pulse (commonly called a glitch). This effect is actually quite important in video DACs, since the eye is very sensitive to this sort of thing, and an important characteristic for video DAC is total glitch energy.