A photonic analog-to-digital conversion (PADC) based on intensity-to-frequency conversion using a frequency chirp in a semiconductor optical amplifier (SOA) is proposed. The presented PADC has a simple scheme whereby optical quantization is achieved using a single SOA with multiple rectangular bandpass filters placed in parallel. In this Letter, we successfully achieve 10 GSamples/s, eight-level optical quantization using a quantum-dot SOA. The PADC also has much lower input signal pulse power requirements for optical quantization, compared with conventional PADCs.