HEART RATE DATA COLLECTION

One innovative aspect is directed to heartrate data collection. In some implementations, a circuit includes a light detector for generating a detected signal based on received light. The circuit includes a switching circuit configured to receive a first signal based on the detected signal and to switch among a first and a second configuration. In some implementations, the circuit includes a first and a second sampling circuit for sampling a value of the first signal when the switching circuit is in the first configuration and second configurations, respectively. In some implementations, the circuit includes an ambient light cancellation circuit for countering a first component of the first signal while the first switching circuit is in the first configuration. In some implementations, the circuit includes an adjustable gain circuit for adjusting a gain of the first signal while the first switching circuit is in the first configuration.