A photoplethysmogram circuit, configured to control a light source and N photoelectric converters to sense biological characteristics of an object under test, wherein N is an integer greater than 1, and the PPG circuit comprises: a transmitting channel, configured to control the light source to perform light emitting operations during pulse repetition cycles; K receiving channels (104_1~104_K), wherein K is an integer greater than 1, and the N photoelectric converters are divided into K sets of photoelectric converter sets (110_1~110_K), the K receiving channels (104_1~104_K) respectively correspond to the K sets of photoelectric converter sets (110_1~110_K); and a controller, configured to control the PPG circuit to operate in a partial sampling phase or a full sampling phase, when the controller controls the PPG circuit to operate in the partial sampling phase, the controller activates J receiving channels of the K receiving channels (104_1~104_K) as current receiving channels and activates J sets of photoelectric converter sets of the K sets of photoelectric converter sets (110_1~110_K) corresponding to the J receiving channels to sense received light for performing the sampling operation, so as to generate J biological characteristics sampling results (D R) during each of the pulse repetition cycles, wherein J is smaller than K; and when the controller controls the PPG circuit to operate in the full sampling phase, the controller activates all receiving channels of the K receiving channels (104_1~104_K) and activates all of the K sets of photoelectric converter sets (110_1~110_K) to sense the received light for performing the sampling operation, so as to generate K biological characteristics sampling results (D R) during each of the pulse repetition cycles, and from the K receiving channels (104_1~104_K), re-select J receiving channels as the current receiving channels to be activated during the next partial sampling phase according to the K biological characteristics sampling results (D R) generated during each of the pulse repetition cycles of the full sampling phases; wherein the controller controls the PPG circuit to operate in the full sampling phase every preset update interval, and the preset update interval is greater than the pulse repetition cycle. a transmitting channel, configured to control the light source to perform light emitting operations during pulse repetition cycles; K receiving channels (104_1~104_K), wherein K is an integer greater than 1, and the N photoelectric converters are divided into K sets of photoelectric converter sets (110_1~110_K), the K receiving channels (104_1~104_K) respectively correspond to the K sets of photoelectric converter sets (110_1~110_K); and a controller, configured to control the PPG circuit to operate in a partial sampling phase or a full sampling phase, when the controller controls the PPG circuit to operate in the partial sampling phase, the controller activates J receiving channels of the K receiving channels (104_1~104_K) as current receiving channels and activates J sets of photoelectric converter sets of the K sets of photoelectric converter sets (110_1~110_K) corresponding to the J receiving channels to sense received light for performing the sampling operation, so as to generate J biological characteristics sampling results (D R) during each of the pulse repetition cycles, wherein J is smaller than K; and when the controller controls the PPG circuit to operate in the full sampling phase, the controller activates all receiving channels of the K receiving channels (104_1~104_K) and activates all of the K sets of photoelectric converter sets (110_1~110_K) to sense the received light for performing the sampling operation, so as to generate K biological characteristics sampling results (D R) during each of the pulse repetition cycles, and from the K receiving channels (104_1~104_K), re-select J receiving channels as the current receiving channels to be activated during the next partial sampling phase according to the K biological characteristics sampling results (D R) generated during each of the pulse repetition cycles of the full sampling phases; wherein the controller controls the PPG circuit to operate in the full sampling phase every preset update interval, and the preset update interval is greater than the pulse repetition cycle.