An optical heart rate sensor comprising: light source circuitry comprising: a light emitting diode to output a source light signal to a target measurement site, and a current pulse driver communicatively coupled to the LED to modulate the source light signal output from the LED on and off at a predetermined frequency; and detector circuitry comprising: a light detecting photodiode connected to a bias voltage, the light detecting photodiode configured to generate a detector current responsive to detecting, when the LED is on, a target light signal reflected or scattered from the target measurement site in response to the modulated light signal and a background light signal, and detecting, when the LED is off, the background light signal; a bank of switches (320, 322, 324, 326) communicatively coupled to the photodiode; and a capacitor communicatively coupled to the bank of switches (320, 322, 324, 326); characterized in that: the bank of switches (320, 322, 324, 326) is configured to switch on and off in response to pulse signals of the current pulse driver in opposite phases to generate an integrator signal representative of the target light signal with the background light signal substantially removed based on charges integrated on the capacitor by the generated detector current during switch cycles; wherein a first pair of switches (320, 326) of the bank of switches (320, 322, 324, 326) are actuating in phase to connect, when switched on, a first input of the capacitor with the bias voltage via the light detecting photodiode, and connect, when switched on, a second input of the capacitor with a supply voltage (Vdet); and wherein a second pair of switches (322, 324) of the bank of switches (320, 322, 324, 326) are actuating in counter-phase with the first pair of switches (320, 326) to connect, when switched on, the second input of the capacitor with the bias voltage via the light detecting photodiode, and connect, when switched on, the first input of the capacitor with the supply voltage (Vdet) when switched on. light source circuitry comprising: a light emitting diode to output a source light signal to a target measurement site, and a current pulse driver communicatively coupled to the LED to modulate the source light signal output from the LED on and off at a predetermined frequency; and a light emitting diode to output a source light signal to a target measurement site, and a current pulse driver communicatively coupled to the LED to modulate the source light signal output from the LED on and off at a predetermined frequency; and detector circuitry comprising: a light detecting photodiode connected to a bias voltage, the light detecting photodiode configured to generate a detector current responsive to detecting, when the LED is on, a target light signal reflected or scattered from the target measurement site in response to the modulated light signal and a background light signal, and detecting, when the LED is off, the background light signal; a bank of switches (320, 322, 324, 326) communicatively coupled to the photodiode; and a capacitor communicatively coupled to the bank of switches (320, 322, 324, 326); characterized in that: the bank of switches (320, 322, 324, 326) is configured to switch on and off in response to pulse signals of the current pulse driver in opposite phases to generate an integrator signal representative of the target light signal with the background light signal substantially removed based on charges integrated on the capacitor by the generated detector current during switch cycles; a light detecting photodiode connected to a bias voltage, the light detecting photodiode configured to generate a detector current responsive to detecting, when the LED is on, a target light signal reflected or scattered from the target measurement site in response to the modulated light signal and a background light signal, and detecting, when the LED is off, the background light signal; a bank of switches (320, 322, 324, 326) communicatively coupled to the photodiode; and a capacitor communicatively coupled to the bank of switches (320, 322, 324, 326); characterized in that: the bank of switches (320, 322, 324, 326) is configured to switch on and off in response to pulse signals of the current pulse driver in opposite phases to generate an integrator signal representative of the target light signal with the background light signal substantially removed based on charges integrated on the capacitor by the generated detector current during switch cycles; wherein a first pair of switches (320, 326) of the bank of switches (320, 322, 324, 326) are actuating in phase to connect, when switched on, a first input of the capacitor with the bias voltage via the light detecting photodiode, and connect, when switched on, a second input of the capacitor with a supply voltage (Vdet); and wherein a second pair of switches (322, 324) of the bank of switches (320, 322, 324, 326) are actuating in counter-phase with the first pair of switches (320, 326) to connect, when switched on, the second input of the capacitor with the bias voltage via the light detecting photodiode, and connect, when switched on, the first input of the capacitor with the supply voltage (Vdet) when switched on.