Four-stage circuit architecture for detecting pulsed signals

1. An electrical circuit, comprising:

a photodiode that receives a light signal from a light source and generates a photocurrent signal;

a first stage comprising a trans-impedance amplifier that amplifies the photocurrent signal and generates a low noise signal;

a second stage comprising a high pass filter that converts the low noise signal into an alternating current (AC) signal having a positive amplitude, a negative amplitude, and a zero cross-over point between the positive amplitude and the negative amplitude;

a third stage comprising:

a positive integrating amplifier that receives the positive amplitude of the AC signal and generates a positive integrated value over an integration period; and

a negative integrating amplifier that receives the negative amplitude of the AC signal and generates a negative integrated value over the integration period; and

a fourth stage comprising at least an analog-to-digital converter (ADC) that receives the positive and negative integrated values.

2. The electrical circuit of claim 1 , wherein the trans-impedance amplifier includes an operational amplifier with a feedback loop comprising a feedback capacitor and a feedback resistor and a low pass filter.

3. The electrical circuit of claim 2 , where at least some components of the first stage, the second stage, and the third stage are programmable.

4. The electrical circuit of claim 1 , wherein the light source is a synchronous light source.

5. The electrical circuit of claim 4 , wherein a pulse of the synchronized light source has a duration of τ, a bandwidth of the second stage is of the order of approximately 1/τ, and a corner frequency of the high pass filter is set as high as 0.5/τ.

6. The electrical circuit of claim 1 , wherein the second stage includes an AC source and a capacitor that provides AC coupling.

7. The electrical circuit of claim 1 , wherein the third stage includes a switch configured to change from the positive integrating amplifier to the negative integrating amplifier at the occurrence of the zero cross-over point.

8. The electrical circuit of claim 7 , wherein the switching action of the switch is configured by a timer.

9. The electrical circuit of claim 8 , wherein the timer is configured to provide a clock synchronized with a frequency of the light source.

10. The electrical circuit of claim 1 , wherein the negative integrated value is subtracted from the positive integrated value by the ADC.

11. The electrical circuit of claim 1 , wherein the fourth stage includes a differential amplifier that subtracts the negative integrated value from the positive integrated value.

12. The electrical circuit of claim 1 , wherein the electrical circuit can substantially simultaneously provide: low-noise, high dynamic range, high ambient light rejection, high signal extraction of the light source, low-power operation, and different sampling modes.

13. The electrical circuit of claim 12 , wherein the electrical circuit can further substantially simultaneously provide sampling of multiple channels for analog-to-digital conversion.

14. The electrical circuit of claim 1 , further comprising a photoplethysmography system connected thereto.

15. The electrical circuit of claim 14 , wherein light from the light source reflects off, or transmits through an organ before reaching the photodiode, such that the photocurrent signal from the photodiode can provide an indication of a measurement of the organ.

16. The electrical circuit of claim 14 , wherein the photoplethysmography system does not have to include a light blocking enclosure to keep out ambient light for accurate measurements.

17. A system comprising:

a plurality of photodiodes that receive a light signal from a light source, wherein each one of the photodiodes generates a photocurrent signal;

a plurality of trans-impedance amplifiers, wherein each trans-impedance amplifier amplifies the photocurrent signal from one of the photodiodes and generates a low noise signal;

a plurality of high pass filters, wherein each high pass filter converts the low noise signal from each trans-impedance amplifier into an AC signal having a positive amplitude, a negative amplitude, and a zero cross-over point between the positive amplitude and the negative amplitude;

a plurality of integrators, wherein each integrator comprises:

a positive integrating amplifier that receives the positive amplitude of the AC signal from each high pass filter and generates a positive integrated value over an integration period; and

a negative integrating amplifier that receives the negative amplitude of the AC signal from each high pass filter and generates a negative integrated value over the integration period; and

an ADC that receives the positive and negative integrated values from the plurality of integrators.

18. The system of claim 17 , wherein the ADC is a low speed ADC having a total sampling time consistent with a power cycle of the light source.

19. The system of claim 17 , wherein the ADC is a high speed ADC that samples the outputs from each integrator at high frequency.

20. The system of claim 17 , wherein the system is coupled to a Smartphone configured to detect gestures of a user based on optical signals.