Photoplethysmography sensor apparatus and method
The present invention relates to a photoplethysmography sensor apparatus ( 200, 300 ), a photoplethysmography sensor method ( 400 ), and a photoplethysmography sensor computer program product. It is proposed to measure a photoplethysmographic signal without ambient light interference. Ambient light signals are rejected by subtraction of a compensation current at the input of a transimpedance amplifier ( 240, 340 ). The compensation current is controlled via a closed loop, without interfering with the low duty cycle operation of a photoplethysmography LED ( 210, 310 ).
1. A photoplethysmography sensor apparatus comprising:
a light source unit configured to generate a first light signal;
a photodetector unit configured to observe a second light signal, where said second light signal is indicative of an absorption of said first light signal in a target, wherein said photodetector unit is further configured to output a photodetector output signal in response to observing said second light signal; and
an ambient light compensating feedback loop comprising a track-and-hold unit configured to receive said photodetector output signal;
wherein said photoplethysmography sensor apparatus is configured to compare an output signal of said track-and-hold unit to a reference signal (V R ), and wherein said ambient light compensating feedback loop comprises a first amplifying unit configured to provide a compensation current based on said comparison to the track-and-hold unit.
2. The photoplethysmography sensor apparatus of claim 1 , further comprising a second amplifying unite for amplifying a sum of the compensation current and the photodetector output signal and for delivering an amplified signal to an input of the track-and-hold unit.
3. The photoplethysmography sensor apparatus of claim 2 , wherein said second amplifying unit comprises a transimpedance amplifier.
4. The photoplethysmography sensor apparatus of claim 1 , wherein said first amplifying unit comprises a trans-conductance amplifier, wherein said trans-conductance amplifier comprises a resistor and/or a voltage controlled current source.
5. The photoplethysmography sensor apparatus of claim 1 , wherein said photoplethysmography sensor apparatus comprises an analog-to-digital converter configured to receive an output signal from said second amplifying unit.
6. The photoplethysmography sensor apparatus of claim 1 , wherein said photoplethysmography sensor apparatus further comprises a light source driver unit configured to control said light source unit.
7. The photoplethysmography sensor apparatus of claim 1 , wherein said photoplethysmography sensor apparatus further comprises a synchronous detector.
8. The photoplethysmography sensor apparatus of claim 7 , wherein said photoplethysmography sensor apparatus further comprises a light source driver unit configured to control said light source unit and wherein said synchronous detector is configured to operate synchronously with said light source driver unit.
9. The photoplethysmography sensor apparatus of claim 7 , wherein said photoplethysmography sensor apparatus comprises an analog-to-digital converter configured to receive the output signal from said synchronous detector.
10. The photoplethysmography sensor apparatus of claim 7 , wherein said synchronous detector comprises a synchronous rectifier followed by a low pass filter.
11. The photoplethysmography sensor apparatus of claim 10 , wherein said synchronous rectifier multiplies the signal with +1 or −1.
12. The photoplethysmography sensor apparatus of claim 1 , wherein said photoplethysmography sensor apparatus further comprises a microcontroller configured to adjust said reference signal (V R ).
13. A photoplethysmography sensor method comprising the steps of:
generating a first light signal by a light source unit;
observing a second light signal by a photodetector unit, wherein said second light signal is indicative of an absorption of said first light signal in a target;
receiving, in response to observing said second light signal, a photodetector output signal by an ambient light compensating feedback loop comprising a track-and-hold unit and an amplifying unit;
comparing an output signal of the track-and-hold unit to a reference signal (V R ), wherein said output signal of said track-and-hold unit is based on said photodetector output signal; and
providing a compensation current by the amplifying unit to the track-and-hold unit based on said comparison.
14. A photoplethysmography sensor computer program product comprising a computer readable memory storing computer program code means for causing a photoplethysmography sensor apparatus to carry out the following steps, when the program is run on a computer controlling the photoplethysmography sensor apparatus:
generate a first light signal by a light source unit;
observe a second light signal by a photodetector unit, wherein said second light signal is indicative of an absorption of said first light signal in a target;
receive, in response to observing said second light signal, a photodetector output signal by an ambient light compensating feedback loop comprising a track-and-hold unit and an amplifying unit;
compare an output signal of the track-and-hold unit to a reference signal (V R ), wherein said output signal of said track-and-hold unit is based on said photodetector output signal; and
provide a compensation current by the amplifying unit to the track-and-hold unit based on said comparison.