Using an AC-coupled transimpedance amplifier can greatly improve green LED-illuminated heart beat sensors' immunity to noise and other extraneous environmental factors.
Thanks to their simplicity and low cost, green LED-illuminated heart beat sensors are now nearly ubiquitous, found in most consumer health products as well as many phones and wrist watches. But, for all their advantages, I’ve observed that they are frequently affected by environmental factors that reduce their accuracy and, in some cases, their ability to take measurements at all.
These factors include sensitivity to finger placement, variations in the distance between the LED source and the sensor, and ambient light incursion. This led me to think about making an AC-coupled transimpedance amplifier (TIA) that would be much less susceptible to bright ambient light, such as sunlight or a 100W incandescent bulb in close proximity to the photodiode sensor.
Simulation showed that the idea worked well, and that DC current in the photodiode (modeled as an AC current source with DC offset bias to simulate the ambient light response) was rejected by the AC coupling to the TIA inputs.
Figure 1 AC coupled TIA with low-noise 2.5V reference level.
To verify the simulation, I breadboarded a preliminary circuit using the components I already had at hand—most notably a Yi T1-3/4 or 5mm “bullet” photodiode and a generic green LED. Both devices were housed in surface-mount packages that would work as well for close distances. If longer distance pulse sensing is needed, it might be necessary to use parts that include lenses in the SMD packages to decrease the field of view.
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The resulting circuit demonstrated good pulse sensitivity with my wrist at a distance of up to six inches from the photodiode/LED setup, even when a 100W incandescent bulb was positioned as close to the photodiode as possible (without blocking the optical path to my wrist between the photodiode and the LED). Likewise, the new circuit’s sensitivity to body motion (my wrist) was minimal.
This is the full circuit as simulated, including a high-pass and low-pass filter, built and tested for the above results:
Figure 2 The complete TIA circuit includes a band-pass filter, which helps to reduce its sensitivity to body motion and fast ambient light variations. As shown, the circuit’s response is -3dB from 48bpm to 390bpm.
Figure 3 Filter output response to a 1nA photodiode AC current at 0.5Hz or 120bpm stimulus.
The TIA implemented here is fairly insensitive to component variations. I used resistors with 5% tolerance and capacitors with 10% tolerance. The balanced AC input and low pass filter affords excellent rejection of AC line induced noise.
As a result, I saw no “hum” in the output signal as I brought my skin closer to, and eventually in contact with the plastic photodiode package. There is no 120Hz response to the background light produced by a nearby 100 W light bulb. Nor did the circuit demonstrate any 60Hz response to the close proximity of the bulb to the photodiode during the test for ambient light rejection.
Many common problems associated with green-LED-based pulse sensors can be reduced or eliminated by using an AC-coupled transimpedance amplifier in the photodetector circuit. For most applications, any possible additional costs related to the TIA’s components are far outweighed by the gains in noise immunity, positional tolerance, and sensing range.
Figure 4 Relative positions of LED, photodiode, and skin surface.
This article was originally published on EDN.
Dave Conrad is a retired electronics engineer with experience in power, video, analog, digital, mixed signal, and software design.