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Pulse Indicator light is blinking without anyone being connected to the emWave2 (aka: Ghosting)

May 31, 2018

 

Sometime, you may notice that your emWave2 will run by itself without being connected to anyone. It may even change Coherence levels and appear that there is something wrong with the device.
This issue is referred to as “ghosting” and is the result of feedback from the aggressive automatic gain function created by our engineers to deal with a wide range of pulse strengths, skin types and physiologies. All efforts to prevent it led to reductions in sensitivity. We also did not want to increase the cost of the product to our customers. Since it only happens when the sensors are not covered, we decided it was worth it for the higher sensitivity.

The problem with “ghosting” is related to a number of factors. To capture HRV, our devices work at much higher sample rates than typical averaging heart rate monitors and SPO2 devices. Standard average devices sample at 20 - 40 samples/sec. We sample at 125 samples/second to 350 (depending on device and application.) The higher frequency provides more opportunity for random beat frequencies to be established. Averaging devices also do not have to run at the same gain variations required for HRV.

When in contact with a body part, there is no real back ground signal to eliminate. We essentially work in a closed loop condition. To deal with the widest possible range of skin types and pulse strength, the sensor aggressively tries to find the optimum signal strength by both boosting LED output using PWM (pulse width modulation), while the input analog stage adjusts the gain across 32 possible amplification settings. In contact with a real person, the system locks to the pulse with a large range of people. However, due to the extended gain range, it is possible for the emWave sensor to lock on to low level reflections from the plastic surfaces protecting the unit, when not in contact with a body part. This is often triggered under the influence of external light sources. Since the loop results in sinusoidal signal slowly hunting across the gain range, it might even result as appearing “coherent” from an HRV perspective.

As this is a normal response from the electronics in the emWave2, it in no way harms the unit nor does it make it defective. As soon as you place your thumb on the sensor, it will have a small red light over the small blue one for a few seconds. Then the red light will fade away leaving only the blue light blinking to indicate that the unit is now registering your actual pulse.

 






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