Lamp Current Noise

In November 2014, before we shipped the ISL5s, we put one in water with a head fixture and obverved up to 10 mV of switching noise on the EEG monitor when we flashed the lamp. The amplitude of the noise did not correlate well with proximity of the lamp and the EEG leads. After half an hour of work, we ran out of time to examine the problem. Since then, Rob Wykes has implanted No2.5 and observes the following noise on the EEG signal during what appears to be 18-Hz, 10-ms optical pulses of 0-100% power.


Figure: Lamp Flashing Noise on EEG Input. ISL No2.5 is implanted in a rat. One-second intervals, full 27-mV voltage range.

Detail of the 20% power noise, shown below, shows a rising and falling edge on the pulse start and stop, and another 130-Hz interference that we see also when the light power is 0%.



Figure: Lamp Flashing Noise on EEG Input for 20% Power, Detail. ISL No2.5 is implanted in a rat.

We believe both these types of noise are electrical and magnetic, rather than photovoltaic. The boost regulator runs even at 0% power, so its switching noise is most likely the source of the persistent 130-Hz. The large spikes when the lamp switches on and off may be due to proximity of the four leads, or be electrical through the body fluids. We do not see such noise when the devices are unencapsulated and in air.