I have designed a circuit that uses the TI CC1310 microcontroller, and has a daughtboard that has the PIR sensor and amplification circuitry from the from the TIDA-00489


When I look at the output of the amplification circuitry of the amplified PIR signal seems to look sensible. I have a nice stable line that sits at Vcc/2, until I move near the PIR sensor, at which point it goes to either Vcc or GND. All as expected.

This testing was done with my circuit being powered by a CR123A battery, and sat on a grounded anti-static mat. If I remove the grounding from the mat, or move the PCB off of the matt, the signal becomes very noisy. Rather than a nice thin line on the scope, I see a thick line that has ~0.5V thickness.

Additionally, I have an LED on the PCB, and I flash it every 1 second. When this happens, I see a sharp drop in the output signal of my amplifier every 1 second, seemingly corresponding with when my LED comes on.

Can someone please explain the causes of these noise sources to me, and let me know how they can be remedied?

  • \$\begingroup\$ Do you have a 'scope? If you look at the frequency components of the noise that can be a help. For me, ~25kHz is the overhead lights, ~50kHz is interference from 'scope display.. etc. \$\endgroup\$ – George Herold Jul 25 '17 at 16:09
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    \$\begingroup\$ Provide a schematic, PCB layout, and a photo of your setup if you have anything off the PCB. \$\endgroup\$ – Jack B Jul 25 '17 at 16:28
  • \$\begingroup\$ It looks like the noise is 50Hz. There is small amplitude 50Hz noise on the signal when the board is sat on the grounded mat, but when it is removed from the mat the amplitude increases. Why might this be? Also, the sudden drop in voltage of the signal when the LED flashes is something I would like to uderstand. Thanks \$\endgroup\$ – Steve Jul 25 '17 at 19:36
  • \$\begingroup\$ Broadly speaking, 50Hz comes from the mains and often couples inductively into long wires or comes through power supply capacitors. The sharp drops when the LED flashes could be inductive coupling to from power wires to adjacent signal wires, or could be that the supply rails drop when the LED comes on, and that comes through the sensor or amp because they have inadequate decoupling capacitors. But we can't work out which unless you post the schematic and layout. \$\endgroup\$ – Jack B Jul 27 '17 at 13:13

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