PIC ADC and Bluetooth Pairing Causing Voltage Spikes

[Veramacor]

Hello all,

I got a problem and wondering if there any suggestions out there...

I have a PIC16F886 micro, a sensor, and bluetooth module.

The circuit gets its power from a +5V 500ma power supply. All chips are running 3.3v thru a 1 amp 3.3v Regulator,  with 100uF and 10uF filter caps on the power supply.

When I read voltages with the bluetooth module off,  everything works as expected.  But when I have the bluetooth turned on,  it goes in to a pairing mode (searching to pair to a bluetooth dongle). 

When in this pairing mode I get a 300mv voltage dip every 500 milliseconds on the ADC line.  I looked at the specs and the maximum output of the bluetooth dongle is about 10millamps.

Is there any way to filter this spike out without adding a separate power supply to the bluetooth module?

tx

J

[jahonen]

Could this be a EMI (self-compliance) problem? Bluetooth RF-signal being inadvertently rectified on nonlinearities on the ADC input? Have you verified the voltage dip with DMM etc? Also, if that is not the case, try to measure the actual pulse current requirement of the bluetooth module (with a scope using a small (0.1 ohms or less) series resistor in bluetooth module supply line) if pulse current is not specified in the datasheet, and decide what is best to do.

Regards,
Janne

[scrat]

Could you please post some other information (through hole or SMD components? layout or at least a schematic)? It would be much more easy to find a solution, there are lots of experienced people here (not me).
Although you may already have done, I'd suggest putting small value caps between supply and gnd pins of all the ICs, with very short leads/tracks. If the voltage dip is the cause, a big capacitor on the supply of the bluetooth module could help, also together with a small resistance or inductance in series to split the two supplies (I'd put it on the side which has less current transients, so I suppose on the PIC side):

           heavy cap || bluetooth module
supply /
         \
          inductor/ferrite bead/1-10 Ohm resistance ->  heavy cap  || (PIC&friends, each || with small cap)