I am currently working on a SiPM detector design. The photocurrent is converted into a voltage by a two stage transimpedance amplifier which is based on a single LTC6229 OPV (Schematic shows LTC6228, since the 6229 is not available in LTspice):
After successfull simulation, I have designed a PCB. The oszilloscope shows small unexpected periodic peaks on the output signal of the detector:
My first guess was that those peaks were caused by distortions on the power supply line from e.g. a switching regulator. But both, positive and negative 5 V supply lines do not show any distortions. The peak-peak voltage of both rails is approximately 10 mVpp without any significant spikes. So the problem has to be somewhere else. I recognized that the frequency of the distortions depends on the supply voltage of the photodetector. So I thought that the spikes are actual "current spikes" of the SiPM detector. To double check this, I disconnected the supply voltage of the SiPM, and soldered a 2k4 resistor between the inverting input of the first stage OPV and ground. So that the first stage is an inverting amplifier, instead of a transimpedance amplifier. However, the oszilloscope still shows some siginficant peaks on the output signal (Frequency and amplitude of the peaks have changed):
Do you have any idea, where those spikes come from? What can I try to prevent this behaviour?
Thank you in advance.