Hi! Welcome to the PMT-combiner repository.
This project got started when there was the need to combine the signals coming out of two photo multiplier tubes (PMTs) installed in one of the 2-photon microscopes at the microscopy unit in Sussex Neuroscience.
The particular PMTs installed were models PM2101 from Thorlabs. This means that they are actually a "combo" of PMT plus current to voltage converter, and the output signal from the unit is voltage.
Having to combine two voltage signals together, we implemented a simple system with two Operational amplifiers. The first one combines the two voltage signals, but because we didn't want to amplify the signal, this opamp is used in a signal inverting configuration. After this first stage, the signal is combined, but also inverted, so a second stage is used to invert the signal again.
The whole system is powered up with 12 volts, which makes it suitable for battery driven applications. The power signal is split into +-6V as the PMT units output +- voltage as well.
Each channel has a switch which connects the PMT signal to the combining circuitry. If the combination is not needed, the switch can be turned so that ground is connected to the circuitry, allowing for just one PMT to be used at a time.