Shortly after I finished my Geiger Counter I learned that even when you calculate the Sievert based on the area of the geiger tube, it's no correct. So a Gamma Spectrometer had to be
I needed -1kV. I developed a circuit for that with a ccfl inverter regulated over its supply voltage. The I/U converter circuit is a standard one described in many application notes. The isolation of the signal is done with a IL300. The circuit for that is taken from its datasheet. The the signal goes into a USB soundcard plugged directly onto to PCB.
The whole spectrometer is USB powered, over a isolated DC-DC converter. The main HV regulator consists of a HV divider, a TL084 as the regulator and a TL431 as a reference. The negative voltage needed is produced by a ICL7660.
High voltage wise there is a delon doubler at the output, the a 1M and a 330n cap as filter.
Everything is isolated very good, so that the PC/laptop or tablet attached, doesn't get damaged in case of a failure.
Until now I tested the HV supply wich works perfectly fine. It outputs nice smooth -1kV.
I still need to calculate the right values for the converter and isolation circuit based and then tweek them, after I've done measurements on the signal amplitude after and befor the optocoupler.
A short update: the circuit works, but in the case of a failure, the protection circuit wouldnt work, because I forgot to add separate ground planes, so no real galcanic separation. I will re design the board and order new ones.