Hi all.
Today I saw a strange thing. I was helping a customer cure his inrush current issue. The test house measured about 100A inrush current into DC circuit from an external power supply. The external power supply would crowbar a couple of times before coming up when the highly capacitive load was connected. I reduced the inrush current at switch on to about 10A using an inush current limiter (an NTC thermistor), which worked fine.
Before the fix, my 300MHz, 2GS/s Tektronix CRO showed a peak inrush current of about 100 to 120A. The test house reported a similar inrush current. But the customer's Siglent Digital Oscilloscope Model Sds1102cml, 100mhz CRO showed a peak current of 350A. In all cases the bandwidth was not attentuated. Measurement was voltage across a 10 milliohm load ground return path, non differential as one side is earthed. So the voltage spike was 3X the other CRO.
I suspect the Siglent CRO has a problem. It seems odd that my Tektronix CRO and the NATA approved test house's CRO has similar results, but the Siglent is way, way out. I did not spend time testing the rest of his Siglent CRO, as I am just too busy.
I have seem this sort of thing before with a low end Rigol CRO where the measurements was about 20% out from that of a high quality calibrated instrument. The same Tek probe was used in the Siglent and vice versa - the 200% error stayed with the Siglent. Happens with X1 and X10 probe settings.
Is this a case of getting what you pay for? Does anyone have any clues on why the Siglent CRO could be so out?