I think it's in the cable. The user manual shows a cable with a big chunky adapter that slots into a modular port on the back of the meter. It seems that port a

I think it's in the cable. The user manual shows a cable with a big chunky adapter that slots into a modular port on the back of the meter. It seems that port also supports a Bluetooth module, so I'd guess it does direct digital signaling and the adapter in the cable contains the optical isolation.