I am searching for a good idea to sense the current of 0 to 100 mA at a voltage level of 0V to 135V. The result should be a current to ground or a voltage respecive to ground. The first problem is the high voltage of 135V, the second, that this voltage can be nearly zero as well. I investigated several possibilities already:
As an additional point i have to concider that only little space is left on the pcb.
Does anyone have a (good) idea how to solve the problem?
have you looked at the AD8212?, If you can affort to "burn" some bias current this should be a good solutiuon.
You can also use a floating op-amp powered by a Zener similar to the one built into the AD8212.
Another option is the AD629.
Or an isolated ADC like the AD7400 as described in this Circuit Note
There are tradeoffs in all cases, such as space, accuracy, additional bias current, speed, etc. So hopefully these circuits will inspire you to find the right solution for you.