I want to do some high-precision current measurement. (Voltage up to 20V and requires current to be shown in 0.1mA resolution, current can be up to 4A). The simplest is low-side sampling by MCP3421:
This one is easy and accurate. But this breaks the ground connection.
Using MCP3421 for high-side sampling is not possible, because it will break its input voltage limit(VCC + 0.3V). Our upstream voltage can be up to 20V.
Another possible solution is by using OP-Amps:
Page 18 describes how to use OP-Amps to do the conversions. But I wonder the resistors can be inaccurate and ageing is a killer of accuracy.
There's one more solution: using high side current sense amplifier like TSC101 or LTC6101. But this sort of ICs often has a high Input Offset Voltage (Vos) up to approx. 1mV. 1mA on 15mOhm sampling resistor is 15uV, so the offset seems to be unacceptable.
Are the above solutions possible? Or are there any other solutions?