To simplify my question lets assume:
I need to convert +/-7.07A RMS AC current into +/-707mV RMS voltage with high acurracy (0.1% or better).
It may be also higher voltage, up to +/-2.5Vpk.
For now I just measure voltage on Ohmite 15FR100E resistor:
When I'm trying to measure 5A RMS current It probably heats up by about 87.5°C (calculated from thermal resistance and power) and that gives about 0.788% resistance change (calculated: 87.5°C * 90PPM/°C).
I want to improve this and I'm thinking about two options:
OPTION 1. Use expensive resistor: Powertron FPR 4-T221 0R100 S 1% Q
- 3°C/W heatsink
And this should give me 0.024% resistance change (from 5A RMS).
OPTION 2. Use INA250 current sense amplifier with fixed gain and integrated 2mOhm shunt.
INA250 op amp features from datasheet:
I think, that I can assume
- I can forgot about thermal problems because shunt R is very small
- I can compensate 0.3% gain error
- If I pick 200mV/A version - I'll get 2x higher voltage vs my shunt and this is fine
But I'm not sure about (QUESTIONS):
- Can I just ignore 50mA offset error (this is ~0,7% of my 7.07A peak current) when I need to measure sine RMS acurrately?
- should I expect some extra sources of errors when I replace shunt with INA250?
- is this that simple? really? $4 op-amp replaces big expensive shunt just like that?
Cheap Ohmite resistor which I'm using now:
Expensive Powertron resistor:
If someone asks why I'm not using resistor like 10mOhm to reduce heat - the answer is because I tried to avoid 10x gain op-amp in circuit, that would be extra source of errors. Maybe I was wrong. Actually this is not about measurements, I'm building current source similar to this: