# Current sense op-amp with integrated shunt vs expensive resistor

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:

• 100mOhm
• 90PPM/°C
• 35°C/W
• 5W

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

• 100mOhm
• 25PPM/°C
• 4.8°C/W
• 3°C/W heatsink
• 15W
• 4-terminal

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?

Attachments:

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:

http://www.kswichit.com/VCCS/vccs.htm

• "...with high acurracy (0.1% or better)." - why? Commented May 26, 2020 at 6:57
• Because this circuit is supposed to be used as calibrator for some product (measurement device) which is capable to archive 0.1% acurracy. With 0.1% acurracy calibrator I will be able to reach about 0.2% product acurracy. This is good enough for product purpose, but if it is possible to get 0.05% calibrator for few bucks (lke 20-50USD) more - why not. And maybe I will be able to measure smaller currents more accurately on single range (one resistor) instead of making range switch. Commented May 26, 2020 at 11:42