# Meassurement of low currents 0-30 mA

Hello all,

I’m trying to find a current sensor that allows prefearbly precise meassurements between 0-30 mA (for my Arduino UNO).
In my opinion this was the best one i could find (because of the gain stage):

I would be very grateful if somebody could tell me if this would work, or if there are other possibilities for this specific range of current.

Thank you very much

It depends on what you mean by "precise". The ACS712 sensor on the board you reference has certain inaccuracies specified: 1.5% typical nonlinearity (but no specified min-max), 185 mv/A sensitivity (180-190 min-max), 21mV of typical peak-to-peak noise, etc. etc. If you need microamp resolution then this is not going to cut it.

Also note that at 30mA the ACS712 is going to put out mid-scale voltage (2.5V) plus 185mV/A * 0.03A = 5.5mV. That's a pretty low voltage, and could get swamped in the 21mV of peak-to-peak noise. Yes, the op-amp on the Sparkfun board can be used to amplify this by up to 47x, but that's still only 0.26V, not really matching the resolution of your Arduino's A/D converter (0V-5V). At 10-bit resolution for this A/D converter, the range 0V-0.26V is broken up into 4.88mV "chunks", so there will only be 0.26V/0.00488V = 53 chunks. This means the 0mA - 30mA range you're trying to measure will only have 0.56mA resolution. And that's assuming an ideal case, not including the 1.5% nonlinearity, 180-190mV/A sensitivity range, peak-to-peak noise, etc. etc.

Is that good enough? If so, go for it...it should work.

-- The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected

Thank you very much for this detailed answer (cause I´m not an electrical engineer and didn't get all the facts from the datasheet, but you explained very well)...so i think the resolution is really to low for my application. I´m wondering how multimeters detect such low currents and if there exist some other out of the box possibillities like this one from sparkfun for this small currents.

I looked around recently for pre-made low-current modules and didn't find any, so I'm interested in something like this too.

If you know the current range ahead of time it's not hard to design a custom circuit to measure this precisely, it's just that the Sparkfun module was designed for 0-5A, more like measuring motor currents and such, not for precision measurements.

-- The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

You can look at Dave L. Jones' wonderful EEvblog (more specificaly his µCurrent device) for how he measures micro amps with his device. Also his video blogs are very well done and I especially like his aussie accent and humor! They are very informative even if you are a multi year in industry engineer, or a beginner looking for a good place to learn :) link to uCurrent video (how to design a product) http://www.eevblog.com/2010/04/08/eevblog-72-%e2%80%93-let%e2%80%99s-design-a-product/

For low level currents to ground a virtual-ground inverting op-amp circuit is one precise way to measure current, but you need dual voltage supply for that. http://en.wikipedia.org/wiki/Current-to-voltage_converter#Op-amp_implementation