Hey all.
Im looking for op amp that could deliver me a great performance for amplifying voltage from a schunt. Im gonna be meassuring max current of about 200mA and I need to use as small schunt as I can. I cant get too big volage drop. Would somebody know of a decent op amp which isnt too expensive and is widely available. I wanted to use the ina199 but they are not available where I live, actually they dont seem to sell any type of INA. Thats the reason I need something known... Also I want an op amp, not a ic that could do it.
Thanks for any suggestions 
Use a shunt that has a 75mv output there are tons of them available on Aliexpress if you are able to order from there.
As far as an Op-Amp to boost the voltage there's a OP-835, OP-2835 or the Op-Amps that are in the the LM-10 and LT-1635 are great for ground referenced current monitoring.
High side or low side?
Why not something like this:
Max9934
Im not sure whats the way to go. Low or high side? Are there any majour diferences? I want to meassure current going through few devices at a time (AVR, ESP, some other stuff - that is all conected to one LDO - I will measure current of all of this together). I was browsing through my local e-shops and only LM-10 is sold here. 
Guess Ill have to order it from china or somewhere else...
Wojta:
Im not sure whats the way to go. Low or high side? Are there any majour diferences? I want to meassure current going through few devices at a time (AVR, ESP, some other stuff - that is all conected to one LDO - I will measure current of all of this together). I was browsing through my local e-shops and only LM-10 is sold here.
Guess Ill have to order it from china or somewhere else...
High side then. You can put the shunt on the input side of the LDO, and it will measure all the current going into the regulator, including its quiescent current.
Awesome, thanks.
Ive come up with what Im gonna do:
Op amp is gonna be the INA199 at the end. I cant seem to find any other solution that I could buy localy so I have to order it elsewhere.
Shunt should be 0.1Ohm as the current will be 0.2A. Am I right?
So supposedly the voltage output should be 2V at max. Are my calculations correct? 0.1Ohms * 0.2A = 0.02V voltage drop at max. Than it is amplified by the INA199 (Gain 100) to 2V which is safely under the voltage my system will be running at. (3.3V)
Or the shunt could be 0.3Ohm and gain of the amp lowered to 50 --> 3V at the output at max.
Vref of the amp should be on Gnd for high side sensing. Right? Same as in the datasheet (Figure 20)
Hope Im at least close at all of these calculations.
Schematic as the attachment should be the final current measuring part.
High side monitoring is difficult unless you use chips specifically designed for this like the Max9934 suggested in a previous post. The reason for this is that you will need to track the power sources voltage and if you do that the easiest way is to connect the V+ of the Current Monitor to the V+ of the Source Voltage.
This can create two problems.
1 - Now you once again need an Op-Amp that can run close to the rail voltage just like low side monitoring but best is to use chips specifically made for high side monitoring. Max9934, LT6101 etc. etc.
2 - If the circuit is not properly designed with standard Op-Amps you will not be able to tie the Arduino ground to the other sources ground. It's always best to tie ground to ground rather than V+ to V+ and I would say it should never be done.
I've used a well known trick in the past where an LM337L has it's adjustment pin tied to ground to get an output of -1.25V. Connect a five volt power supplies V+ to the ground and run the LM337's input from the V- of the 5V PS. This gives you -1.25V and if you tie the V- of the Op-Amp to this rail you are well within the input rail voltage problems that most standard Op-Amps have. Two things to consider when doing this are a resistor from V- to Ground so the lM337 is under a bit of a load and therefore more stable. Last is something like a Schottky diode or resisters to the Arduino Input so it can not go below ground.
One can also use the 7905 or 79L05 which is much easier to get your hands on however now you must protect the Arduino's input from -5V instead of -1.25V.
The INA199 is a much better, simpler solution than the suggestion in my previous post.
If it's easy to get then use it.
As far as the Shunt resister goes, always go as low as you possibly can as you do not want to lose any more voltage than you have to.
My previous suggestion was used in a situation where it had to be low side monitoring and a 600A - 75mv Shunt was used. It's resistance is 0.000125 ohms so gain was required to get any readings at all with lower currents. This also alleviated any ground loop issues.
Thanks
So going with INA199 conected as in the schematic in this post and a 0.1Ohm Shunt should be ok?
Also the zener shouldnt cause any problems. Right? Ive thown it in just for safety...
Thanks for help
Looks god to me.
If your running the INA199 off the same PS as the Arduino you don't need the Zener.
If your running it off of a separate PS with a V+ that's higher than the Arduino's V+ there's the potential to damage the output of the INA199. If you want to keep it in there just add something like a 100ohm resister in series with the output to stop this from occurring.
Thanks
Oh you are right.
Thanks to all of you.
Try an INA219 breakout board (Adafruit and ebay).
Not just an opamp like the INA199, but also inbuild 12-bit A/D for current and voltage.
Easier than trying to interface an INA199 or other opamp with Arduino's 10-bit A/D.
And it has an onboard 0.1ohm shunt.
Leo..
Thats a great call. Im not using a breakout board or module, its gonna be embedded into my board but still, its probably easier than INA199.
Thanks