Measuring shunt current and opamp

Hello,

I have a 50A 75mV shunt and I'm trying to measure current using arduino.

Here's the schematic of what I currently have on the breadboard:

U1 = TL071
R1 = 270
R2 = 47k
So this gives a gain of 174.

Everything seems almost OK but even though there is no load, the opamp output is not 3.3v but ~3.47v. What is the cause of this?

Also, is the 3.3v arduino output stable for using as aref for adc? Or is it better to use internal 1.1v ?

Thanks

Probably the input offset voltage of the op amp. At a gain of 174 it only needs to be 1 mV to produce 170 mV of offset.

Also, your schematic is hard to read. I cannot tell what are connections and what are crossovers.

KeithRB:
Probably the input offset voltage of the op amp. At a gain of 174 it only needs to be 1 mV to produce 170 mV of offset.

Also, your schematic is hard to read. I cannot tell what are connections and what are crossovers.

Didn't know about such thing as input offset voltage. Now I get what's happening here.

There aren't any connections on the schematic, only crossovers. I didn't put dots on T connections though.

Thanks

TL071 is not a good choice for you. You need a low voltage, single voltage, rail-to-rail opamp instead.

pito:
TL071 is not a good choice for you. You need a low voltage, single voltage, rail-to-rail opamp instead.

Could you recommend something that is widely available?

You need a precision opamp for that gain. An instrumentation amp might
be best but it depends what sort of bandwidth you want. There are thousands
of op amp parts, "widely available" isn't so meaningful, you need to find a supplier
you can use and checkout what they have.

There are also chips specifically designed for this application, they are most likely
to have the best combination of features. "High side current sense amp" is probably
the kind of search term you need.

With that kind of gain, better to use two Op Amps.

MarkT:
You need a precision opamp for that gain. An instrumentation amp might
be best but it depends what sort of bandwidth you want. There are thousands
of op amp parts, "widely available" isn't so meaningful, you need to find a supplier
you can use and checkout what they have.

There are also chips specifically designed for this application, they are most likely
to have the best combination of features. "High side current sense amp" is probably
the kind of search term you need.

I didn't really understand the bandwidth thing. Does it matter if I'm measuring DC current?
I tried searching for current sense amps but couldn't find any available from the local supplier. But I found LT1167 instrumentation amp there.
http://www.stanford.edu/class/ee122/Parts_Info/datasheets/LT1167.pdf

One thing in a datasheet is bothering me:

Single Supply Operation
For single supply operation, the REF pin can be at the same
potential as the negative supply (Pin 4) provided the
output of the instrumentation amplifier remains inside the
specified operating range and that one of the inputs is at
least 2.5V above ground.

Would it work If I pull one of the inputs above 2.5v with two resistors between ground and 5v?

I don't think you are going to find an adequate solution for your problem with an op-amp or an IA unless you gain a LOT more electronics knowledge... that being said an Allegro Hall effect device for measuring... Look at an ACS712... Link:
http://www.allegromicro.com/en/Products/Current-Sensor-ICs/Zero-To-Fifty-Amp-Integrated-Conductor-Sensor-ICs.aspx
A simple device to use but it's output is centered at Vcc/2 and changes amplitude and polarity (+ Vcc/2 or -Vcc/2) depending on current flow and direction.
Diagrams of opamps to form level shifters should be within your search capacity. I personally would first try an LMC662.
R/R I/O, Low voltage and a low input current and low IOS. if required the second amp can be used as a gain block to scale the output of the level shifter.. A pot can be used to set the level shifter output to zero for zero current and the second amp can set the gain to something usable, If required.
The device is nearly foolproof to use BUT DO read the data sheet until you understand it completely BEFORE you begin.

Doc

For your reading pleasure: