Interfacing Current Shunt

Go To Last Post
14 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi all

Just a couple of simple questions.

I am trying to interface a current shunt (300A 75mV FSD) to the ADC but i know that the voltage is too low and I need to amplify it.

I have been looking at Instumentation Op-Amps but there is so much info i have got myself a bit lost in what i need.

I will be needing up to the 300A as I am testing very high current industrial alternators.

Can someone please help me or give me a direction as to where i can find this information.

Thanks in advance.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

some series of avr have programmable adc gain up to 100x (correct me if im wrong). this will shurly give you enough gain eliminating the need for designing, building and testing an instrumentation op-ap cct. the only extra design that may be needed is to add some passives for filtering the incomming signal; more filtering can then be done through software.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

There are indeed a bewildering number of instrumentation amps out there!

Try the INA122. It is programmable gain, modestly priced, rail-to-rail, and has a wide operating voltage range. I use them and they make me happy 8-)

edit:

Even an ordinary opamp configuration can do your job, too. My favorite is the LTC1152 for high accuracy applications. Even in a 10 bit system, the negligible offset and input current makes for a headache-free design.

Tom Pappano
Tulsa, Oklahoma

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thanks very much for the responses.

I didnt realize that the ADC had that much gain will have to look into that for my device.

Thanks for the info for the op-amps to it will be helpful.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You could always try a hall effect sensor - this will give you galvanic isolation as well.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

INA139 is a current shunt monitor. I have used it in a design and it worked fine. Not too high voltage thou. Max 60V or something.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You could try the LT1167. It's expensive though.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Malvino Microelectronics or Sedra/Smith Microelectronics. Learn how to use an opamp.

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thanks everyone for the help it is much appreciated.

It has given me more option to look into.

Thanks again

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Check your solution against the tolerance you need. Parts like the INA family are trimmed on-chip in production to give extremely high common mode rejection. This just means you can measure a very small voltage difference across its inputs relative to the absolute DC voltage. If you want to measure 100mV on a 12V line you can use an every-day op-amp with external 1% resistors and probably be OK. If you want to measure a small delta with a high DC voltage with good accuracy than an INA is what you probably want.

Go with an everyday op-amp if you can, lots of second-sourced parts out there. Instrumentation Amps are more far and few between but they're beautiful performance-wise. The old Burr-Brown group is famous for having the best INAs in the world (I used to work there so I'm biased).

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

garboui wrote:
some series of avr have programmable adc gain up to 100x (correct me if im wrong). this will shurly give you enough gain eliminating the need for designing, building and testing an instrumentation op-ap cct. the only extra design that may be needed is to add some passives for filtering the incomming signal; more filtering can then be done through software.

Hi Everybody,
I am using this AD8215 Current shunt monitor for my project and i have my presentation in couple of weeks. I am trying to understand the inside circuitry for this device and it seems like its very complicated to figure it out. In anybody can help me in this to understand, it would be very helpful for me. I am attaching the datasheet for this component and you can see the inside circuitry diagram with some explanation for it.

Attachment(s): 

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

A moderator may want to split this off, the hijacked thread is 4 years old.

How it works is proprietary. How to use it is quite clear in the data sheet.

It all starts with a mental vision.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

The internal workings are hardly proprietary, except for the offset circuit apparently. Other current sense amplifier ICs work in the exact same way.

Basically it tries to keep to voltage between the + and - pins zero. The voltage at the load side of the sense resistor pin is lower than the input side of the sense resistor. The amplifier tries to keep the voltage difference between its + and - inputs zero by drawing current through the resistor that's between the sense resistor and the + input. When current flows through this resistor the voltage at the + becomes lower until it matches the voltage at the - input. The current drawn through the resistor flows to another resistor that converts it back to a voltage, that in turn is amplified.

Assume:

* 12V input
* 1A load
* 0.1 ohm sense resistor
* input resistor 100 ohm
* output resistor 220 ohm

Voltage across sense resistor = 1A*0.1R=0.1V. Voltage at feed side of Rsense is 12V, at load side 11.9V when ground is taken as reference. That's a difference of 0.1V that has to be regulated 'away'. To create this 0.1V across the 100R input resistor, the amplifier draws 0.1V/100R=1mA through it. This 1mA also flows through the 1K output resistor, and this gives a voltage of 220*1mA=0.22V over it, amplified by 20 gives 4.4V which is the final output.

I just repeated the explanation of the datasheet, of course :roll:

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

And if the magic smoke that handles the special offset is released by exceeding the maximum specifications, it will fail. :)

It all starts with a mental vision.