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INA818: Design REview

Ismail SAHIN
Prodigy 30 points

Part Number: INA818
Other Parts Discussed in Thread: INA146

Tool/software:

Hi Everyone,

I want to design current monitor with INA818 . My input  ± 20V DC  and 15V AC ,100mA maximum current flow.Using  1 Ohm Shunt resistor for measuring low current about uA range.

Pls give suggestion may i change opamp use another one.

Thank for suggestions.

 

  • 0
    Prodigy 30 points

    We have 4 point voltage source and we want to measure their current flow each other.

    First Use Case

    Point 1 : -20V DC 

    Point 2 : -15V DC 

    Point 3 : +15V DC 

    Point 4 : +20V DC 

    For example measure between Point1 and Point2  approx. %1 accuracy

    Second Use Case

    All that point have AC current with in range(10VAC) and measure their current with each other.

    My main question is Vcm voltage of INA818 and input protection enough?

    Best Regards

  • 0 in reply to Ismail SAHIN
    TI__Mastermind 29258 points

    Ismsil,

    1. The common mode range of instrumentation amplifiers can be determined using the "Analog Engineer's Calculator" software tool, "INA Vcm vs Vout" calculator.  I did this for your circuit (see below).  For your case you get good common mode range from -5V to +5V, however, at -5V you are nearly non-linear.
    2. The INA818 has built in input protection.  This will allow overstress signals up to ±60V without damaging the device.  The device will not function while the overstress signal is applied, but will recover to a functional state afterwards.
    3. I do not understand all the cases labeled "point 1" to "point 4".  In order to understand the operation of this circuit I need to know what common mode voltage is connected to CURR_A or to CURR_B.  Also, I need to know  what current is flowing through the shunt.  In your original post you indicate the current to be 100mA.  This would develop a 0.1V input signal.  Your configuration has a gain of 99V/V so the output would be 0.1V x 99 V/V + 2.5V.  Of course the actual output would depend on the polarity of the current through the shunt, so you need to indicate this to properly understand the output range.  I suggest using TINA or PSPICE to simulate the circuit.  

    Best regards, Art

  • 0 in reply to Art Kay
    Prodigy 30 points

    Dear Art,

    I want to make 4 way ammeter ,inputs  must be support ± 20V DC or 15V AC current bidirectional.And maximum current 100mA with 10 Ohm Shunt accuracy %1 enough

    Now our schematic  with INA818 but if we supply  INA818  ±15V  Vcm not support our voltage range , i check it with Analog Engineers Tool

    I try to find best solution for our requirements.

    Best Regards

     

    Art Kay said:
    rough the shunt, so you
  • 0 in reply to Ismail SAHIN
    TI__Mastermind 29258 points

    Ismail,

    It is difficult on an instrumentation amplifier to get common mode range beyond the power supply voltages.  This is where a difference amplifier can be helpful.  A difference amplifier uses resistors on the input whereas instrumentation amplifiers use op amps on the input (see below). The advantage of the instrumentation amplifier is that they have very high input impedance.  The disadvantage of instrumentation amplifiers is that they have limited common mode range. The resistor network in difference amplifiers will divide the common mode down, so these amplifiers often have very wide common mode range.  You should look at INA146.  This device is a difference amplifier followed by a gain block.  So the input signal and input common mode are attenuated by a factor of 0.1 V/V but the output of is gained up by a post amplifier.  This allows for a overall gain of 100 V/V because the second amp can have a gain of  100V/V.  On a ±15V supply this allows for a common mode range of ±100V.  The input impedance of the INA146 is 100kΩ which is will not introduce any significant loading error since your shunt is only 1Ω.   Note that shunt monitoring is one of the main applications of difference amplifiers, so that is probably the direction you should look.  If you don't like that INA146 there are many other difference amplifier options that would work.  I just selected this one because of the post gain option. 

    Best regards, Art

  • 0 in reply to Art Kay
    Prodigy 30 points

    Dear Art,

    Thanks for your suggestion.I check difference amplifier solutions and INA146 may solve our problem but need to test it.

    Best Regards

    Ismail