Instrumentation Amp Issues???

Discussion in 'General Electronics Chat' started by pjk1939, Feb 29, 2012.

  1. pjk1939

    Thread Starter New Member

    Feb 29, 2012
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    Hello all,

    I am new to the forums and hopefully I can be some help to someone in the future; however, I have a question of my own.

    I am testing an instrumentation amplifier (INA128P).
    http://search.digikey.com/us/en/products/INA128P/INA128P-ND/251085

    I am using a regulated 3.3V as the V+ and ground as the V-.
    I am sending 1.0V into Vin+ and hooking Vin- and REF to ground.
    I used both a 50k and 100k between the RG pins.

    The gain of the in-amp is 1+(50k/RG).

    I measured the output with a multimeter and was expecting something around 1.5V and 2V, respectively.

    However, I received .094V for both resistance values. So my questions are:
    1. Is the in-amp bad or blown?
    2. Is this a proper way of testing the in-amp?
    3. If it is not the proper way of testing, what would be a better way? I only have two variable power supplies.

    Thanks in advance.
     
  2. #12

    Expert

    Nov 30, 2010
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    Common mode voltage range is listed on the datasheet as V+ minus 1.4 volts and V- plus 1.7 volts typical. Your input is only 1 volt above the negative supply voltage.
     
  3. pjk1939

    Thread Starter New Member

    Feb 29, 2012
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    Alright I see that in the datasheet; however, if you don't mind me asking, could you explain exactly what that means?

    Am I not allowed to input anything less than 1.4 V into the Vin+?

    Thanks and sorry for my inexperience.
     
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The other problem you have is that the specified unipolar operating supply range is from 4.5V to 36V whereas you are only providing 3.3V.
     
  5. pjk1939

    Thread Starter New Member

    Feb 29, 2012
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    Oh, well I thought that the "Wide Supply Range = +- 2.25V to +-18V" signified that 3.3V is fine.

    It is located in the Features section on the first page.

    Thanks again.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    No. The notation ±2.25V means you would have a bipolar supply providing a total of 4.5V across the device supply terminals. Your supply of 3.3V is insufficient for guaranteed performance per the device specs.
     
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  7. #12

    Expert

    Nov 30, 2010
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    I was busy with the LM431 problem for a while.
     
  8. pjk1939

    Thread Starter New Member

    Feb 29, 2012
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    Alright I understand now. Thank you for your reply.

    #12 did happen to mention the common mode voltage range, could either of you or anyone else explain what V+ minus 1.4 V and V- plus 1.7V means?

    Thank you for all your help. You all are very knowledgeable.
     
  9. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    An instrumentation amp has two input levels of interest. The difference input and the common mode input. The term 'common mode voltage' simply means a signal voltage level that is common to both the ± amplifier inputs. For instance one might want to measure the small difference ΔV=10mV between two signals which have a common DC offset of Vcm=5V. We want the differential amplifier to only amplify the actual difference ΔV and to be unaffected by the 5V common mode DC offset. The ability of the amplifier to reject any influence of the common mode signal level is a measure of its effectiveness.

    There is a range of common mode input level over which the performance of the device can be guaranteed to meet the specification. At worst - in this particular device case, if the common mode level is no greater than 2V below the +ve supply rail or no less than 2V above the negative supply rail then the device common mode performance should meet common mode rejection specifications. With a minimum 4.5V supply rail total, this doesn't leave you much room to maneuver on the input common mode level. The typical values quoted give you a little more scope with a minimum of risk of loss of guaranteed common mode performance.
     
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  10. pjk1939

    Thread Starter New Member

    Feb 29, 2012
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    Thank you so much, I don't completely understand; however, your explanation was very informative and I do a much better understand than I did.

    Thank You.
     
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