Op Amp -Input +Input Resistance

Discussion in 'The Projects Forum' started by Jomar, Oct 4, 2013.

  1. Jomar

    Thread Starter New Member

    Jan 29, 2013
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    Hi Good Day,

    I was wondering what are these parameters (-Input and +Input Resistance per the data sheet) and how will I be able to test it? Is this the resistance on the inverting and non-inverting input of an op amp or is this the resistance on the ±15V supply voltage of the op amp? Thank you!

    http://www.analog.com/static/imported-files/data_sheets/AD844.pdf
     
  2. wayneh

    Expert

    Sep 9, 2010
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    It's referring to the inputs, not the power supply.
     
  3. Jomar

    Thread Starter New Member

    Jan 29, 2013
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    Is there any way possible to measure it? I tried inverting and non-inverting circuit and measured the current with a specific Vin to get the -/+ Input. Is it like, Vin/I (current that goes in the op amp) = -/+ Input Resistance?
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Use an increasingly large resistor between your source voltage and the input. You can then map how the voltage at the input deviates as the resistor gets larger. If the input resistance is small compared to your voltmeter, you could just watch voltage with your meter. But some op-amps have higher input resistance than a cheap meter.
     
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  5. LvW

    Active Member

    Jun 13, 2013
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    Jomar, do you really want to measure currents in the nano-amps range? For which purpose?
    Be aware that there is a huge difference between dc bias currents and ac signal currents.
    More than that, opamps always are used with feedback which changes input currents drastically.
     
  6. Jomar

    Thread Starter New Member

    Jan 29, 2013
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    LvW, I just need to make sure that my part I have here meets the required DC electrical characteristics of the data sheet. That's why I need to understand each of the parameters for this op amp. and yes, if it requires to measure current in nano amps, we have a pico ammeter here and a really good voltmeter.
     
  7. Jomar

    Thread Starter New Member

    Jan 29, 2013
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  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Why would you trust strangers on the Internet describing how to measure quantities so small that it is unlikely any device you own can measure it accurately? On the other hand, why would you trust an Op amp manufacturer that can only be successful if they are able to make products that meet the specifications they set in their own datasheet and would fail from lack of customer trust if they instead sell crappy op amps? That being the case, why would you pick input resistance as the benchmark test to verify the goodness or badness of an op amp?

    An inquiring mind wants to know?
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    One easy way to measure the input impedance of an amplifier is to connect it to a known driving voltage thru an impedance equal to the spec.

    Say you have an amp with a gain of 10 and an input impedance of 1,000,000 ohms. If you connect it to a 1V source thru a 1,000,000 ohm resistance the effective input voltage is then 1/2V, so the output will be 5V.

    Doing this test without the extra resistor gives you the gain. Some simple algebra will let you calculate the exact input impedance without having to measure vanishingly small currents.
     
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  10. wayneh

    Expert

    Sep 9, 2010
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    That's a better description of what I was alluding to in #4.
     
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  11. LvW

    Active Member

    Jun 13, 2013
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    Jomar - what about a method to use the dc output of the opamp as a measure for the dc input currents?

    1.) Ground the pos. input and put a large resistor RN into the feedback path. If you know the dc input offset you can calculate the dc current that goes into the neg. input terminal from the resulting dc output voltage.
    2.) A similar approach applies to the input current into the pos. terminal. Set RN=0 (unity gain feedback) and put a large resistor RP between the pos. input terminal and ground.
    ___________
    In both cases, the wanted quantities can be calculated from Kirchhoff`s voltage law.
    However, as mentioned: A detailed calculation requires to know the offset voltage.
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    From the request of the OP, I understood that he wants to validate the input resistance of an op amp chip, not the resistance of an amplifier circuit.
     
  13. sparko311

    New Member

    Oct 5, 2013
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    Guys - this opamp in question is a current feedback opamp. These are very different than typical voltage mode feedback opamps. They have a very low input impedance on the inverting input. A few tens of ohms, to be exact. They are traditionally used for very high speed circuits, [60MHZ, 2kV/uS slew for this particular opamp] Although this is not exceedingly fast - and voltage mode types can be obtained with these speeds.

    Are you really sure you want a current mode feedback opamp ?

    The datasheet starting on about page 12 explains some of the nuances of current fed back opamps - as well as some of the input impedance anomalies that they have.

    These things will oscillate like mad with even a pF or two of stray C to ground on the input pins. Any ground plane must have a cutout under these devices input pins.

    Hope this helps.

    Andrew
     
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  14. LvW

    Active Member

    Jun 13, 2013
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    Andrew - thanks for clarification. I must confess that, indeed, I have overlooked the link that was given in post#1.
    The AD844 device is a very special current feedback amplifier insofar as it can be (and is in practice) used also as a current conveyor (CCII).
    This is possible because it provides access to a high impedance output node.
     
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