Some of my op-amp questions

Discussion in 'General Electronics Chat' started by kevin.cheung19, Jul 13, 2012.

  1. kevin.cheung19

    Thread Starter New Member

    Nov 22, 2011
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    1) Let's say I'm doing a differential Amp, and besides doing the differential, I also want to have a gain, the question is: Are all the non-ideal characteristic of op-amp will be amplified as well? (I mean the voltage errors that is caused by Vos, I_biased, I_offset, etc...)

    2) I saw one spec said that "output swing to low: 1mV 10mV max
    ", does that mean I can not really get it down to a 0mV output? If so, is this an offset throughout the whole range or is this just a minimum value that I need to exceed to get some real reading? If this is the case, is a dual powered op-amp help in this area?

    3) Is there a reason to use multiple op amps instead of only using one? (I saw some circuit
    people uses multiple op-amps, let's say one for differences, one for gain, and such, when all can be put in the same op-amp.)

    These are some questions that is in my mind for awhile, if you can help me answer any of them, it will be much appreciated.
     
  2. bertus

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  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Those links are for diff-in, diff-out amplifiers. I think the OP is interested in single-ended output.

    Yes.

    Yes.
    It is the latter only.
    That is the standard solution. Any modern op amp can be powered by either a single supply, or dual supplies. The limitations are defined in the datasheet (input common mode range, output voltage swing, etc.)

    If you need very high input impedance, you will need 2 or more op amps. If you can live with some resistive load on your input source(s), you can get by with one op amp.
    Keep in mind that resistor matching becomes critical if you build your own differencing amp. You might want to consider using an instrumentation amplifier, which does all the matching internally.
     
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  4. JMac3108

    Active Member

    Aug 16, 2010
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    Yes, the non-ideal parts get amplified too. For example, a few millivolts of input offset is no big deal in a unity gain buffer. But if the gain is 100, then suddenly the output has a few hundred millivolts of error due to offset!

    Many op-amps are not rail-to-rail. This means they can not drive all the way to the power supply voltages. Even rail-to-rail op-amps may not get to 0V fully. This is not an offset throughout the entire op-amp range but just a limitation on the level that it can drive to at the high and low power supply rails.

    There could be any number of reasons to seperate functions into seperate op-amps. Many times its just bcause the designer did not know how to design what he needed in a single step - he was using standard circuits that he knew and cascading them together.
     
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  5. crutschow

    Expert

    Mar 14, 2008
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    Another reason to cascade op amps is if you need significant gain at some high bandwidth that's beyond the gain-bandwidth product (GBW) of a particular single op amp. Using more than one op amp allows a higher gain for a given frequency response.
     
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  6. kevin.cheung19

    Thread Starter New Member

    Nov 22, 2011
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    Thank you all SO MUCH! Super fast reply, I really appreciate you all for all the great answers! Now I understand a lot more now.

    Now I know why people separate out the functions for the op-amp now. And I'll definitely look into an instrumental Op amp.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    Another reason for cascading them is to increase the orthogonality of the adjustments. It is handy to have controls that, when you adjust them, change only one parameter and not a bunch of them.

    Also, look into "instrumentation" not "instrumental" amplifiers (though the search engines are getting so good that it probably won't matter too much).
     
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  8. SgtWookie

    Expert

    Jul 17, 2007
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    Have a read through this:
    http://www.intersil.com/content/dam/Intersil/documents/an12/an1298.pdf

    It does a good job of explaining Ron_H's statements:
     
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  9. kevin.cheung19

    Thread Starter New Member

    Nov 22, 2011
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  10. SgtWookie

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    If you manage to find one, it'll probably cost $300 or more.

    Why don't you try to explain exactly what it is that you are wanting to do, and what are the signals that you're trying to use as inputs?
     
  11. kevin.cheung19

    Thread Starter New Member

    Nov 22, 2011
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    I need to measure a 0-3VDC differential voltage (Common Mode voltage can vary from 10V-350VDC) coming off from two low impedance source (<0.1Ohm, but they are within <3V apart always) and feed that into a DAQ that has 0-10VDC input. I'm looking to get 1mV accuracy of the differential signal. I'm looking to sample this at least 20Hz minimum. If I can fit this inside a 5x4x1.5" project box, it'd be great, the dimension is actually flexible, but the smaller, the better.

    I've been using AD629, but it's not high enough in voltage and it is a diff amp, rather than an IN-Amp.

    $300 is actually Ok for a workable solution. Although $100 is what I'm targeting, $500 is what I'm considering maximum.
     
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