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Help with instrumentation amplifier set-up

Discussion in 'General Electronics Discussion' started by leeb_965, Apr 29, 2012.

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  1. leeb_965

    leeb_965

    34
    0
    Feb 5, 2012
    I've just started to take circuits I've simulated and try and do them for real on a breadboard but I am having trouble to get an instrumentation amplifier to work.

    I have attached the breadboard and simulation.

    For an input of V2 = 0.2 and V1 = 0.1 to get

    0.1 x gain

    which is 1 + (200k / 10k) = 21

    for an output of 2.1V

    On the oscilloscope I get an amplified sine wave but I increase the difference between V2 and V1, the positive half-cycle clips but the negative remains.

    Im not sure why this happening.

    Can anyone suggest a way to fix this? It works ok in the simulation.
     

    Attached Files:

  2. gto_ron

    gto_ron

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    Oct 5, 2011
    Leeb

    I'm not sure why you're building an instrumentation amp out of dicrete op=amps. You can buy one of several high-quality instrumentation amplifiers from the IC manufacturers all contained in one package. It needs only a gain-setting resistor. Analog Devices makes several and is the source I usually use. I believe they bought out Burr-Brown; who use to be the original instrumentation amp vendors.

    ron
     
  3. leeb_965

    leeb_965

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    Feb 5, 2012
    is this the usual way to use an inst amp? should discrete opamps work if they are all matched? what do you recommend?
     
  4. shrtrnd

    shrtrnd

    3,688
    457
    Jan 15, 2010
    Burr Brown's mfgr facility was in Tucson, AZ (where I am), they got bought out by Texas Instruments. When they did that, they brought in a lot of TI people, and a lot of guys I
    know who used to work for Burr Brown were out a job.
    Burr Brown made some of the best instrumentation op amps around, TI still makes some
    of their most popular components, but dropped a lot of the original Burr Brown line.
    You can still find Burr Brown Instr Op Amps out there, unless this is a do-it-yourself project.
     
  5. TedA

    TedA

    156
    16
    Sep 26, 2011
    leeb_965,

    I believe that you have a very conventional instrumentation amp circuit; it should work OK, within its limitations.

    For production use, a single integrated part usually costs less at any chosen performance level, but it is no sin to make your own using op-amps.

    Any recommendation to you would depend on your particular circumstances. We don't know what your circuit needs to do, or what your expected annual production rate might be.

    That said, I'll try to help with your questions.

    I'm a little confused by your description of what's happening.

    First, V1 and V2 do not appear to be labeled on your drawings.

    And you say they equal pure numbers. Could you mean DC voltages? AC voltages? And relative to what reference point?

    Nowhere in the linked schematic do I see a place with " 0.1 X gain". I see 21 X voltage gain, followed by plus and minus 1X voltage gain.

    The pictorial diagram shows "inputs from ac signal generator". What sort of generator? Does it have a balanced output, with the average fixed at zero volts DC?

    You might read up on "input common mode voltage range"; your asymmetrical clipping on the output may have to do with this.

    This particular circuit, with fairly low power supply voltages, will not accept very large input signals. The LF353 inputs are not guaranteed to work closer to either supply than 4V, so you have only a plus or minus 2 volt range from ground potential. Similarly, the outputs may quit at 3V away from each supply. Divide what's left by 21, and you have an input voltage of only 143mV peak. Real parts will do somewhat better than the MIN numbers off the data sheet, but this is no rail to rail part. ( They really kinda had plus and minus 15 volts in mind for the power.)

    These limitations might be mitigated by higher supply voltages, or by op amps with rail to rail I/O. Moving the gain to the second stage could help improve the common mode input range.

    So I suggest that you keep both the common and differential mode input signals small, for the circuit shown.

    And look out for the DC bias on the inputs if you are working with AC inputs. Op-amps have small but non-zero input currents that have to come from something off the left hand edge of your schematic.

    A good way to see what's going on is to ditch the AC on the inputs, and use a couple of pots connected across plus and minus 300mV, or so. You can measure the inputs and output and see if the result is as expected.

    I hope this is of help to you.

    shrtrnd,

    Yes. Burr Brown was a good outfit. And the LF353 original source was similarly swallowed-up; the ten gallon hat people have been to Santa Clara, too.

    Ted
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    9,543
    1,969
    Nov 17, 2011
    If you look at an instrument amplifier's datasheet, mostly a high resistance resistor from the non-inverting inputs to GND is recommended to have a path for the input bias current. 1 Meg is sufficient.Unless your signal generator is GND-referenced.

    Harald
     
  7. leeb_965

    leeb_965

    34
    0
    Feb 5, 2012
    The instrumentation amplifier is now working fine by itself but when a Phase senstive detector circuit is put at the output, the amplified signal has extra noise added to it. I think I need to look into putting a 1Meg resistor at the inputs like you say.
     
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