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Why are Instrumentation amps so expensive?

Discussion in 'Electronic Design' started by Roger, Jun 16, 2006.

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  1. Roger

    Roger Guest

    It is a question I have been asking myself for a couple of weeks, ever
    scince I got a requirement for a low offset drift high impedance true
    differential amp with high CMRR. I have a solution in the form of the
    $3 AD627, but I cannot really say that I am **satisfied**. There are
    some good cheap low voltage single supply solutions as well, but of
    course they don't let your inputs float around much!

    Can it be possible that a such a device costs more than a 24bit ADC?
    The obvious conclusion is that it is the laser trimming that costs, but
    keen to get to the bottom of this I went back to basics and blew the
    dust off my AOE. Now in ch.7 we have the run down on auto-zero
    techniques, chopper stabalized amps and blow off into differential amps
    and finally instrumentation amps....and things just dont add up. The
    first thing that crossed my mind is why don't FET input amps switch
    around thier inputs to get a zeroing effect. Ha! there is an example,
    the ICL7605 which uses a capacitor to pass a voltage to a chopper op
    amp. The text notes that the drawback is the noise, but I would expect
    that 16 years on somebody would have come up with such a device with a
    low pass buil in. Better still, why not make an ADC with this sort of
    front end built in....go and pick up a voltage somewhere and then eg.
    use it in a capacitive SAR, rotating it in opposite polarities each
    conversion. Alas, not only can I not find such a device, I can't even
    find a ICL7605.

    The second thing I find curious is the theoretical **advantage** of the
    instrumentation amplifier over a differential amplifer with simple
    buffers on each input. The text notes that one of the snags of the
    differential amp is that tight resistor matching is required to achieve
    high CMRR, whilst in the instrumentation amp configuration this is not
    necessary. OK, but let's have a look at the 'INA' range that the text
    quotes...the laser trimmed differential amp devices cost less than the
    true instrumentation amps. How come? Of course the snag with the
    instrumentation amps is the low impedance, of course it is not a
    problem in some apps such as thermocouples but if your measuring a
    bridge, well it is a disadvantage. Were it not for price the true
    instrument amp would win hands down over the diff amps in nearly all
    applications....if nothing else they are also easier to protect. So the
    fact that a range of instrument amps also includes lowwer cost low
    impedance diff amps suggests to me that there is **something** in the
    instrument amp that makes them costly.

    Clearly I have missed something here, or perhaps there is some new
    technique that eclipses all requirements and instrument amps are
    considered legacy?
     


  2. do a search for Guy Macon


    martin
     
  3. Roger

    Roger Guest

    Should I be looking at the insult file?
     
  4. Well, yes, your post was rather verbose, I sort of fell asleep
    reading it. Can you repost in sort of 10 lines or less?


    martin
     
  5. Roger

    Roger Guest

    Sorry if I offended anyone, but your right, it is wordy. Ver II: (7
    lines)

    Why are true instrumentation amps so expensive? I understand from AOE
    that one of the advantages of the instrumentation amp over a difference
    amp is that resistor matching is less critical, and yet if I look at
    e.g. the 'INA' range of amps, the differential amps cost less than the
    instrumentation amps.

    I would have expected technology to have produced some sort of chopper
    amp auto zeroing instr. amp by now, or perhaps there is now a 'better'
    way to capture a floating differential voltage and present it to an
    ADC?
     

  6. Dont know. We will have to wait for Win Hiill or Jim T or other
    guru's to reply
    Sorry , just making a point



    martin
     
  7. John Fields

    John Fields Guest

     
  8. Jim Thompson

    Jim Thompson Guest

    Even from the "verbose" version it's not clear what the OP is trying
    to accomplish ;-)

    Maybe a "flying cap" ??

    What input voltage range?

    ...Jim Thompson
     
  9. Guest


    Wow, up to Version 6.12 ALREADY?

    http://www.guymacon.com/FUN/INSULT/INDEX.HTM

    "You are not ANSI compliant and your markup doesn't validate. You have
    a couple of address lines shorted together. You should be promoted to
    Engineering Manager."

    That's just wrong. ;)
     
  10. Joerg

    Joerg Guest

    Hello Roger,

    It's probably because of the low market volume. They are used in
    measurement setups and university projects and that's not a high volume
    market.

    For high volume products that require a large CM range engineers usually
    spend quite some time to make it happen with jelly bean parts. Often the
    first stage is discrete.
     
  11. Yes, they exist, for less than a couple dollars in 1K. They're not
    cheap because there are really no high volume applications for them.
    I've been able to avoid them in most instrumentation applications, for
    example.
    If it's not moving too fast you can just measure the two voltages and
    subtract digitally.


    Best regards,
    Spehro Pefhany
     
  12. Ok, lets say that you are hunting for components to acomplish certain
    function in known enviroment. You are as free as a bird to use what fits.
    I am test department tech. on hunt for instrumentation amplifiers to
    amplify, for computer recording, your ADC, physical parameters like heat
    (thermocouples from copper/constantan to platinum/platinum-iridium and
    tungsten; strain gauge reading of forces from static to mechanical shock
    and so on and on)
    So I end (early '80s) with instruments of Vishay at $400/each (50 qty)
    and do my testing for more than 8 years without failures. And NOT in lab
    but in enviroments where the Tektronix scope displayed a spiral trace
    with the trace moving in oposite direction to the flow of time. (Real
    SCI-FI).
    And the technical design department want accuracy <1% at ANY reading.
    Actually you pay for ability to use the equipment to specs.

    My 2c.

    Stanislaw
    Slack user from Uladulla.
     
  13. Genome

    Genome Guest

    If true then it's a little bit strange.......

    Guy Macon sort of promotes himself as an 'Engineering Manager'.

    Oh..... I'm being slow :)

    DNA
     
  14. Ban

    Ban Guest

    Don't get confused by AoE.
    Instrumentation Amps have some indispensable advantages over diffamps or
    discrete designs. The input impedance is very high, *equal* for both inputs
    and for *common mode* and *differential mode* signals. This maintains the
    CMRR for any source unbalancing. The internal resistors are trimmed to very
    high precision. A single resistor can vary the gain.
    The AD8225 has a gain error of +/-0.1% and 5ppm/K drift max. Offset error is
    not so much of an issue today, it can be corrected for in the digital
    domain. This amp also has a CMRR of >80dB at 10kHz, very difficult to
    achieve with discrete parts, C-trimmers are needed for that.
    There are quite a bit of applications where IAs are used: RTI, thermocouple
    etc. bridges, professional audio, 4-20mA receivers, EKG front-end....
     
  15. Very well said. I use the AD620 by Analog Devices, and it works very well
    to measure currents as low as 5 amperes on a 1000A 100 mV shunt. This
    corresponds to 5 mV, and my 1% accuracy specification means I must have no
    more than 50 uV of noise or common mode error, where common mode voltage
    could be as high as one or two volts. I pay about $5 to $8 for these (in
    small quantities), but it is quite reasonable in a $3000 instrument that is
    used for calibration purposes. I have also used the INA118 or similar
    devices, which were somewhat cheaper, but had more noise and drift.

    Paul
     
  16. Genome

    Genome Guest

    Yes, me neither. I could build one of these out of an LM324 and use the
    spare one to drive an LED.

    Bastards!

    DNA
     
  17. Roger

    Roger Guest

    Jim Thompson wrote:

    Nothing. In the verbosr version I even mention that I have a solution
    for my app.

    Mine is purely a curiosity question: "Why are instrumentation amps so
    expensive?".

    I could narrow down the field and ask "Why do the instrument amps tend
    to cost more than the diff amps in a range of precision amplifer
    products such as BB's INA range".

    Answers here and elsewhere do not convince me:

    1) Laser trimming. Well, good diff amps also need laser trimming and in
    some respects (according to AoE) the instrument amp cct makes some
    resistors matching less critical.

    2) Circuit complexity. Scince when has this been a major factor in IC
    prices.

    3) Quantity. Instrumentation amps have a potentially much larger
    market, specificaaly they could be used to improve most designs where
    diff amps are used.

    Clearly there is something that makes them so much more expensive to
    produce than similar precision analog devices.

    I also entered into the question of why there are not improved
    "chopper" versions of instrumentation amps or cicuits which can do the
    same. I specifically noted that the ICL1706 device shown in AoE appears
    to be no longer made, nor is there anything like it.
    Let's answer this in one. Most ADC's require an input range within the
    5/3v3 supply range and only offer psuedo differential inputs which have
    zero CMRR characteristics, yet many sources cannot be readily adapted
    to this: Uninsulated thermocouples and bridge sensors optimised for eg.
    10V supplies are common examples. Even ground referenced sources (which
    in some respects includes uninsulated thermocouples) can benefit from
    differential inputs as sensors may be remote and/or in power cct's and
    be subject to large CM noise. In low impedence situations it is
    possible to use diff amps, but I would only bother to make the
    distinction if the diff amp saved me money, one good low priced
    instrument amps could eliminate the requirement for a similar diff amp
    in the same product range.

    But I also mused on the idea that there could be ADC's with a floating
    capacitor input, i.e. FETS which attach a cap to a source, and then
    switch it into the convertor, so the ADC can measure voltages outside
    the supply range which the core converter is running at and, by
    flipping the cap, could null out offset errors at the same time.

    Well, I would love an ADC like this, but perhaps that is because I do
    not know some technique that gets the same results at low cost ;-)
     
  18. Joerg

    Joerg Guest

    Hello Roger,

    Like where? I have used diff amps in designs that went into production
    but I have never used instrumentation amps on any released product.
    There was no need to.

    Ban has brought up good examples but even there cost often rules. Many
    good ECG (EKG) units go sans instrumentation amp.
    Offsets and extended input voltage ranges are often handled by clamping
    circuitry. In the same way that TV sets restore the DC bias for the
    video path from a source that isn't DC coupled.
     
  19. Roger wrote:
    (snip)
    If you need really good common mode range, this is the way to go. One
    pair of reed relays connect the cap to the field voltage, and after
    they are opened, a second pair connect the cap to the A/D and signal
    ground. You need definite break before make contacts.

    I have seen thermocouples, with more than a hundred common mode volts,
    read this way.
     
  20. Roger

    Roger Guest

    Still missing the point :-(

    If a precision instrumentation amp cost pretty much what a precision
    diff amp does then there would seem little point having both in a
    product range.

    So that rules out "quantity" as a factor.
     
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