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Attenuation of op-amp gain

Discussion in 'Electronic Design' started by Dave Boland, Mar 31, 2005.

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  1. Dave Boland

    Dave Boland Guest

    A design I'm doing uses and ADC with a 2.5 volt reference.
    The ADC if buffered with a voltage follower. The problem is
    that some of the signals are >2.5 volts, and have an Rout of
    10K or more. At first thought one would use a voltage
    divider with high resistance values (x10**6 ohms)in front of
    the op-amp. The problems with that are that it increases
    the resistor noise and the Rin varies with each attenuation
    setting (0-2.5v, 0-5v, 0-12v, 0-50v). Any good ideas on a
    simple way to do this and keep a relatively constant Rin at
    10M or more?

    Thanks,
    Dave
     

  2. What are your requirements w.r.t. frequency response
    and accuracy? How many bits are you converting and
    what are you doing now about anti-aliasing? What is
    the expected range of source impedance? Is there a
    big cost concern, or can performance drive the design?
    The answers to your question will depend on such facts.
     
  3. Fred Bloggs

    Fred Bloggs Guest

    His answers to those questions will not influence the outcome of
    receiving useful information from you that can be applied to anything,
    they will only lead to more questions and generalistic bs from you.
    You're so dumb, you can't even so much as outline a simple constant
    input impedance attenuator.
     
  4. I read in sci.electronics.design that Dave Boland
    Buffer the high-impedance signals individually and attenuate the buffer
    outputs? No high impedance input required.
     
  5. Dave Boland

    Dave Boland Guest

    Oh my, it looks like I've stepped into a hornets nest.
    Didn't mean to start an argument!

    The system runs under a RTOS, and the fastest readings are
    500uS. The accuracy should be better than 1%, and yes there
    is a filter between the op-amp and the ADC.

    Perhaps thinking of this as an intelligent DVM would be
    helpful. I looked for DVM schematics and the only on I
    found was for the Intersil chip. I'll spend some more time
    on this later this afternoon.

    I have seen some designs where the Rf/Ri for an op-amp stage
    was less than one, making the gain less than one. I have
    not been able to find the design or the part (op-amp). The
    op-amps I have seen so far are not stable for gains less than 1.

    Thanks for the help so far.

    Dave,
     
  6. There is unlikely to be any argument. For reasons not
    relevant here, I have acquired a vile shadow who feels
    obliged to post some vitriol in response to my posts.
    A brief review of his recent posting history should be
    enough to help you understand his purposes adequately.
    I will take that to mean that you are only looking for
    accurate response from DC to something less than 1 KHz.
    That eases the problem considerably since getting accurate
    frequency response from high impedance attenuators is an
    interesting challenge above a few 10's of KHz.
    What would be most helpful is to understand the real
    requirement. Without that, "solutions" offered here are
    very likely to solve a different problem than your own.
    If the op-amp is designed to be stable in the unity gain
    configuration, it will be stable for those Rf/Ri values.
    To see why, consider the loop gain rather than the
    closed loop gain. (None of this is to diminuate the
    issue with feedback lag due to shunt capacitance at
    the inverting input.)

    If the inverting configuration solves your problem,
    except for your stability concerns, I suggest you go
    ahead with it. To get a "better" solution would still
    require knowing your actual SNR requirement, the
    range of source impedances, cost goals, any gain
    switching requirement, and active input ranges.

    If you use the inverting configuration, you can place
    a cap around the feedback resistor to stabilize the
    circuit (assuming it is unity gain stable) and form one
    (real) pole of your anti-aliasing filter.

    Given what you've stated so far, I do not understand
    your concern with resistor noise. Unless you have
    conversion accuracy greatly exceeding the 1% you
    mention above, (and plan to acquire much greater
    accuracy also), that noise should not be a problem.
    HTH.
     
  7. Ted Edwards

    Ted Edwards Guest

    The first thing that comes to mind is use a unity gain voltage follower
    op-amp followed by a gain of (say) 0.5 inverting op-amp. Something like
    a TL082 and a couple of resistors could do the whole job. IIRC, the
    TL082 is unity gain stable. If not there are certainly bi-fet amps that
    are.

    Ted
     
  8. Fred Bloggs

    Fred Bloggs Guest

    You have quite a few shadows being the pretentious,pompous, and
    unaccomplished phony that you are.
    Apparently not enough "easing" for your sorry and confused excused for a
    brain...
    You already have enough information, the OP has given you the various
    attenuation ranges required, the output impedance of his sources, the
    sampling rate, the input voltage range, and the accuracy requirement.
    But you still insist on more information because you're clueless and
    need to stall.
    You already have enough information- it is a simple DVM- but still you
    have nothing to offer. Getting anything to work at even DC does not seem
    to be one of your strong points.
    No kidding...
    It should be obvious the OP does not know very much- resistor noise is
    just something he has "heard" of.
    That is a lot of hoping- most of your posts consist of non-informational
    and low-quality garbage. BTW - I really enjoyed your dog simple
    confusion over power line induction of currents into that copper pipe-
    too damned dumb to understand the OP said it was overhead and 100'
    removed- but not a bad try for a pedantic pseudo-intellectual- at least
    you have those elementary trig function derivatives on tap.
     
  9. Pooh Bear

    Pooh Bear Guest

    Quite highish input offset voltage though.

    Graham
     
  10. Ted Edwards

    Ted Edwards Guest

    STMicroelectronics show a version of the TL082B with 1mv typ/3mvmax Vio
    and 10uV/C. 2{log}2.5÷1E-3 = 11.3 bits so there should be no problem
    for 10 bits or less. With an offset adjustment on the second stage, the
    offset could be further reduced enough for a few more bits.

    Ted
     
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