Precision op-amp buffer

Discussion in 'The Projects Forum' started by sgrover, May 29, 2013.

  1. sgrover

    Thread Starter New Member

    Feb 1, 2013
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    I'm trying to build an circuit in which I feed a bipolar -10 to +10 V signal to an ADC. As a first step, I need to buffer the incoming signal (before level shifting)

    I want to do this as precisely as possible. So I chose to use a precision op amp with a low offset voltage.

    However, if the supplies to the op amps even slightly mismatched, that'll cause a DC offset in the op amp output.

    I only have a positive supply available for the opamp (this project being arduino-based). If I use the standard method of using a DC inverter (such as the 7660) to get the negative supply, the output impedance of the converter will cause a mismatch between the + & - supplies and a DC offset to appear.

    Is there another way of generating a precise negative voltage for use as an op amp supply?

    Thanks.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    A small mismatch between the positive and negative supplies will have only a small effect on the offset for a precision op amp with a high common-mode-rejection.
     
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  3. tindel

    Active Member

    Sep 16, 2012
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    crutschow is right - The power rail voltage offsets are no big deal. Opamps are generally current based devices, so as long as you're running enough voltage the current will be regulated correctly.

    Much more important in truly precision op-amp circuits are the input offset voltage, input bias current, input offset current, and noise density. Two amplifiers I use alot are the OP497 and the LT1078. Both offer superb Vos, Ios, Ib, and ND parameters. The OP497 is great for precision applications where you don't care about power consumption (22mW per amplifier, max). The LT1078 is great for low power (2.55mW per amplifier, max) applications and can be run on a single 5V supply rail-to-rail, but you sacrifice some precision.

    Another thing to point out is that it sounds like you're wanting to use two stages. Depending on how accurate you really want your measurements, then I would discourage this. Generally speaking the more amplifiers you use, the more error you introduce into your circuit. I would suggest offsetting your voltage on a single op-amp using an adder. Do note that your offset voltage MUST be a precision voltage for any type of accuracy. I generally use a LT1009 or a AD588 to accomplish this. The LT1009 is great for a single 2.5V reference... the AD588 is great for higher voltage and bipolar referances, but beware that it is a power HOG (300mW, max).

    All that said - The most important thing about high precision is to have a high Vin/Vos ratio. This can be difficult depending on the task. I try to shoot for a 100:1 ratio for 1% error at my lowest input voltage.
     
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  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    It is also no problem to ad another power supply or battery. For the negative voltage supply. And they do not have to be identical. If you use a good power supply without to much noise. You will get a much better solution than using the inverter IC. As with such a solution you will always have some inherent noise problems. If you want a precession system. You also have to take noise into your design considerations
     
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  5. crutschow

    Expert

    Mar 14, 2008
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    A good ground plane on the circuit board with good decoupling on all ICs is a big help in minimizing noise as well as a careful layout to keep any noise sources, digital signals, and the output signal well away from the sensitive input. Use a separate or split ground plane for the analog and digital and connect them together only at one small point (typically at the A/D converter which has both analog and digital signals).

    How many bits does the A/D have?
     
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  6. sgrover

    Thread Starter New Member

    Feb 1, 2013
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    Thank you very much for the clarification. I was mistakenly under the impression that mismatched supplies would cause a DC offset at the output. I now realize that all it does is result in a common mode voltage at the input.

    @tindel: Thanks for the op amp and voltage reference recommendations.

    The A/D I'm planning to use is the quad MCP3208 (12 bits resolution).
     
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