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Integrated Resistor accuracy

Discussion in 'General Electronics Discussion' started by ryz, Aug 16, 2012.

  1. ryz

    ryz

    6
    0
    Aug 16, 2012
    Greetings dear forum members!

    Today I was going through the specifications of an ASIC with a colleague, and I was shocked to hear that the manufacturer was only able to grant a 30% tolerance on the integrated resistors. Anything more accurate would have cost a whole lot more additional money. This ASIC however was created several years ago, and I was wondering how is the situation today?
    Is it still normal to have such low accuracy for integrated resistors (without trimming and additional procedures) or did the industry develop since then?
    I was trying to google this but no real relevant result came up. I guess it is only known by those who work with these things dayly, and I was hoping that I can find some of those people here :).

    Thank you in advance for answering my silly question!
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,129
    1,842
    Nov 17, 2011
    Not so silly a question at all. At first glance it is quite unexpected if you're not familiar with the IC fabrication process.

    This is normal and absolutely no reason to be shocked. The reason is the IC technology that is not well suited to building resistors, let alone accurate ones.
    If you need a resistive element, try to find a circuit that doesn't use absolute resistor values but resistor ratios instead. Whereas a single resistor can be fairly inaccurate, the ratio of two resistors can be controlled much better, because the resistors on a chip tend to tolerate all into the same direction (either all too high or all too low).

    Or try to avoid resistors at all. The inaccuracy of integrated resistors was one of the reasons that led to the invention of "switched capacitors" (and capacitors can be much more easily integrated into the fabrication process of an IC. But even capacitors on ICs have large tolerances, therefore e.g. switched capacitor filters rely on capacitor ratios, too.

    Looking at modern ICs that require precision resistors you will very often find that an external resistor has to be added (e.g.adjustable voltage regulators, settting the current for LED drivers etc.)
     
  3. bountyhunter

    bountyhunter

    11
    0
    Mar 9, 2012
    For IC's, 30% absolute tolerance is typical but resistors on the same IC track EACH OTHER extremely accurately so resistor ratios are very accurate. That is the reality of IC design.
     
  4. ryz

    ryz

    6
    0
    Aug 16, 2012
    Thank both of you for your answers! I have studied about IC manufacturing technologies but this must have skipped my mind.
     
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