Single-supply vs. Rail-to-Rail?

Discussion in 'The Projects Forum' started by blah2222, Nov 27, 2013.

  1. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Hi all,

    Silly thought that has been bugging me for the last while.

    I have an AD620 (instrumentation amplifier) that requires a split-supply. I am using a 9V battery and have created a virtual ground at the battery's mid-point (+4.5V) using a TLC277 (single-supply) op-amp to attach to the AD620's reference pin.

    My question is, if I pass the output of the AD620 through a passive high-pass filter and then through an inverting amplifier (using the TLC277) does the resistor in the high-pass filter need to be tied to the virtual ground (+4.5V) or can it be connected to the negative battery terminal and the same question regarding the positive input of the op-amp?

    From what I have understood, a single-supply op-amp is pretty much the same as a split-supply op-amp but it can be operated much closer to the supply rails for inputs and outputs.

    Would it be fair to say that "rail-to-rail" and "single-supply" are synonymous?

    Thank you,
    JP
     
  2. LvW

    Active Member

    Jun 13, 2013
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  3. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
    554
    33
    Sorry, the page didn't load after submission and had to retype question in aforementioned thread.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,392
    1,606
    Cannot say without a schematic.

    Errr... it hurts my ears to see that, though it is true "rail-to-rail" types are recommended for "single-supply" situations.

    The terms mean different things. "Single-supply" describes the power being supplied. "Rail-to-rail" describes the permissible voltage range of an input or output.

    Thus it's possible to fine a rail-to-rail amp that works with a split-supply. Also rail to rail is not a requirement to get an amp to work with a single supply.
     
  5. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Schematic attached. Note that the LT1920 is a drop-in replacement for the AD620 that I am using and is also a split-supply instrumentation amplifier.

    I have currently biased the LT1492s (drop-ins for TLC277 single-supply dual op-amps) so that R2 and R10 are connected to Vcc/2 (+2.5V).

    Would there be a significant difference if I switched their connections both to GND rather than Vcc/2? I'm assuming the negative peaks will saturate the amplifier.

    I have seen some schematics for single-supply op amps that interface with ADCs that have GND connections rather than Vcc/2 for similar circuitry. Is this because the inputs and outputs are always contained between 0 and 5V and are just pulses?
     
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  6. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    **EDIT**

    Sorry, the gain stage had an incorrectly labelled V- rail.

    Attached is fixed schematic.
     
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  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Not exactly. Single supply devices typically means the inputs can go down to ground (or the negative rail like the LM358 does). Rail-to-rail means the inputs or outputs can go all the way to either the neg or pos supply.
     
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