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
    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,
  2. LvW

    Active Member

    Jun 13, 2013
  3. blah2222

    Thread Starter Well-Known Member

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

    AAC Fanatic!

    Apr 24, 2011
    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
    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

    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
    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.