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Opamp/Circuit Recommendation? — Parallax Forums

Opamp/Circuit Recommendation?

JonnyMacJonnyMac Posts: 8,923
edited 2015-06-28 11:50 in General Discussion
I need to bump a 0-to-5v signal by 2.5 volts so the output is 2.5v to 7.5v. The Propeller side of things has the 5v supply; the other device can provide ground, 2.5v, and 7.5v.

I'm open to suggestions!

Thanks.

Comments

  • jmgjmg Posts: 15,144
    edited 2015-06-21 14:55
    If you have 7.5V power and need 7.5V out, then almost any Rail-Rail IO opamp should do.
    Other selection numbers are offset & drift and bandwidth, and price...
  • Heater.Heater. Posts: 21,230
    edited 2015-06-21 15:39
    So basically you need to add 2.5 volts to whatever signal you have.

    You need an op amp summing amplifier: http://www.electronics-tutorials.ws/opamp/opamp_4.html and a source of 2.5 volts.

    Assuming your devices have a common ground and your power rails have enough head room I'm sure you can work it out.

    Warning: I have never done this but my confidence in op amps is great :)
  • frank freedmanfrank freedman Posts: 1,975
    edited 2015-06-21 17:40
    When I was playing with my multichannel sampler, the samples were sent (for testing) out parallel to a homebuilt r2r DAC. The DAC used an NTE928 Dual op amp. This is a single supply device that was powered from a nine volt battery. the input side of the R2R array was directly from the propeller IO pins. Check also the TI publication "Op Amps for Everyone" probably more than you want to know but it's really good reading with a fairly broad footprint. Google "OP AMP level shifter" as that sounds like what you are trying to accomplish......

    FF


    Some of the older med tv systems used to use an inverting level shifter to essentially shave off the dark current signal from the actual live TV signal. Dark current would be sampled when x-ray was off, and subtracted when x-ray was on.





    "
  • JonnyMacJonnyMac Posts: 8,923
    edited 2015-06-21 17:45
    Thanks, gents! And Happy Father's Day to the dads amongst you!

    Check also the TI publication "Op Amps for Everyone" probably more than you want to know but it's really good reading with a fairly broad footprint.

    Will go find it now.
  • frank freedmanfrank freedman Posts: 1,975
    edited 2015-06-21 18:03
    JonnyMac wrote: »
    Thanks, gents! And Happy Father's Day to the dads amongst you!

    .

    And to you as well Sir.
  • macrobeakmacrobeak Posts: 354
    edited 2015-06-21 18:12
    Frank,
    Thank you for that TI publication "Op Amps for Everyone" tip.
    It is pure gold!
  • DomanikDomanik Posts: 233
    edited 2015-06-21 23:09
    You might be able to drive 1 side of a 2.5V reference pulled high through a 1K to +12V. The other side of the 2.5V reference would be offset from the Propeller output, giving you 2.5V to 7.5V. I've seen this done with zeners.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2015-06-21 23:17
    Most op amp ICs have the same pinout. You most certainly will need a circuit board to build anything reliable. And most opamp solutions retain linear gain by multiple stages. So a 2x or 4x solution (for stereo audio) are more common than just 1x.

    Elliot Sound Products has IC boards for a versitle use of a 4x op amp that may keep the cost down. In this case you might only use one side of it (just 2 stages, intended for stereo preamp).

    sound.westhost.com

    I know you may be thinking this is overkill, but it is just about the only good mail order source for op-amp project IC boards.

    These days it is actually harder to buy a 1x op amp IC than a 2x or a 4x. Elliot Sound Products Project 88 board can pretty much adapt to any project.

    http://sound.westhost.com/project88.htm
  • jmgjmg Posts: 15,144
    edited 2015-06-21 23:50
    JonnyMac wrote: »
    I'm open to suggestions!

    It can be this simple
                            +5V
                         |
                        .-.
                        | |
                        | |100k
                        '-'
                         |
                   ___   |     |\
           0~5V---|___|--o-----|+\
                    50k        |  >---o------- 2.5~7.5V
                           .---|-/    |
                           |   |/     |
                           |    ___   |
                           o---|___|--'
                           |     50k
                          .-.
                          | |100k
                          | |
                          '-'
                           |
                          GND
  • Heater.Heater. Posts: 21,230
    edited 2015-06-22 01:57
    Ah yes, better still a non-inverting summing amplifier.

    Here is a nice article, full of juicy maths, explaining how that works:
    http://masteringelectronicsdesign.com/the-transfer-function-of-the-summing-amplifier-with-n-input-signals
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2015-06-22 07:23
    Much clever boffinry going on there, jmg.

    Testing it out now. Op amps like to keep their + and - inputs the same voltage. So plugging in some random input voltages, and the output voltages seem to work out right. A cunning circuit!
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2015-06-22 07:54
    Well, here is another ready to go printed circuit board for Op-amp projects. This uses the 2x op-amp, not the 4x

    http://www.pmillett.com/opamp_pcb.html

    Ebay has a few others, but I am not willing to investigate all the pros and cons of each board.
  • frank freedmanfrank freedman Posts: 1,975
    edited 2015-06-27 15:35
    So, how did this one turn out? What circuit did you go with?

    Thx

    frank
  • Duane DegnDuane Degn Posts: 10,588
    edited 2015-06-27 16:20
    Jon, make sure and watch the video I linked to in the Art of Electronics thread starting at 14:14. Paul Horowitz explains a graph about op amps. I recommend the entire video but I thought the section about op amps was particularly interesting.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2015-06-28 05:46
    As far as the OP's original project, I'd have to say 95% of the task is properly selecting an op-amp for the project.

    The vast majority of op-amps are intended for dual voltage supplies and provide better gain and better linearity if +15V/-15V supply is provided.

    So working with a single sided supply and in a lower voltage range is going to mean researching a specialized op-amp. It also may mean that the need for multiple stages to get the right amount of gain should be considered. (The Art of Electronics, 2ed by Horowitz, et al does have a selection table for op amps, but I fear that is a bit old and out-dated.)

    So an output swing from 0 to 7.5 Volt DC is less than ideal for many op-amps. It may even be better to use a digital device that is similar to an op-amp - a comparitor. For those not aware of it, many comparitors are willing to operate far beyond the +5VDC for the sake of providing a higher voltage output.

    In my experience, using an op amp for a digital frontend creates more work than is necessary. Considering a comparitor might make the whole design cleaner - (a) the device is intended for a single voltage supply, (b) hystersis is easier to deploy.
  • Heater.Heater. Posts: 21,230
    edited 2015-06-28 06:16
    Loopy,

    How on Earth is a comparator supposed to translate an input of 0-to-5v to an output of 2.5v to 7.5v ?

    This is not a digital requirement and I'm sure hysteresis is not desirable. Mind you JonnyMac has not said exactly what he want's to do with this only given us an abstract problem.

    There must be a ton of rail to rail single ended supply opamps that can do this now a days.
  • JonnyMacJonnyMac Posts: 8,923
    edited 2015-06-28 08:16
    I do require analog output that moves between 2.5v and 7.5v. Need to pop over to All Electronics (yes, it's open on Sundays!) for some parts. Will have an idea later today.
  • User NameUser Name Posts: 1,451
    edited 2015-06-28 08:27
    Goes without saying that if the signal you need to level-shift is AC, a capacitor is your friend. Fair-sized electrolytic caps have for decades been used on the output transistors of audio power amplifiers to remove DC bias - functionally indistinguishable from adding DC bias, which is what you want to do.
  • BeanBean Posts: 8,129
    edited 2015-06-28 08:43
    Jon, If you do NOT need a common ground, I would be tempted to cheat and connect the 2.5V from the other circuit to the propeller circuit ground.
    You just need to make absolutely sure that the grounds do not connect through any other circuit.

    Bean
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2015-06-28 09:33
    Heater. wrote: »
    Loopy,

    How on Earth is a comparator supposed to translate an input of 0-to-5v to an output of 2.5v to 7.5v ?

    This is not a digital requirement and I'm sure hysteresis is not desirable. Mind you JonnyMac has not said exactly what he want's to do with this only given us an abstract problem.

    There must be a ton of rail to rail single ended supply opamps that can do this now a days.

    Okay, I misunderstood the original goals were not a conversion to an digital input.

    It still remains a rather special op-amp as most use a greater voltage range than 5 volts to provide better gain, roughly a range of 24-30V and usually dual supply.

    Rail to rail single suppy op amps tend to be the less common product, not so much demand. Not impossible to locate, but more less to choose from and maybe more expensive.
  • Beau SchwabeBeau Schwabe Posts: 6,547
    edited 2015-06-28 11:50
    It still remains a rather special op-amp as most use a greater voltage range than 5 volts to provide better gain, roughly a range of 24-30V and usually dual supply.

    Rail to rail single suppy op amps tend to be the less common product, not so much demand. Not impossible to locate, but more less to choose from and maybe more expensive.

    I completely disagree ... Most Op-Amps can be configured for single or dual supply by how you externally supply the voltage to them. To the Op-Amp all it "sees" is a relative voltage to it's supply pins... And as far as rail to rail versions, they are easier to find as well.
  • base2designbase2design Posts: 78
    Jon, make sure and watch the video I linked to in the Art of Electronics thread starting at 14:14. Paul Horowitz explains a graph about op amps. I recommend the entire video but I thought the section about op amps was particularly interesting.

    Argh... I was intrigued by your post Duane, but the link seems broken in the new forum. I'd tried searching for this on YouTube but couldn't seem to find the video... any new links are appreciated!
  • Heater.Heater. Posts: 21,230
    edited 2015-06-30 13:23
    The only graph about opamps in that Horowitz video is all about how to select the best opamp for various source impedances when really low noise is a priority requirement.
    I'm guessing Jon is not working at that level and could get away with something a lot cheaper and easily available.
    That video is well worth a watch though and is here: 
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