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Project, How to find chips?

  1. Oct 8, 2013 #1
    Hi, Im a Mechanical Engineer and I really love the field of mechatronics so I got myself a nice team and we're currently working on a project that requires me to redesign a PCB, I have the circuit schematics and the original PCB, but I have little to no experience with actually designing them. (I know how to use SMT and PNP machines and I've done rework and reflow myself).

    I was looking over the schematics and there are 3 OP Amps per channel, among other things but it uses LM324D chips which have 4 OP Amps in them, so it leaves one unused OP Amp. The thing is this device needs 5 channels, so I was thinking is there a chip with 15 Op amps in it? if so how can I look for it? where do I find it? I tried googling that but I didnt find anything useful. I really want to reduce the size of the board so a 15 OP amp chip (or maybe 2 8 OP Amp chips?) will probably help me reduce this drastically. Or maybe is it possible to cut out one of the op amps? so it only requires 2 op amps per channel, and so I can do with a 10 channel chip? (still where do I find one?)
    Some more info:

    The circuit has 3 OP AMPS, one is a transimpedance amplifier, followed by a 2nd order butterworth filter fc= 39Hz and the third one is an inverting amplifier.


    -Thank you!
     
    Last edited: Oct 8, 2013
  2. jcsd
  3. Oct 8, 2013 #2

    UltrafastPED

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    Try Mouser.com, op amps, and select 10 channels:
    http://www.mouser.com/Semiconductors/Integrated-Circuits-ICs/Amplifier-ICs/Operational-Amplifiers-Op-Amps/_/N-6j73m?P=1z0lvrn&SAP=true&gclid=CMivxpSAiLoCFfFDMgodHDkANg [Broken]

    There were 15 hits. But the minimum purchase quantities, and the chip package reduces your options to maybe one ... which is not carried in stock.

    But why not just use two quads? You will have to change the layout anyway ...
     
    Last edited by a moderator: May 6, 2017
  4. Oct 8, 2013 #3

    SteamKing

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    This is a list of LM series ICs:
    http://en.wikipedia.org/wiki/List_of_LM-series_integrated_circuits#Operational_amplifiers

    Quad OP-AMPS seems to be the most on a single chip. You realize that 15 OP-AMPS on a single chip would mean 3*15 + 2 = 47 pins in a DIP? That would be 1 big chip not to be a CPU.

    How about arranging the circuit wiring to use the fourth OP-AMP? You may not get all of the elements for a single channel on one device, but possibly you could use the spares from several devices for a single channel, leftovers, as it were.
     
  5. Oct 8, 2013 #4
    yeah that would work too! but since I am not too experience with electrical engineering (Im a mechanical as I said) I really dont know much about how to do that, I mean are these op amps all the same on the chip? is it 4 exactly equal op amps? or do they vary somehow? would it matter which one goes first? (I mean as long as the other components are wired correctly I would assume which op amp wouldnt matter would it?

    heres 1 channel, as I said I need there to be 5 channels, so I was thinking maybe taking one OP amp off? (i dont know if thats possible?)
    this one uses 3 op amps, which is 1 chip, and theres a leftover unused. . .so maybe I can do the whole 5 channels with 4 chips overall, 16 channels, 1 left over total. . .that could work actually. . .

    thanks for the replies BTW!

    http://img818.imageshack.us/img818/4913/y8f5.png [Broken]
     
    Last edited by a moderator: May 6, 2017
  6. Oct 8, 2013 #5

    UltrafastPED

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    All of the op amps on a quad IC are "identical" - that is, they all share the same spec sheet.

    You will find that the layout is simpler if you use a quad for each channel.
     
  7. Oct 8, 2013 #6
    but wouldnt that mean I'd have to use 5 chips which would in turn become a bigger PCB right? Im trying to reduce the size of the current PCB
     
  8. Oct 8, 2013 #7

    UltrafastPED

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    Use both sides ... use smaller components ... pay somebody to do it for you ...
     
  9. Oct 8, 2013 #8

    AlephZero

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    Life will be a lot simpler when you are testing and debugging if all 5 channels have the same physical layout, at least for the first prototype(s). If you want to squeeze it onto the minimum number of chips later, at least you know it worked before you redesigned the layout!

    Looking at your circuit for one channel, you might be able to get away with 2 op amps not 3. The third one seems to be a fairly low gain inverting amplifier. You can get the "inverting" functionality just by swapping the input leads over. You might be able to get make up the gain in other parts of the circuit - e.g. make your filter stage have some gain, and/or increase VR1.
     
  10. Oct 23, 2013 #9
    Thanks for all the responses, they've been really helpful, I've been reading a lot about circuits and am starting to get the hang of it, Im just a little confused since I am entering this project kind of midway so I have to catch up to it. Anyways you already saw the schematics, there is a problem though, as I am looking at the PCB thats linked to that schematic (in EAGLE), I dont see C1 or C2, they are nowhere to be found. . . I already checked the inner specs of the op amp chips and they dont have integrated capacitors or anything. So I was wondering what happened there? is that capacitor not a capacitor? is it part of a Variable resistor?
     
  11. Oct 24, 2013 #10
    * CANT EDIT PREVIOUS POST :(*

    I finally saw those capacitors, they were indeed in the bottom part. Anyways. I have a question for AlephZero:

    You mentioned I might be able to get away with 2 op amps and not 3, how would I remove the third one? my question is, should I remove that capacitor and resistor that are right before the third OP amp, or should I leave them alone and just cut the third op amp?
     
  12. Oct 24, 2013 #11

    meBigGuy

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    You would need to understand whether you need the independence of the gain control (and changing frequency corner) on the first amplifier as well as the gain control of the last amplifier. The filter is best left as fixed gain.

    For example, it is possible that the gain of the 1st stage needs to be adjustable, but low enough so it and the filter will not clip for any given input device (devices vary from unit to unit), and the output needs to be variable because what it is feeding may have different gain requirements, or maybe it is adjusted often.

    You can't remove the resistor as it is part of the gain control, and the capacitor is probably there for a reason. Could be the dc offset in the input device, or the dc offset in the 1st 2 stages, or the desire to have a 1.5Hz low frequency corner. Without it you will gain up the DC from wherever it came.
     
    Last edited: Oct 24, 2013
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