Op amps!

Discussion in 'General Electronics Chat' started by geoffers, Mar 14, 2015.

  1. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    Hi all,
    I've recently started fiddling in the murky world of analogue signals and op amps, as usual I started fiddling with out any more than a rough idea what was going on. After a lot of googling and poking stuff with a scope (pc picoscope, if you need a scope and haven't got one I'd recommend it!) I now have the bare bones of a rfid reader. I've yet to program a pic to decode the signal but have it 'extracted' from the carrier.

    This brings me to my questions, firstly I'm using tl082 op amps (because their cheap and it was what other folks have used:)) I've an idea they might not be exactly the best for my application though, I read on this forum they are only spec'd to get within three volts of the rails, I have gnd, 12v and 5v at the moment, 12v runs a mosfet driver ic, 5v logic which controls the driver. I could use comparators on the pic and a protection diode to read the signal but think a rail to rail op amp would be best.

    This is a long winded way of asking if anyone has a favourite rail to rail op amp?

    I also read somewhere that for a active bandpass filter you should have 100x bandwidth op amp, is this right? The tl082 is 3MHz my signal is 134kHz and it works?! Have I just been lucky.

    The carrier wave is also quite big, I've put clamp diodes on the first op amp to stop it getting cooked, are there any op amps that will stick around 20x supply at the inputs?

    Last thing (maybe!) one of the op amps is working as a comparator, I read on one web page this isn't a good idea? I like it because it cuts down on the number of different chips I need but would I be better off with a dedicated comparator?

    So basically I'm looking for a cheap rail to rail (12v-15v) op amp that can stick high inputs and work reliably as a comparator. There seems so many to choose from and I know there is a lot of experience on this site:eek:.

    Cheers Geoff
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    The last I heard you figured out bandwidth using bandwidth product (BWP).
    BWP=(signal frequency)x(gain)
    You have 3 MHz bandwidth and 134 kHz signal.
    3,000,000 Hz divided by 134,000 Hz equal to gain of 22.388.
    As long as you op amp has gain less than 22.388, you will be fine.
    Which brings us to the question.
    In that particular example (3 MHz, 134 kHz), what is the gain that TL082 provides?
    geoffers likes this.
  3. crutschow


    Mar 14, 2008
    An op amp is fine as a comparator as long as you don't mind the slow response time (due to their internal compensation) as compared to a dedicated comparator.
    geoffers likes this.
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    Part of the joy of designing is making your own decisions. If you succeed, great, if you fail, you learn. My favorite op-amp may not be your favorite op-amp. Select your op-amp and own the design.
  5. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    Thanks chaps,
    My gain is 5 so I should be fine :), I shall look into some comparators, although I suppose if the op amp works fast enough.....
    I get what your saying about learning from my mistakes, my desk is littered with education! In farming circles we say its good to learn from your own mistakes but cheaper if you can learn from someone elses! ;) What I'm concerned about is I read on this forum that the tl082 can vary from chip to chip, some will get closer to the rails than others, so my design might work with good ones then not with others, advice like that isn't on many data sheets. There are lots to choose from and I was hoping for so hints on pitfalls others may have found.
    Cheers Geoff
  6. PlasmaT


    Feb 19, 2015
    Very true.