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The dual amp front-end and other things....

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  • The dual amp front-end and other things....

    First of all...Howdy to all of the "old souls" still on the forum (and, of course, to all of you just starting out).

    While I may have been AWOL for a few years (?), I'm back.

    Soooo....
    Dual front ends and my take. I have replaced my NE5534 single op-amp front end with a dual op-amp LM4562 ( http://www.ti.com/lit/ds/symlink/lm4562.pdf ) as per my old post on the "show us your HH build" thread ( http://www.geotech1.com/forums/showt...ed-HH-detector ) {P.S.- don't forget to remove R13-1Meg!}.

    I also replaced the 'back-end', IC8-TL072/082 with another LM4562. I may be suffering from the," I washed my car so no wonder it runs better" complex - OR - Having matched front and rear amps really does make a difference. The scope says the noise is lower, but did I really GAIN anything? Hmmmm. The lower first stage gain did help the noise issue. What should I look for on the scope in time domain to empiricaly define the difference between the setups? Just a thought...

    I suspect that the LME49710 single amp version ( http://www.ti.com/lit/ds/snas376b/snas376b.pdf ) would be a great alternative to the NE5534 for a single amp front end. Just pricey.

    As of this post the LM4562's are not cheap (about $4.20 U.S.}. But if you are going for the shortest samples and quietest background noise, they might be the answer for your app.


    Just my thoughts... GTB

  • #2
    Hi GTB, I've been using a OPA1611 for the frontend amp in my detector, it has lower noise spec's than the LME49710.

    Low noise opamps are a waste if your using the standard 1k resistor and clamping diode frontend. The 1k resistor generates more noise than the opamp.

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    • #3
      Actually... it is a shunt, so ... no, it is not that bad.

      Comment


      • #4
        Hey there, Mickstv:
        I checked the specs on the OPA1611/12's. Pretty sweet. I'll try a pair of OPA1612's and check 'em out. I do have to agree with Davor, however. I also run a pass filter on the first amp stage. ATM, I'm playing with a virtual ground power supply based on the TLE2426 to quiet the op-amp PS feed a bit.

        Take care, GTB

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        • #5
          mickstv...on the other hand...

          Hey mickstv....perhaps I spoke a bit prematurely,

          The OPA1611/12 series is only available in SOIC packaging. While I don't mind building custom 'carriers' from DIP headers, I would bet that most here would rather not.

          I will try them, but personally, unless you are building Carl's SMT version (or you like to play the "SMT to DIP" conversion game {which I do}), I would pass on these. Just me...

          GTB

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          • #6
            Hi GTB, sorry about that I should have mentioned that they are SMD. It's not too hard to direct solder them to a standard DIP format using really short wires.

            With regards to the powersupply if your using the typical 7660 and it's synced to the TX pulse you shouldn't really have a problem with noise on the power rails.

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            • #7
              Hey mick..

              True enough about mod-ing them in. I just feel that a lot of the newbies would find that a bit dicey. I do like the specs. on them though.

              As far as the pwr supply goes, there are serveral variables effecting overall preformance directly linked to the 7660/coil 'pull down'/and the effects of a 'variable ground' as posed by useing VB+ as a ground reference. With the 'coupling' effect of running the pwr supply timing (N1) with the coil timing (freq&width), AND coupling the sample timing delays (i.e.-N1 again), you have slaved the circuit to the same unstable VB+ ground. This is not a big thing for most folks. However, with the timing delays and coils that I am useing and testing, it is a big deal for me.
              I prefer a solid and stable ground reference and power supply for what will happen later in the circuit and I believe that most op-amps would agree with me (although the only time electronics talk to me is when they're 'blowing smoke'). Your mileage may vary, I dunno...

              P.S.- Do you folks in NZ really have crayfish (river-roaches, crawdads, etc.) that get to 20+ LBS.?! mmmm....

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              • #8
                Hi GTB, re the 7660, I had the same problem I also tried a LT1054 but was no better. So my design is now using a LMR62014XMFE (element14 part number 206-4695) it's a SMD also. I set it up with a 200k trimpot in the feedback circuit to adjust the output voltage. This IC also has a shutdown pin which is tied to the gate of the TX, so it's off during receive. It's only been installed for a few hours and haven't done any field testing but waving big iron objects over the coil no-longer causes any problems.


                I'm actually no in NZ I just did that to trick another user ages ago.

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                • #9
                  Originally posted by mickstv View Post
                  ...... So my design is now using a LMR62014XMFE (element14 part number 206-4695) it's a SMD also. I set it up with a 200k trimpot in the feedback circuit to adjust the output voltage. This IC also has a shutdown pin which is tied to the gate of the TX, so it's off during receive. It's only been installed for a few hours and haven't done any field testing but waving big iron objects over the coil no-longer causes any problems.
                  Hi mick

                  Did you stay with using this option ? Was it an improvement overall ?

                  grey.

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                  • #10
                    Hi grey, no I don't use that IC anymore. My detector uses a TC4424 mosfet driver, only one half of the IC is used so I configured the other half as a charge pump triggered by the TX pulse from the micro. The output of the charge pump goes to a LP2950 (5v reg) which then powers part of the electronics i.e. frontend amp, sample switches and integrator circuit's. The micro and other output circuits are powered via a LT1054 which is synced with the TX pulse.

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                    • #11
                      Cheers, thanks for the info. Got any videos of your creation in operation ?

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                      • #12
                        Originally posted by greylourie View Post
                        Cheers, thanks for the info. Got any videos of your creation in operation ?

                        The design works on the bench and in my backyard, but I won't get a chance to test it out in the field until after Christmas. So no video's ATM.

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