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Thread: What programmable gain amp to use?

  1. #1
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    What programmable gain amp to use?

    I want to add a programmable gain amplifier to my Teensy-based guitar "pedal", so that I can use guitars with different pickup levels.
    I already have the buffer with gain 1, so I'll put the amp right after it.
    Should be low-noise and 3.3V power.

    And I need it to be controlled via SPI or UART, something Teensy can do. Don't want any knobs on my pedal, only the touch screen.

    Thanks.

  2. #2
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    I2C is also a good option for PGAs, and there are a good number of programmable pots available that can provide the same outcome. Lots of manufacturers (TI, MAXIM, Microchip, etc) all have audio-rated digital control PGAs or digital pots.

    The old Intersil (renesas) X9C103 series remembers its settings when powered down. It's not I2c but has a simple up/down three wire interface.

    Happy hunting!

  3. #3
    If you want to be particularly clever, you can just use a set of cmos switches to change the bias resistors around on demand.

  4. #4
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    Just found this: https://www.mouser.in/datasheet/2/25...20-1095341.pdf
    Looks like what I need. $3.22 on DigiKey, not bad: https://www.digikey.com/product-deta...20N-ND/1769257

    I think I'll try it.

  5. #5
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    Yes, that's the kind of thing I was suggesting, and at a good price.

    I'm not a big fan of TDFN packages - a really tiny footprint, narrow pad spacing and no pins out the side.

    Beware, TDFN comes in 0.4mm and 0.5mm pad spacing - the DS4420 datasheet is unclear which is used here.

    I downloaded the Eagle footprint and it appears to be 0.4mm. Have fun with it!

  6. #6
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    Thanks @palmerr,

    I'm pretty good with the heat gun and low-temp paste.
    The trick is to put a little paste all around, then heat up the whole chip and move it around the pad a little.
    Then the paste spread evenly and the chip "snaps" into place, with each pad getting sucked into its place by the tiny droplets of paste on them.

    The problem is with the PCB's. Can't pay $15 for one. Use to get it for $0.40 each from China, but you know where that went.
    I'll probably wait for the new Teensy 4.1 (if it ever comes, given everything), redo my PCB and order a couple for $15 each, crying.

    The real problem is that sometimes you just can't make some chips to work. I just wonted to do something already proven to work.
    Last edited by Andy Belov; 03-18-2020 at 02:12 AM.

  7. #7
    Never trust an Eagle footprint until you have verified it yourself. Been burned on that one.

    For that size of chip, I'd use a stencil. Solder bridges are pretty hard to fix when they are under the chip.

  8. #8
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    Use the low-temp paste. Takes 10 seconds to heat up the chip, remove it, wipe off the extra paste, move the chip around to make the paste adhere to the pads only and put it back on.
    I've done it lots of times and never damaged anything. The temperature is 130C, a little more than a boiling water.
    And it's just as strong as anything else.
    And if you see bridges between the pads, just go with a sharp solderer along the edge and the excess paste sticks to the solderer.
    Magic! Just be quick not to heat up the whole chip and move it again. But you can always put it back in 10 seconds!

    Just try it, you will never go back: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
    And the blower: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

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