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I want to build an oscilloscope preamp and wonder what sort of op amp should I use? What's the maximum input level a smartphone can handle and should I bias the signal (for instance -5V/+5v to something like 0-1V)? What means of circuit protection should I use? Can I use diodes?

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    \$\begingroup\$ At the very least you should current limit the signal. If you have a high impedance scope, buy a wired headset and probe on the microphone line to see what kind of voltages you get. Neat project! \$\endgroup\$ – HL-SDK Jan 6 '14 at 16:28
  • \$\begingroup\$ Which input of the smartphone do you intend to use? \$\endgroup\$ – alexan_e Jan 6 '14 at 17:02
  • \$\begingroup\$ Uh, sorry me, the mic input. I guess it would have a voltage divider, some sort of range selector, an op amp buffer and some circuit protection measures. I wonder which op amp would suit this project? \$\endgroup\$ – alkopop79 Jan 6 '14 at 17:18
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    \$\begingroup\$ There is insurance too: cell phone replacement plan from wireless carrier. :) \$\endgroup\$ – Kaz Jan 6 '14 at 18:04
  • \$\begingroup\$ Check also here for variable gain and dual channel: marzocchi.net/Olafsen/Hardware/Oscilloscopio?setview=en \$\endgroup\$ – FarO Aug 19 '16 at 16:36
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The microphone input will be sensitive to signals of millivolts, and definitely should be limited to 1V or less. It will probably be AC coupled internally.

See http://www.instructables.com/id/A-Preamplifier-for-Smartphone-Oscilloscopes/ and http://hackaday.com/2012/07/14/android-oscilloscope-built-from-parts-just-laying-around/ ; the latter of those is just an attenuator, relying on the built-in preamp. The former adds a unity gain buffer as an impedance converter, and will probably produce better results. In both cases it's only suitable for audio frequency ranges.

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  • \$\begingroup\$ I've seen both and I'll go for the buffered one. I fonder though if I can use a generic op amp? \$\endgroup\$ – alkopop79 Jan 6 '14 at 17:38
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    \$\begingroup\$ You need one that's stable at unity gain (not all are, e.g. LM386) \$\endgroup\$ – pjc50 Jan 6 '14 at 18:47
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    \$\begingroup\$ Check the special op-amps called "instrumentation amplifiers" or in-amps. They have high impedance and differential inputs and use a single external resistor to select gain which you can switch for range. Check the application notes for very low cost jameco.com/Jameco/Products/ProdDS/825934.pdf I prefer a little more, like AD620 jameco.com/Jameco/Products/ProdDS/1780862.pdf or go all out with precision single supply jameco.com/Jameco/Products/ProdDS/1729936.pdf (Not pushing JameCo. They have easy to find data sheets). \$\endgroup\$ – C. Towne Springer Jan 6 '14 at 19:28

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