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I hope someone with experience in maxim operational amplifiers could assist me. I try to use MAX412 operational amplifier to amplify a signal with the following parameters:

  • Signal Type : AC
  • Amplitude: (1-4) mV Peak-Peak
  • Frequency : (0.6-30) Hz
  • Constraints : Power supply of 3.3v (negative portion is ignored.

Output needed :

  • Signal Type : AC
  • Amplitude: (1 up to 2) V (The higher the better)
  • Frequency : (0.6-30) Hz without additional frequency components.

I checked MAX412 datasheet and upon the design, I made a simulation for gain of 2000.

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Although the output was promising, when I implemented it, nothing was shown :) . I was testing using function generator (50 ohm output impedance) and an oscilloscope.

Your help is appreciated. Thanks

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  • \$\begingroup\$ The datasheet says the minimum supply voltage is ±2.4V=4.8V, not 3V. Models are often inaccurate outside of the maximum ratings. \$\endgroup\$
    – Janka
    Mar 22 '18 at 21:14
  • \$\begingroup\$ @Janka . The datasheet states that it ranges from ±2.4V to ±5V. So any power supply in between is acceptable. \$\endgroup\$
    – wbadry
    Mar 22 '18 at 22:04
  • \$\begingroup\$ Yes, but 3V is only ±1.5V. \$\endgroup\$
    – Janka
    Mar 22 '18 at 22:05
  • \$\begingroup\$ @Janka. Check the negative terminal. it is grounded. \$\endgroup\$
    – wbadry
    Mar 22 '18 at 22:06
  • \$\begingroup\$ If you think simply disabling the negative half of the amplifier this way would work … I'd check its inner schematic first … \$\endgroup\$
    – Janka
    Mar 22 '18 at 22:15
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Three suggestions.

  1. Power: As Janka pointed out in his comments, your power supply must be within spec per the datasheet. As you've shown it, it doesn't meet that spec. Now, you can power it with a single positive supply (4.8V <= V+ <= 10V) on V+ and 0V at V-, but then you will have to bias the input to accommodate the asymmetrical power, and AC couple your source. It's all about tradeoffs.

  2. Offset: Your gain is 2k (2001). Per the datasheet, max input offset voltage can be as much as +/- 0.25mV. Assuming worst case, that will add 0.5V to your output. With a signal swing of 1mv pk, your 1Hz output is 2.0v pk. Add the 0.5V offset, and if you're powering with +/- 2.4V, you will be in saturation on signal peaks. If your signal is 1mv p-p, then you have head room.

  3. Input Balance: Add a 1k resistor at the opamp's (+) signal input. This will minimize additional offset caused by input bias current. With gains this high, these seemingly small values can stack up.

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  • \$\begingroup\$ Thanks to you and @Janka , I was able eventually getting it. My apology to Janka if I totally misinterpreted his guide. \$\endgroup\$
    – wbadry
    Mar 25 '18 at 15:55
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I followed both Janka's note and AlmostDone's. I created a DC offset and made the amplifier biased to the same offset with resorting to single DC supply as recommended. Your help is much appreciated.

enter image description here

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