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I have been looking around at different front end implementations for a DIY oscilloscope and came across a variant that I found simple enough as a starting point.

I have a question about protecting the inputs though. Please look at the attached schematic. AFE design

The way I understand it, there is no protection for the input of the OPA659 buffer. As per the datasheet, the maximum input can be 0.4V above 5V. Would this not mean that a higher signal mistakenly connected to the oscilloscope could damage it?

Would it make sense to clamp the input voltages to the +5V and -5V rails using clamping diodes?

Could I use a +-15V supply to increase the range of my input signals and modify the subsequent attenuation to bring it within range of my ADC?

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  • \$\begingroup\$ Any series resistor would risk becoming a low-pass filter with the first stage, but it can be reasonable. Then it could be interesting to put some low-leakage clamp diodes or half BJT using the B-E junction, or even an N-JFET with S-D shorted together. Clamping to supplies, or pre-polarized Zeners (to lower the capacitance). Are you sure of your tiny 220 pF DC input blocker? Looks to be quite tiny, as if you were considering only relatively high-frequency AC \$\endgroup\$
    – LuC
    May 5 at 12:10
  • \$\begingroup\$ This is a design I found online and not my own, hence I cannot say much about the design choices made. I am open to suggestions though \$\endgroup\$
    – ACBlue
    May 5 at 13:33

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Would this not mean that a higher signal mistakenly connected to the oscilloscope could damage it?

It could immediately damage it or, it could weaken it in such a way that performance is degraded.

Would it make sense to clamp the input voltages to the +5V and -5V rails using clamping diodes?

That makes sense plus, add a series resistor to the non-inverting input so that current is limited at +/- 5.4 volts. However, upon checking the data sheet, the absolute maximum rating is the supply rail and not 0.4 volts beyond the supply rails: -

enter image description here

So, using protection diodes to the op-amp supply rails is not going to work. However, the input common-mode range is +/- 3 volts on a +/- 6 volt supply as per this data sheet extract: -

enter image description here

So, on a +/- 5 volt supply, the usable input range will be about +/- 2 volts hence, you could use steering diodes to +/- 3.3 volts (if you have those supply rails.

But you have another problem here: -

enter image description here

  • You have no bleed resistor to ground when coupling via the capacitor
  • The capacitor will remain charged when you revert to DC coupling
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    \$\begingroup\$ Excellent points. Note that since the input is buffered, input clamping can be done through a bootstrapped diode pair, and 3.3V supplies can be furnished with zener shunt regulators. This keeps capacitance low / bandwidth high. \$\endgroup\$
    – Tim Williams
    May 5 at 12:02
  • \$\begingroup\$ The datasheet has a section 8.2.1 where ESD protection diodes are mentioned. It says "These diodes provide moderate protection to input overdrive voltages above the supplies as well": I read in another op-amp datasheet that these diodes have a typical forward voltage of 0.7V so a Schottky Diode will clamp larger inputs to just 5.4 keeping it below the threshold of 5V + 0.7V. This was my assumption when I mentioned 0.4V. \$\endgroup\$
    – ACBlue
    May 5 at 13:41
  • \$\begingroup\$ @ACBlue the picture in 8.2.1 show internal diodes and, the protection they offer is for ESD events (very short-lived events) and not suitable for "a higher signal mistakenly connected" (as you wrote in your question). I would certainly consider adding a resistor of maybe 100 ohms directly in series with the non-inverting input though. \$\endgroup\$
    – Andy aka
    May 5 at 15:30

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