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Please refer to AD828 or any other bipolar op-amp not unity gain stable. Which of the two below cases is better? Any other suggestion? Simulation suggests the second, but I need to understand if the first (which I built) is still safe.

Be aware that I have not easily access to ground in my PCB close to the opamp.

Please read also note 1 in the following article, which suggested me to include a high value of Rf in the second case: "What shall we do with an unused op-amp?"

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The crosstalk test circuit in the AD828's datasheet shows it as a unity gain follower. \$\endgroup\$
    – Colin
    Dec 16 '20 at 11:02
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    \$\begingroup\$ If you go on Analog Device site, under Operational Amplifiers (Op Amps) Product Selection Table, and you filter for AD828 adding "Min Gain Stable" parameter column, it shows 2. \$\endgroup\$
    – Gianluca G
    Dec 16 '20 at 13:10
  • \$\begingroup\$ @Colin that fooled me too. It looks like it might be stable at unity gain with lower supply voltages but not with bipolar 15V supplies. Take a look at the open-loop gain and phase margin graph (TPC 10). \$\endgroup\$
    – mkeith
    Dec 17 '20 at 3:57
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When an opamp is not unity gain stable then you should not do this as this is the unity gain configuration:

schematic

simulate this circuit – Schematic created using CircuitLab

With the risk of oscillations and instabilities.

Your Case 2 could be an option but still, you'd have to check that it is really stable. I'm too lazy for that so here's what I would do. I would simply get rid of the feedback and use the opamp as a comparator:

So this is what I would do:

schematic

simulate this circuit

This applies a small input voltage (~280 mV) at the input. All opamps should be able to handle this for a very long time without issues. The output of the opamp will be pulled high. If swapping the inputs of the opamp is more convenient than that's OK as well, then the output will be pulled low. Since there is no feedback, there can be no oscillations and instabilities.

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  • \$\begingroup\$ Will try Case 3. Simulation is ok even if feeding with huge amount of noise. \$\endgroup\$
    – Gianluca G
    Dec 16 '20 at 13:00
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I'm sorry, but I can't see in the first circuit by what measure the unity gain amplifier is stabilized. It is the classical unity-gain configuration which will not work if the opamp is not unity-gain stable.

In the second circuit, however, the loop gain is reduced by the resistor R9 and the loop gains zero crossing is shifted to lower frequencies (less phase shift). This allows stable operation with unity gain. It is possible to set the loop gain (variation of R9) to each desired value without touching the closed-loop gain - however, you have to live with a reduced frequency range of operation.

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  • \$\begingroup\$ No worry regarding reduced frequency range of operation, as it is unused opamp \$\endgroup\$
    – Gianluca G
    Dec 16 '20 at 13:01

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