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I saw a zener diode protection circuit on an Arduino analog pin, and it had a 1k ohm resistor in series. I want to protect an op-amp, and I don't see why I need the 1k resistor, so the question is whether this circuit will work, protecting the input with a zener diode. Note that the transistors controlling the gain are actually 2n7002 MOSFETs, the symbol is incorrect in the diagram because of an unrelated problem. enter image description here

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    \$\begingroup\$ The amplifier inputs seem backward - you have positive feedback; is that what you intended? \$\endgroup\$ Apr 30 '16 at 15:39
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Without a resistor in series to IN1, the current through the Z-diode is not limited. It will still protect from ESD, but any permanent voltage from a low impedance source below -0.7V or above 5.6V will destroy the diode.

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  • \$\begingroup\$ so a 1k resistor in series is standard practice? I don't want to hurt the sensitivity of the amplifier too much \$\endgroup\$
    – Dov
    Apr 30 '16 at 15:09
  • \$\begingroup\$ Any op amp has an input resistance way above 1kOhm. Of course, there are also the capacitances of the op amp input and of the Z diode. But unless you are dealing with RF, these capacitances will not hurt the performance of the circuit including the 1kOhm series resistor. The exact value does not matter much. \$\endgroup\$ Apr 30 '16 at 15:22
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The LMV324 has interesting specifications; in the absolute maximum ratings table, we usually see the maximum input voltages at the IN(+, -) pins relative to Vcc and to ground, but that is not quite the case here:

LMV324 maximum ratings table

This implies that the input can handle 5.7V to -0.2V on any single supply, but I suspect that is incorrect as the internal ESD diodes will turn on under the conditions of a 3V supply (for which it is rated) at about 3.6V (see functional diagram below).

My personal view is that the voltage at the V(+,-) inputs should be no greater than Vcc + 0.2V

LMV324 functional diagram

As there are already internal ESD diodes, you may be able to use those provided the input current is limited to a few milliamps (typical ESD diodes cannot withstand more than about 3mA to 5mA for any length of time).

An input resistor achieves that; that said I would probably drop an ultralow \$V_{on}\$ schottky (there are some nice devices that have < 0.2V \$V_f\$ if the input current is suitably controlled) from the input pin to Vcc and one to ground with a suitable series resistor.

As already noted, Zener diodes are relatively slow; in addition, their full reverse voltage can end up significantly higher than the rated Zener voltage \$V_Z\$ depending on current.

schematic

simulate this circuit – Schematic created using CircuitLab

As the input resistance of the non-inverting terminal is usually measured in 100s of \$M\Omega\$, the only effect R3 has is to limit the current into the diodes.

Size R3 to maintain a low \$V_f\$ for the external diodes.

[Update]

Added notes on selecting an appropriate device.

First, evaluate the threat voltage (ESD discharge is already handled by the internal diodes); I will arbitrarily choose 10V above Vcc as an example.

Then we have to clamp the input below Vcc + 0.2; I will also arbitrarily assume 5V operation. Note that some short time above 0.2V will probably be ok - I will use 0.25V.

I can now choose a device; I will use a 30V device that has really low \$V_f\$ and low reverse leakage.

Now select a schottky that has the required performance; this device looks promising.

Now size the resistor so that we keep the forward voltage below 0.25V and so that any leakage current does not significantly affect the linearity of the circuit.

At 25C with 5V reverse, the part has \$4\mu A\$ of leakage and we need to keep the forward current below 20mA to achieve \$V_f \le 0.25V\$.

This device has a typical forward voltage of 0.25V at 20mA forward current.

At 10V for 20mA, we get \$500\Omega\$ for the series resistor, and in reverse at 5V (normal operating conditions) we get \$4\mu A \cdot 500\Omega\$ = 2mV offset voltage due to leakage.

If that is too high for the gains you want, then use a follower stage as an input buffer with an offset adjustment.

Note that at mid-span (2.5V for a 5V supply), both diodes will be slightly leaky and tend to cancel, reducing the effective offset voltage at the non-inverting input; i.e. the leakage current in the device to Vcc is also flowing through the device to ground.

Choosing these devices is always a trade-off; that is why so many different devices are available to accommodate varying requirements.

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First, there is a special hell for those who draw GND above positive signals.

About the protection: zeners are relatively slow. Sometimes it's enough, but more common protection is shottky diodes: one with cathode on VCC and anode on the signal, and the other one with cathode on the signal and anode on VSS.

Of course a current limiting resistor is required, unless for any reason you are sure the diodes (and voltage source) can take all excessive power.

Actually, in many opamps those clamping diodes are built in on the silicone level. In that case you can find it in the datasheet. They will recommend maximum current.

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