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Over-voltage protection for inverting amplifier

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pjhenley:
Is this a legit approach to over-voltage protection? The purpose of this circuit is to take a voltage in the -20V to 20V range and fit it inside of 0V to 5V for output to an ADC. I am familiar with the idea of using diodes to clamp the voltage to the positive or negative rails, but it seemed to me that with an inverting amplifier there is no need to let the voltage deviate that far! Holding the voltage within a diode drop of the usual summing point would seem to provide some headroom that can only be beneficial.

I have put this circuit together and it seems to work just fine, but I have some concerns that I can't easily test. For example, would the transistors be too slow in the event of a sudden high-voltage transient? Is there concern for oscillation with the transistors and op amp feeding back to each other? The use of BJTs causes the voltage on the non-inverting input to be pulled up or down when they are conducting, but it seems to pull them in a direction that is beneficial to regulating voltage on the inverting input. Still, would it be better for any reason to use FETs?

Zero999:
It should not be necessary as the MCP602 already has protection diodes.

If the inversion isn't required and the bandwidth is low, get rid of the op-amp and perform the function passively, using only resistors.



Note the above are ideal component values.

For E24, use:
R1 = 82k
R2 = 110k
R3 = 330k

For E96 use:
R1 = 25k5
R2 = 34k
R3 = 102k

Of course it's fine to multiply or divide by 10.

If this is driving an ADC directly, add a 1nF capacitor between the output and 0V, but this will reduce the bandwidth.

pjhenley:
I think you may be right that the MCP602 is robust enough to handle over-voltages all by itself given a 1Mohm resistor in front of it. I see the discussion of that now in the datasheet, although my search terms didn't turn it up before.

I wouldn't want to get rid of the op-amp and go passive, though, for a few reasons. First, the sample-and-hold ADC doesn't want an impedance that big in front of it and I'd kind of like to keep it. Second, the ESD protection on the ADC is much less robust than the MCP602's, especially if we are reducing the input impedance. Lastly, using the op-amp gives me an opportunity to filter the signal between the op-amp and the ADC. Also, it is pretty trivial to implement range switching by switching in a different feedback resistor value.

The inverting bit doesn't bother me because you just fix it in firmware.

Even if it's academic, however, is this a protection scheme that is in use?

spec:
Hi  pjhenley,

It's a clever circuit and it would definitely protect the inverting input of the opamp.

But there is a downside- you would be injecting leakage currents and noise into the most sensitive and sacrosanct point on the circuit- sorry. ::)

There are other problems too, but not necessary to explore those.

Zero999:

--- Quote from: spec on November 28, 2018, 06:28:40 pm ---Hi  pjhenley,

It's a clever circuit and it would definitely protect the inverting input of the opamp.

But there is a downside- you would be injecting leakage currents and noise into the most sensitive and sacrosanct point on the circuit- sorry. ::)

There are other problems too, but not necessary to explore those.

--- End quote ---

What are the other problems?

I can't see any issues with that circuit. Both of the transistors will be off during normal operation and their leakage currents will be tiny. Assuming a 10-bit DAC, 1 count is 5/1024 = 4.883mV, so with a 100k resistor, the leakage current would have to exceed 48.83nA to cause a count of error and the transistors will leak much less than that.

--- Quote from: pjhenley on November 28, 2018, 04:36:57 pm ---I think you may be right that the MCP602 is robust enough to handle over-voltages all by itself given a 1Mohm resistor in front of it. I see the discussion of that now in the datasheet, although my search terms didn't turn it up before.

I wouldn't want to get rid of the op-amp and go passive, though, for a few reasons. First, the sample-and-hold ADC doesn't want an impedance that big in front of it and I'd kind of like to keep it. Second, the ESD protection on the ADC is much less robust than the MCP602's, especially if we are reducing the input impedance. Lastly, using the op-amp gives me an opportunity to filter the signal between the op-amp and the ADC. Also, it is pretty trivial to implement range switching by switching in a different feedback resistor value.

The inverting bit doesn't bother me because you just fix it in firmware.

Even if it's academic, however, is this a protection scheme that is in use?

--- End quote ---

What sort of overvoltages are you expecting?

For future reference, an ADC only requires a low impedance to transfer charge to the sample and hold capacitor. The problem is, if the DC impedance is high, the voltage will sag/rise, when a sample is taken, as the sample and hold capacitor transfers charge to or from the source, depending on the voltage. Adding a capacitor between the ADC's input and 0V will act as a reservoir and smooth out and voltage changes due to the transfer of charge in the sample and hold capacitor i.e. give the source a low AC impedance. I should have drawn a 1nF to 10nF capacitor on my schematic but forgot.

See the links below for more information.
https://www.embeddedrelated.com/showarticle/110.php
https://www.st.com/content/ccc/resource/technical/document/application_note/9d/56/66/74/4e/97/48/93/CD00004444.pdf/files/CD00004444.pdf/jcr:content/translations/en.CD00004444.pdf
http://www.ti.com/lit/an/spna088/spna088.pdf

I've never seen this protection scheme used before but it seems perfectly sensible to me.

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