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Over-voltage protection for inverting amplifier
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spec:

--- Quote from: Hero999 on November 29, 2018, 12:16:25 pm ---
--- Quote from: spec on November 29, 2018, 01:33:44 am ---
--- Quote from: imo on November 28, 2018, 08:03:09 pm ---there is not such an opamp which can do full 0..Vcc output (maybe towards zero, but not Vcc).
PS: MCP6401 used in the below simulation.

--- End quote ---
Are you sure about this? Take a look at the TSX711 for example
https://www.st.com/en/amplifiers-and-comparators/tsx711.html

--- End quote ---
He's right. No op-amp's output can fully reach either of its supply rails. The example you've provided can get close, but it's still nearly 30mV out on the high side and just over 20mV out on the low side, when sourcing or sinking 1mA respectively. Refer to the table at the bottom of page 5 and top of page 6 of the data sheet. Whether this is an issue or not depends on what errors are acceptable.


--- Quote from: spec on November 29, 2018, 03:13:19 am ---
--- Quote from: Hero999 on November 28, 2018, 06:57:20 pm ---
--- 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.

--- End quote ---

Both of the inputs to an opamp are very sensitive to any kind of disturbance. There is the unnecessary leakage current which you seem to dismiss, but there could possibly be distortion introduced by the non linear characteristics of the emitter base junctions, but most importantly you would be introducing unnecessary inductance and capacitance. I would also suggest that it is bad practice.

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The summing node is not as sensitive as you believe because it has a very low impedance. The op-amp's output voltage will change to keep both of its inputs the same, as long as the loop is closed. The value of the feedback resistor affects how sensitive the inverting input is to stray currents flowing through Q1 and Q3. I dismissed the leakage current because it's negligible, as my calculations show. The worst case cut-off current for the 2N3904 and 2N3906 is 50nA, but that's specified with a VCE of 30V and in this circuit VCE = 2.5V, so it will be a tiny fraction of that. Perhaps if he's using a 16-bit DAC it might become an issue, but the op-amp's saturation voltage would start to be problematic before then.

https://www.onsemi.com/pub/Collateral/2N3903-D.PDF
https://www.sparkfun.com/datasheets/Components/2N3906.pdf

The non-linear characteristics of the base-emitter junctions is a non-issue because there's under 1µV across them and the additional capacitance and inductance is negligible at the frequencies the MCP602 cares about.

--- End quote ---

I give up |O
nsrmagazin:
The standard circuit used is the one in post 2. But without the large value resistor in the middle, just 1k and 9.1k.
Zero999:

--- Quote from: spec on November 29, 2018, 01:11:04 pm ---I give up |O

--- End quote ---
You'll never learn anything if you give up.


--- Quote from: nsrmagazin on November 29, 2018, 01:12:40 pm ---The standard circuit used is the one in post 2. But without the large value resistor in the middle, just 1k and 9.1k.

--- End quote ---
Which circuit are you referring to? Mine?

A simple potential divider will make the output voltage dependant on the source resistance. It will work if the source resistance is constant, known and can be factored into the calculation.
coppercone2:
keep in mind internal protection diodes are small. you can do better with consequences.
rstofer:
There's a very methodical procedure for fitting an input range into an output range including offset using op amps.  It does not, however, protect for voltages outside the input range specified.

http://www.ti.com/lit/an/sloa097/sloa097.pdf

A couple of days ago I was working on this for another thread so I'll just repost the MATLAB script.  This is specific to the case where both 'm' and 'b' are positive as discussed in the paper.

You could specify the output range to avoid getting close to the rails.  Maybe leave 0.1V either side when using a rail-to-rail op amp.

Unfortunately, I can't post a filetype of .m so I had to post it as .txt  The only reason I wrote this was to document the process.  The calculations are easy on a calculator.
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