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how to protect analog inputs ?

Associate III

I've seen the discussion in an old question here

Accordingly, ADC inputs are not 5v tolerant in ADC mode.

I started studying some Zener diodes, in particular 1n4727 Vz 3v

What I noticed in the graphs is that instead of what we commonly assumed of a reverse Zener voltage

it looks instead like a conventional forward biased diode curve

with a turn on voltage around 1.5v

This is quite curious and interesting as it implies that if I use this diode as a shunt or voltage limiter to GND, below 1.5v I get a 'linear' response as the diode don't conduct. Above 1.5v, it becomes a non-linear 'square root' style response. If I'm willing to make do with the fuss about that 'non-linear-ness', it would seem I can achieve much higher 'Vin' ?

Another question though

I can't seem to find a 'Shockley' equation to represent the Zener breakdown region.

Any hints about an equation representing that part? i.e. the reverse Zener breakdown region.

Most cases seem to assume 'vertically down', but this 'weird' case doesn't seem to fit anywhere


found something, it seemed the Zener reverse breakdown region is governed by something different - tunnelling, and more complicated than that Shockley equation

ST Employee

First of all, normal Zeners are not a good choice for the protection of electronic devices, especially because of their not very sharp characteristics. TVS (transient voltage suppressors) do a better job.

However, you are about to stumble across two effects that can be found in diodes commonly known as Zener: the Zener effect and the avalanche breakdown effect. Both are also described in the Wikipedia article you mentioned. With your zener diode you can see the limit between 5...6V.



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Associate III

Oh, wow, found something interesting, it seemed this is relevant to that 'Zener diode'. I'm not sure if it'd exhibit 'negative resistance' and oscillate


Put a low offset op amp follower to protect and impedence adapt the input. What is the analog electric diagram upfront the mcu? Which input impedence the adc is having?

Associate III

Just to give a little more input on the application, the MCU is feeding in from a current sense amplifier

This has 50x as amplification and I choose a shunt resistor that gives about 2 amp full range, the danger is if it goes above 2 amps, quite likely it'd go above the rated Vin for the ADC and that port is not 5v tolerant, so it'd fry the MCU at the ADC I'd suppose. Hence, I looked at Zener 'clamping', but these findings is quite a surprise, as the reverse voltage doesn't look like a 'conventional' straight down, but some sort of 'power curve' (e.g. x^2), at least at the 'knee' region.

Below 1.5V it is a 'straight line' the diode doesn't conduct. Above that is where it gets funky. I'm thinking if i 'limit' the currents, say place a 1k ohm in front of that, would that 'small breakdown' part look like the Shockley equation or such. If it does, I can fit the values there to determine the parameters / coefficients, and I've a 'linear' + 'non-linear' ADC and I can go above the linear 'dynamic' range

The op amp is a good idea, I'd try that. In addition, I'd think I'd need to experiment to see if the current sense amplifier would produce output voltage above VCC (denoted as V+ in data sheet) supplied, as if it doesn't do that, then this is safe