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I have bought a couple of BZX55C3V3 diodes to protect the ADC inputs of the microconroller but I noticed they all behave strange (and each one in a different way). This is a ST 3V3 diode (http://www-3.unipv.it/lde/strumentazione_componentistica/datasheet/ZenerBZX55C33.pdf).

The test connection is simple: Vcc -> 1kOhm resistor -> cathode -|<|- anode -> GND.

I measure the voltage drop (Vd) on the diode and it is the following:

Vcc Vd
1V  1.00V
2V  2.00V
3V  2.55V
4V  2.85V
5V  3.05V
6V  3.20V
7V  3.30V
8V  3.40V
9V  3.45V
....
15V 3.70V
...
20V 3.85V
...
30V 4.00V

When it is positively polarized (anode +, cathode -) the voltage drop is more less 0.65-0.8V (between 1V and 30V), which looks ok.

What might be wrong with these diodes? The problem is that in my local shop the only 3V3 Zener diodes are exacly BZX55C3V3 and I have no idea whether this specific model is that poor or maybe all my diodes come from a failed batch.

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That looks entirely normal to me. 3.3V zeners have a very soft knee and are unsuitable for this purpose. The zener voltage is guaranteed to be between 3.1V and 3.5V at 5mA test current, which agrees with your number with an 8V input (3.2 \$\le\$ 3.4 \$\le\$ 3.5V) with zener current approximately 4.6mA. In other words, it's behaving exactly like a 3.3V zener diode, which in this case is not good at all.

A diode to a clamp will do better, but without more information it's hard to make recommendations. A TL431 with a 1mA current (from a 5V circuit) and a diode to your ADC input should work, but you'll have to keep the clamp voltage to maybe 3V, and some current still will flow into the ADC input. A R-R IO op-amp will clamp it more positively.

If your actual input voltage never exceeds 1 volt, the datasheet I linked above guarantees that the current at 25°C will not exceed 2uA, so your error will be less than 2mV, but at 125°C it could be as bad as 40mV error. Chances are you want to use more than the bottom 1/3 of your ADC range.

Edit:-

"Zener" diodes of greater than about 5.6V are actually avalanche diodes, and they have a much sharper knee and a positive tempco. Here is a set of curves from a totally different family of small zener diodes (lifted from a 20-year-old paper Toshiba databook). As you can see, at about 8.2V and above, they are really, really good, with little voltage change from 1uA to 10mA. The "3.3V" one changes from about 1.25V to about 3V. This is a consequence of the physics involved, and you'll find some zeners shift the curves up and down a bit, but the shapes will be similar. Active circuits are required to do much better.

enter image description here

Unfortunately, for some reason, this kind of info is often omitted from modern datasheets, even though it was considered important enough to murder trees for in the old days.

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  • \$\begingroup\$ I checked the 5V Zener and it is very stable: at 0-5V it has Vd of exactly Vcc (0-5V) while for Vcc of more than 5V it drops 5V. Is that really so much difference between how 3.3V and 5V Zener diodes work? \$\endgroup\$ – tml May 19 '14 at 17:57
  • \$\begingroup\$ Yes, there is a big difference. I'll see if I can scare up a set of curves, as the datasheets are a bit sparse. \$\endgroup\$ – Spehro Pefhany May 19 '14 at 17:58
  • \$\begingroup\$ Those are cool charts, wish they would include them in modern datasheets. Seems like a pretty important parameter to take note of. \$\endgroup\$ – dext0rb May 19 '14 at 18:41
  • \$\begingroup\$ I've seen them in current On Semi datasheets. Could have been for a 25-year-old part number though. \$\endgroup\$ – The Photon May 19 '14 at 18:45
  • \$\begingroup\$ @ThePhoton Yes, I couldn't find my 30 year old Motorola databook as fast as the Toshiba one. The 1N5xxx 500mW series was specified pretty well, IIRC. \$\endgroup\$ – Spehro Pefhany May 19 '14 at 18:52
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There is something called dynamic impedance of a zener. A practical diode can be assumed as an ideal diode in series with this dynamic impedance. This value is expressed as \$Z_Z\$ in the datasheet. This impedance is the reason for your misbehavior of your diode. The value of \$Z_Z\$ changes with \$I_Z\$.

\$eg\$: at \$20mA,\ Z_Z = 20\Omega\$ (from graph in the datasheet) and \$\Delta V_Z = 20\Omega\times 20mA = 0.4V\$

So if you can not allow such variation in voltage, then go for a diode which have allowable \$Z_Z\$ in your operating currents.

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