- on Di 12 Mai 2015
Author: d-yu
ADC protection circuit design
5/12/2015 8:56:36 PM 方案全部错误，因为输入大电压时，运放无法正常工作！
There was a time when I thought zener diodes were great. Now I know that they're not. As a matter of fact they stink. This diode has a 4% tolerance at 250 µA, so you may lose the top 200 mV of your reading, but it gets worse: at 10 µA zener voltage is only 4.3 V, that's a 14 % error. If your input comes from a relatively high impedance source, like a resistor divider you may lose the top 700 mV.
Most microcontrollers have clamping diodes on their I/O pins:
enter image description here
You can use those. If your signal comes from a low impedance output you'll want to add a series resistor to protect the clamping diode against a too high current. 50 mA is often specified as Absolute Maximum Rating. If you use a 15 kΩ resistor you'll limit the current to 1 mA for a 20 V input. The Photon rightly points out that the current shouldn't be too high. That's because you're injecting current where the voltage comes from a voltage regulator, and that can only source current, not sink it. So if an external source injects current the regulator's load should be able to drain it to ground.
As PetPaulsen points out there's debate whether this is acceptable practice. The datasheet may say maximum input is Vcc + 0.3 V, but it may also say maximum 20 mA for the clamping diodes (for instance this PIC controller. That may mean that the clamping diode voltage drop is less than 0.3 V, for instance if they're Schottky's. Anyway, you can always use your own external diode to clamp to Vcc. This Schottky diode only drops 100 mV at 10 mA, so it will clamp the input to a safe value. Don't forget the 15 kΩ resistor for low output impedance sources.
If your input voltage doesn't go negative then the ground clamp isn't required.
大电流的话，Zenner二极管容忍度小，出错率低。就是压降要小很多
LINK: