poorchava
Advanced Member level 1
- Joined
- May 21, 2009
- Messages
- 429
- Helped
- 71
- Reputation
- 142
- Reaction score
- 71
- Trophy points
- 1,318
- Location
- Wrocław, Poland
- Activity points
- 4,780
Ok, detailed explanation following:
-use a back-to-back transient voltage suppressor on the input. This kind of device will shunt voltage spikes to ground. TVS is nice because it typically triggers in the order of single picoseconds. It will protect your device from static discharges as from operating connectors and handling the device without ESD protection gear. TVSs have typically much "sharper" characteristic than zener which means less leakage at voltages below triggering threshold
-use input filtering. I'd say that RF choke or resistor in series + capacitor to gnd (10n, 100n or else depending on frequency of sampled signal, you don't want to attenuate useful signal). Also, use voltage divider to divide voltage in manner which doesn't allow full swing. This will protect your input, minimize effect of clamping circuitry on input value and allow for better accuracy, as ADCs usually have some input range, in which they perform the best and that is definitely NOT the range near supply rails. Moreover, reference of say: 2.56V or 1.28V allows for much, much easier convertion result processing.
-use schottky clamping diodes (as alexan_e wrote). If voltage exceeds power rail by more than diode voltage drop, then current will flow through that diode from input to power supply. Same with ground, if voltage drops more than diode voltage drop below ground potential, then diode begins conducting current from gnd to input. This is important, because MCU has such clamping diodes inside, but they are usually very fragile. This means that you want your external protective diodes to start conducting earlier than the ones inside MCU. Internal MCU clamping diodes are always schottky type, so you cannot use silicon type, because they wuill be totally ineffective (when silcon clamps would begin to conduct, the clamps inside MCU would be toasted a long time ago)
-use a back-to-back transient voltage suppressor on the input. This kind of device will shunt voltage spikes to ground. TVS is nice because it typically triggers in the order of single picoseconds. It will protect your device from static discharges as from operating connectors and handling the device without ESD protection gear. TVSs have typically much "sharper" characteristic than zener which means less leakage at voltages below triggering threshold
-use input filtering. I'd say that RF choke or resistor in series + capacitor to gnd (10n, 100n or else depending on frequency of sampled signal, you don't want to attenuate useful signal). Also, use voltage divider to divide voltage in manner which doesn't allow full swing. This will protect your input, minimize effect of clamping circuitry on input value and allow for better accuracy, as ADCs usually have some input range, in which they perform the best and that is definitely NOT the range near supply rails. Moreover, reference of say: 2.56V or 1.28V allows for much, much easier convertion result processing.
-use schottky clamping diodes (as alexan_e wrote). If voltage exceeds power rail by more than diode voltage drop, then current will flow through that diode from input to power supply. Same with ground, if voltage drops more than diode voltage drop below ground potential, then diode begins conducting current from gnd to input. This is important, because MCU has such clamping diodes inside, but they are usually very fragile. This means that you want your external protective diodes to start conducting earlier than the ones inside MCU. Internal MCU clamping diodes are always schottky type, so you cannot use silicon type, because they wuill be totally ineffective (when silcon clamps would begin to conduct, the clamps inside MCU would be toasted a long time ago)