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Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Hey all, I need help in deciding what kind of circuits I could use to protect a micro-controller ( Inputs will be from analogue sensors, a break wire chrono (signal for this will be easy), and B-Dot probes. B-Dot probes are magnetic field sensors, they give a voltage output. I will be using this with a railgun, which means fast current rise times, and high currents. What should I do to protect the micro controller from these inputs, incase of bad inputs? "In single ended conversion mode, the analogue input valid conversion range is 0V to 1.2V."
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Usually the analog inputs can take voltages up to the positive rail without problems. In that case you use two ultra fast diodes, One to ground and one to the positive rail. Then add a suitable series resistor to limit the current. Also add decoupling with a low ESR capacitor so spikes does not find its way into th emicrocontroller through the power supply.
In many cases a series resistor is good enough since the microcontroller has built in diodes but when you are dealing with uncertain voltages it is a good idea to add external diodes.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Don't expect a zener to clip at a clearly defined voltage, they work over a range. Same with diodes but they tend to be better defined. I have a 3.6V zener protecting one of my microcontrollers and it is eats up 8-10 mA at 3.3V.
Is 0-1.2 V the valid conversion voltage range or the maximum range that does not cause damage? All my microcontrollers can go outside the valid conversion range without problems, they will just not return the correct value. Do you have a link to the datasheet?
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The valid conversion range is 0 - 1.2 V because it has a 1.2 V reference voltage. The power supply to the ADC is AVDD which is 1.8 V. So the internal protection diodes will not come into play until you reach 1.8 V + 0.3 V drop over the diodes. So as long as you stay below 2.1 V you will be fine.
If you have access to AVDD you can connect the diode to that. If not you can connect several ultra fast diodes in series to get the required voltage drop.
There are also many other methods, like using an op amp buffer, a linear optocoupler and so on.
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how to protecta microcontroller from data sampling
