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Thread: Protecting analog inputs from overvoltage

  1. #1
    Member Rena's Avatar
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    Protecting analog inputs from overvoltage

    As I understand, the analog inputs on Teensy 3.2 are rated for 3.3V maximum and can't handle a 5V input. In my project I want to defend against accidentally connecting a 5V source.

    I've found this circuit, but the comments suggest it's not safe. However it seems like the concern is mainly with long-term use and causing glitches. I'm not too concerned if the Teensy malfunctions when someone connects 5V, so long as it doesn't fry. (Even better if the circuit can be modified to eg light an LED to tell them to disconnect that ASAP!)

    Is that design suitable for protecting from short overvoltage spikes? Or can anyone suggest a better method, that doesn't compromise the accuracy of in-range signals?

    Even better if it could be adjusted, so that I could eg turn a knob to scale the input down to 3.3V and still be able to analyze a 5V signal that way, without risk of blowing things up.

  2. #2
    Senior Member Epyon's Avatar
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    It of course depends on what exactly you want to protect your analog inputs from, but I can confirm that this circuit successfully protects T3.2 analog inputs from regular accidental overvoltages, provided you use Schottky diodes with a forward voltage drop of 300mV or less.

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    If you look up the maximum ratings of the analog input of the K20 chip the Teensy 3.2 uses, you see that you can go from -0.3V to VDD+0.3V on the analog inputs. By placing external Schottky diodes on the input you make sure that the input voltage never exceeds this limit under normal circumstances. E.g. I have some applications with instrumentation amplifiers fed with a +/-5V power supply. Under normal circumstances the amplified output signal will not exceed 3.1V, but if some lazy engineer left the input terminals of the amplifier disconnected it could go into saturation and put -5V or +5V on the analog inputs of the Teensy. In that case the Schottky diodes clamp the signal to +/-0.3V of the VDD/GND rail, thereby protecting the Teensy inputs.

    This is of course only an emergency solution. The Schottky diode shunts the excess voltage to the power rail. If the shunted power exceeds the power the Teensy and other connected components use, the voltage of the power rail will rise and potentially damage the Teensy and said other connected components. This can happen if multiple inputs protected by diodes go into saturation. You should design your analog circuitry so that under normal circumstances this does not happen.

    Schottky diode clamps do not offer full protection against transients and large voltage surges, e.g. from static discharges. You need additional components like TVS or MOV for that.
    Last edited by Epyon; 11-23-2017 at 08:12 PM.

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    The minimum - a 1k resistor in series (therefore limiting the current through the internal clamping diodes).

    Downside:
    Less S&H charge current for the ADC so might increase the variance in readings.

    Epyon's circuit makes an assumption - the source of your analog signal doesn't have a very low impedance therefore the clamp diodes won't fail with high current. Assuming decent clamping diodes, then I'd probably add a 100ohm resistor in series to Epyon's circuit.

  4. #4
    Member Rena's Avatar
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    Alright, I put that circuit together on a breadboard and it seems to do the job. Is there an IC that combines several of those circuits?

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    Quote Originally Posted by Epyon View Post
    It of course depends on what exactly you want to protect your analog inputs from, but I can confirm that this circuit successfully protects T3.2 analog inputs from regular accidental overvoltages, provided you use Schottky diodes with a forward voltage drop of 300mV or less.

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    Also (slightly OT, but useful under the thread title), using anti-parallel diodes to ground (i.e. both going to ground) is a common protection method to interface to AC-coupled ceramic sensors. Again max useful analogue voltage is limited to the forward voltage drop.

  6. #6
    Senior Member Epyon's Avatar
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    Quote Originally Posted by Rena View Post
    Alright, I put that circuit together on a breadboard and it seems to do the job. Is there an IC that combines several of those circuits?
    A BAT54S contains two Schottky series diodes with a center tap, but it is only available in SMD.

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