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How to protect analog inputs on low power device from overvoltage?
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NotAVirus:
Hi,

Normaly, to protect my ADC-inputs from overvoltage I would use diode clamps to the supply rail, and use a series resistor to limit the current into my supply rail. As long as the current injected into the supply-rail is lower than the total current-consumption of the device, everything should be fine. However, in low power applications with sleep-modes only consuming microamps, I dont think clamping the input-signal to the supply rail is viable.

Another way to clamp the signal would be to use a zener diode instead of diodes to the supply-rail. The problem with this solution is that low voltage Zener diodes have a terrible reverse leakage current, and requires a significant reverse current to achieve its reverse voltage. This will ruin the accuracy of the analog input.

A third solution, might be to make another voltage rail, and then diode-clamp inputs to this. This rail should have a voltage slightly lower voltage than VCC to ensure that current will be clamped to this rail instead of VCC. This rail should have some kind of shunt regulator, making it possible to dissipate the the injected current. My VCC voltage may vary between 3.0 and 3.6V depending on battery voltage. Therefore the shunt regulator should be referenced to VCC instead of a fixed value. This is where I am stuck |O I cannot find a simple circuit that can dissipate current when voltage on this clamping rail is approaching VCC.

have anybody else struggled with similar problems, and found a solution? Is there perhaps another way to protect analog inputs from overvoltage?
DBecker:
A diode-isolated protection rail, with zener clamping on the rail.

I've used a SRV05-4-P-T7 as a low capacitance steering diode array primarily because JLCPCB assembly has that as a basic part.  It is intended for USB data line protection and has 3.5pf capacitance to ground.  You'll probably want to treat the internal TVS as secondary and provide your own zener clamp.   Bias that rail to minimize the loading and capacitance.

David Hess:
Transistors can be used with the base/gate connected to a reference while the collector/drain directs the current at the emitter/source to a sink point.

A low zener diode can be prebiased to provide a stable reference for diode or transistor based protection.  Transistors and diodes can be paired to remove the effect of Vbe or diode forward voltage drop.
EEEnthusiast:
Does the ADC have input drivers? If so, limit the voltage of the OPAMP driving the ADC so that it is just below the max input limits. This would protect the ADC at all times. Ensure that the OPAMP can drive rail to rail.
NotAVirus:
Thank you so much for the replies.

"diode-isolated protection rail" was the thing I was thinking on. Combining this with a prebiased zener-diode, I would get an accurate clamping voltage. I am a bit conserned of the current required to bias a zener. Most zeners I have looked at requires several mA of current to reach their rated voltage. A shunt-regulator like ATL431 requires a bias of 35uA. This is still a bit high for a small IoT-device spending most of its life in deep sleep.

I have come up withe the attached circuit, which uses VCC as a voltage reference, and transistors to shunt the overvoltage. This way the clamping-voltage will follow VCC to some degree. The circuit works i LTSpice and only draws about 2uA from VCC. Is there any reason why this should'nt work in practice?


--- Quote from: EEEnthusiast on December 12, 2019, 02:19:36 am ---Does the ADC have input drivers? If so, limit the voltage of the OPAMP driving the ADC so that it is just below the max input limits. This would protect the ADC at all times. Ensure that the OPAMP can drive rail to rail.

--- End quote ---
There is no external buffering on the inputs. The ADC I am using(ADS124S06), have internal input buffering.
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