How to protect low current analog line from overvoltage

[tarmogr]

I want to reliably connect a small current source to a ADC pin of an MCU.  I need to protect the MCU in case of overvoltage. First taught was to use a diode connected to supply, but most diodes have an unacceptable reverse leakage current. And If I go to low leakage diode (like BAS716) the forward drop becomes too large to protect anything.

Some spec:
Analog line is the Imon line of TPS1663 https://www.ti.com/lit/ds/symlink/tps1663.pdf?ts=1744045354555
The protection circuit shall not leak more than 300nA (-25C to +85C ambient).
The TPS part has internal 5V zener, but my MCU runs at 3.3V. The abs max allowed on ADC pin is 4.0V and recommended is 3.6V

[Ian.M]

Replace R_Imon with a potential divider to feed the ADC.  Make the lower resistor whatever value you need for your ADC scaling, and the upper resistor half that value to limit the max. input voltage to 3.3V

[Doctorandus_P]

One method is to add an opamp.
during normal operation, the opamp is configured as a voltage follower, and there is no voltage over the diode, and thus no current. During clamping, the voltage and current levels do not need to be accurate, so it's relatively easy do design something for that.

[mtwieg]

You should definitely use an opamp to buffer the signal. The ADC input itself would likely draw more than your allowed 300nA.

Though the 300nA spec seems excessively low, considering the poor starting accuracy of the IMON signal...

[Ian.M]

Whether or not you need an OPAMP buffer depends on if R_Imon is less than your MCU's ADC's max. source impedance.  If you do need a buffer amp, a low input bias current lowvoltage RRIO OPAMP configured as a unit gain buffer should do the job.  The TPS1663's internal 5V Zener will clamp its Imon pin to 5V, so simply choose the upper resistor of the potential divider to limit the max. input voltage to the OPAMP.   It may be preferable to limit the input voltage to slightly over 3.3V so that the Zener doesnt start conducting before the desired 3.3V FSD is reached.

[David Hess]

A pair of diodes, or even better transistors or a diode and transistor, can be used to avoid the forward voltage drop of the diode.  One junction provides a voltage offset so that the other junction starts conducting at the negative or positive supply voltage.

[radiolistener]

To safeguard the ADC input from voltage transients exceeding the recommended input range, it is common practice to implement an external clamp circuit using two diodes connected in an anti-parallel configuration. The forward voltage drop of typical fast recovery diodes ranges from approximately 0.2 V to 0.7 V, depending on the selected diode technology (e.g., Schottky diodes for lower forward voltage and minimal capacitance).

When the input voltage exceeds this threshold, the corresponding diode conducts according to the input polarity, effectively redirecting the excessive current away from the ADC input to a safe reference potential (such as ground or the ADC supply rail). Consequently, the voltage seen by the ADC input remains within safe operating limits, determined primarily by the forward voltage of the conducting diode.

For high-speed or high-precision ADCs, it is critical to use protection diodes with ultra-low junction capacitance to minimize impact on signal integrity at high frequencies.

In applications requiring higher clamping voltages (e.g., up to 4 V), it is advisable to select protection devices specifically rated for the desired breakdown voltage, such as TVS diodes or integrated protection ICs, rather than connecting multiple diodes in series, which may degrade performance due to increased capacitance and reduced response speed.

[tszaboo]

the forward drop becomes too large to protect anything.

That doesn't happen in my experience. You need some power, and not voltage alone to damage an IO pin of a microcontroller or an ADC pin. BAV199 would be my default choice for this. You can also add a zener to the power supply of the ADC, if you route some current into the supply. It depends on the voltage levels and current consumption.