I was asked how an active clamp could look like:



This circuit prevents the adc input voltage from exceeding Vdd by a scheme that is called "active zener": The TLC274 at the right tries to keep the inverting input at the same potential as the non-inverting input, which is Vdd. For adc input voltages below Vdd the OPamp toggles the output high, which is without any effect due to the reverse biased Schottky diode. But at the moment, when the potential at the inverting input tries to exceeds Vdd, the output of OPamp goes low to keep the potential at inverting input at Vdd. The OPamp output is one forward voltage drop of Schottky diode lower than Vdd, then.

The only possible negative effect of this active clamp is the leakage current that might flow through the reverse biased Schottky diode, and this only at rather high ambient temperatures. You can check this bad influence: Apply Vin=0V and measure the voltage drop across the 1k resistor. The voltage drop should be less than 1LSB of ADC, which is 2.43V / 4096 = 0.6mV. Usually, no problem, but check it.

A last hint: If leakage current with glass body diodes is an issue, take care that they are properly shielded against ambient light and heat radiation!

Kai