I need to measure precise voltage in the range from 0.375 volt to 10.1 volts from a source with 100ohm impedance. Maximum voltage however, may be well above 10.1V, so I need to be sure I do not exceed the ADC maximum. The ADC I am using measures from 0V to 4.096V with an absolute input maximum input of 5.3V. Impedance is about 15 Mohm. Target accuracy is on the order of 100uV.
I tried using a resistor voltage divider with a combined resistance of about 1k ohm to reduce the 10.1V input to about 4.0V, and added a 5.1V zener diode as protection against excessive input voltage. However, at 4V divided voltage, the zener has enough reverse leakage to drop the measured voltage by 10mV.
Is there a better way of using zeners?. I simply put it in reverse between the divided voltage and ground. Alternately, I could use an opamp to scale the voltage instead of a resistor divider, but that adds parts and complexity.