After many days of investigation, design and simulations I finally design this circuit used as part of my arduino voltmeter.

This circuit uses AD8605 (https://www.mouser.com/datasheet/2/609/AD8605_8606_8608-1716254.pdf) operation amplifier configured as differential amplifier in order to prevent reverse polarity protection.

The voltmeter is deisgned to read 0v-20,5v max. In this two pictures I put the simulations with it maximum voltage. Last one in reverse polarity situation.



Focusing in the last picture (reverse polarity scenario) as you can see V+ input has -212mv. I read carefully the datasheet of this op amplifier and its says common mode input voltage goes from 0v to 5v maximum.

Anyone with vast experience in electronic design could tell me if this minimum negative voltage in V+ will damage the op amp or I simply ignore this voltage in this particular scenario.


1 Answer 1


Data sheet for AD8605 includes the following paragraph:

Phase reversal can cause permanent damage to the amplifier; it can also cause system lockups in feedback loops. The AD8605 does not exhibit phase reversal even for inputs exceeding the supply voltage by more than 2 V.

So negative input voltage of -0.2V, even though outside the supply range of 0V to +5V shouldn't cause problems. But it doesn't fully answer your question, because bias currents appear to rise precipitously for input voltage below 0V.
Data sheet goes on to suggest adding series resistance to limit bias current:

The AD860x has internal protective circuitry. However, if the voltage applied at either input exceeds the supplies by more than 0.5 V, external resistors should be placed in series with the inputs. The resistor values can be determined by:
\$ {{V_{in}-V_s}\over{R_s}}\lt 5mA\$
The remarkable low input offset current of the AD860x (<1 pA) allows the use of larger value resistors. With a 10 kΩ resistor at the input, the output voltage has less than 10 nV of error voltage. A 10 kΩ resistor has less than 13 nV/√Hz of thermal noise at room temperature.

Your diode D1A shouldn't exceed the 0.5V threshold mentioned in this paragraph. Even so, you might consider moving D1A's cathode to the junction point between R4 (1k) and R3 (10k). Doing so allows R3//R2 to limit input current.


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