Analog inputs - Sergey89

[Sergey89]

[kb1gtt]

With the clamping diodes I don't see much of need for the op-amp. I guess you get the drop from 5V to 3V, however you can do that with a resistor divider. I don't see much of a need for R9 or C5. Perhaps that op-amp is sensitive to RF noise on the front end. C1 - C4 are not ESD protected, they could be damaged.

[erich]

R5 - C5 are a nice RC input filter, takes out a lot of hash.
Ditch the opamp and clamp diodes, use a resistor divider and a transorb. I used these http://www.semtech.com/images/datasheet/srda3.3-4.pdf for my i/o protection.

[Sergey89]

Another option without using an operational amplifier. Various configurations of the resistors can be used for different sensors.

[kb1gtt]

Remember your 5V has to be push / pull, as over voltage will dump current into the 5V rail. If you don't drain that energy, the 5V will rise. I think I see you use a lightly loaded zener else where, to keep it pulled down. Also you have to make sure the diodes are low leakage, 10uA of leakage would generate .1V.

[Sergey89]

Yet another idea. We can raise the power supply voltage of the op amp and provide a full range of the output voltage. Op amp has a good power supply rejection ratio. LC-filter provides an additional suppression of the high frequency components of the supply voltage.

[kb1gtt]

Hmmmm, sounds like this would be more ratio metric as well. Well ratiometric if you have a 12V sensor.