I'm working on an android multigauge for cars based of a pic microcontroller. Some inputs from sensors are not fed by my circuit so after stumbling upon this thread: http://www.microchip.com/forums/m612024.aspx, I had to make some changes to the circuit to be able to respect the fact that an input can't have more voltage then the voltage supply + 0.3V. So this was fixed with a schottky diode array on the input and a tvs diode on the supply line.

But I started to check the specs on other chips on my circuit. The AD8495 on it's datasheet on page 3 calls for an input voltage range of -Vs - 0.2V to +VS - 1.6V. http://www.analog.com/media/en/technical-documentation/data-sheets/AD8494_8495_8496_8497.pdf

Am I correct in thinking that if my circuit is not powered but is grounded (fuse blown or something) and that there is voltage on the inputs (+in to -in) then it's not safe? Thermocouple input voltage is tiny but if Vs is 0V and the input is 30mV, that is more than 0V - 1.6V... And if I want to protect against a wrong connection, let's say someone connects 12V by mistake on the input. Then even if Vs is 5V and if I have diodes protecting the inputs, voltage would be limited to about 5.2V and that would still be above VS - 1.6V...

Am I correctly understanding this? And if so, what would be the proper way to make sure that the input is always at Vs - 1.6V max?


1 Answer 1


No you're not correctly understanding this. The Vs-1.6V is an operational specification, so the part will not necessarily function if the input is closer to Vs than 1.6V (at 25 degrees C only, so you will want to leave more margin). This is the common mode range of the amplifier.

Damage may occur if you exceed the absolute maximum ratings, which appear- max and min look to be reversed- to be quite generous, so you won't be anywhere near them- a good thing for abs max ratings:

enter image description here

In general, it's not a bad idea to have some series resistance on inputs if your circuit can handle it. Even a few K can help.

Also see this section of the datasheet:

enter image description here


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.