Protecting inputs when powered down

??? If they are powered from the pi, how can they still have voltage? ?? if they are on the I2C bus, how are they connected to the A2D inputs on the Pi?

I suspect those already exist internally. It is more relevant to prevent those getting smacked with high current.

a small resistor would limit any current

If the external signals are strong enough to feed a pull-up that's low enough in value to be fast enough, one way is to feed the external signal into the emitter of an NPN bipolar transistor, with a pull-up from the collector to the Pi's 3.3V supply, and a pull-up from the base of something like 20 times the resistance, also to the Pi's 3.3V supply.

It's unusual, but it's just a common-base saturated level shifter.

David

They are ADs1116 I2C devices on the I2C bus. Their VDD suppy comes from the Pi, their inputs come from whataver they are measuring which doesn't get turned off whe n the Pi is turned off.

Yes, the ADS1115 has internal diode clamps but it does say in the datasheet that external Schottky diode clamps are recommended.

That's an idea, thank you, for logic inputs it'll be fine.