ADC input greater than 3.3-3.6V

I may need to input and convert to digital a voltage greater than the specification input voltage. It could be 5v or up to 12V depending on the sensor I use. I’m not too sure about I2C lag or polling delay so I might prefer to read directly from the board’s ADC instead of using an I2c device. What is the ADC input impedance? Any suggestions?

Thanks in advance, M.A. Simon

ISTR impedance is ~30k. You will want to use a resistor divider and put a cap on the midpoint as there’s charge injection when it’s sampled.

To prevent current draw through this divider, we suggest you use a high side switch to isolate this when it’s not being used. See the hannah reference design (which reads the 6v battery pack voltage through a divider) for more info.

On this general topic of input pin voltage … sort of a loosely related question, about the Nora ref design and sampling battery voltage.

Studying the Nora schematic, I note Vbat is connected directly to an input pin, yet device Vdd is pegged to 3.0V (or 2.85V) during TX mode … indicating pin voltage > Vdd. Any issue with this? Is this a potential flaw with the design (or my thinking!)

(By the way … am I just missing the circuitry where the Hannah battery voltage is sampled? Can’t see this).

It’ll just rail the ADC input. You’re generally ok to VDD+0.3v before the ESD diodes do anything.

If you’re using 2xNiMH or 2xAlkaline you’re likely to be 3.0v or under. 2xAA lithium… well, you actually don’t really need a power supply for those babies :slight_smile:

Thanks for the comment on the ESD diodes @Hugo … kinda figured it was likely fine to be a little over Vdd on the inputs.

The plan for me is to try out 2xAAA lithiums on a little project I’ve got going that really doesn’t need to wake all that often … should be more than enough capacity. Have already replicated the Nora battery circuitry, and plan to run at 2.85V … but that’s likely overkill … probably could have just stuck with the batteries as you mention.

The main thing is to have a decent size bulk cap across the batteries; even with the lithiums, the impedance from the battery contacts can result in reduced battery life.

Will do … thanks for the tip. (I’ve typically been using a 470u cap in my circuits on the power/battery line).

Found this holder to use with AAAs …

Darn tightest battery fit I think I’ve ever experienced with a holder, so that likely helps mitigate contact impedance a bit. (Pricey holder though).