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?
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
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.