ADC Input voltage

[the_mutt]

I saw that the ADC voltage range is 0-0.7 V. Does this mean that we should adjust our voltage to that range or is there circuitry in the extension board that does this for us from a 5 V scale?

[slipperyseal]

according to this page:

https://developer.sony.com/develop/spresense/developer-tools/hardware-documentation#_differences_between_spresense_and_arduino_uno

"[2] Analog Input Pins: Since the analog input range of CXD5602 is 0.7 V, resistance voltage division is adopted to lower the voltage from 5 V to 0.7 V. Since this method may cause problems depending on the connected device, when using the AIN terminal on the expansion board, pay attention to the output impedance of the connected device."

so it appears there is voltage scaling in place.

[the_mutt]

Thanks! I decided to play it safe last night and used a voltage divider to ensure my measurements were already from 0-0.7 V. The ADC readings only went up to 140 or so, which for 10-bit ADCs is very low, so it was reasonable to assume the extension board had built-in voltage dividers.

My concern stemmed from looking at the schematics and seeing voltage dividers for the high-performance ADCs that only scaled down to about 1.4 V, while the other 4 channels scaled down to 0.7 V. Anyways, I now know how it works. Note that the mainboard only accepts up to 0.7 V, so if you use the bare mainboard you will need external voltage dividers.

[slipperyseal]

yes a good idea to start with low voltages and actually see how it behaves.

as for scaling on the extension board only, that makes sense that the extension board is for Arduino compatibility and the main board is as "pure" as possible.

i was a bit frustrated to find the ADCs are only mapped to the B2B connector of the main board. but i guess there are only so many pins available.

i think the spresense has a lot of potential. i'm developing a few projects with the hope to base them on the spresense main board (but preferably not the extension board, for the sake of cost and simplified design). One is a marine robotics project which currently uses a Raspberry Pi A+ but would be more optimised and reliable using something like the spresense, ( http://robotics.catchpole.net/ - the camera integration, onboard GPS and low power consumption are the main attractions). It's a shame the chip set it only currently available on that board right now. Sony invite requests to purchase the MCU but only for large orders. so maybe the spresense will be used for general ARM cortex development but then actual devices will need to use something else which is more readily available but without as many features. <rant complete>