Analog Inputs on Panda

NETMF & Gadgeteer
1

Hello,

I apologize if this has been asked before, but apparently the ‘search’ function is being upgraded right now, and we need some info as soon as possible.

According to NXP documentation, and the e-book, the analog inputs on the Panda are limited to 0 - 3.3V. The FEZ component shield (as well as many other component/sensor shields wire the analog connections with +5V supply. So, when I connect a 3-pin component (say for example, the potentiometer), the input voltage presented to the analog input will be 0 to +5V.

Am I not understanding something? Are we expected to convert the signals of the components from 0-5V to 0-3.3V ourselves? If so, wouldn’t it be easier to allow the component shield the ability to select either 3.3V or 5V for the analog inputs?

Thanks,
Paul

2

Good question. You are correct and this is why we have a jumper to select 3.3V vs. 5V on FEZ Connect. See jumper next to analog sockets http://www.ghielectronics.com/images/catalog/261-2_large.jpg

Connecting 5V will not damage your board but it will give wrong results and even mess up the values of the other analog inputs. So for experimentation this is fine but for actual use then you have to limit the voltage to 3.3V

3

If you feed the ADC with a simple voltage divider…

4

Since i learned about net mf i regret every day i didn’t pay attention to physics classes in high school :wink:

5

At last!
Very happy with my new Panda II board, but new to this, I couldn’t understand why my KeyPad shield was giving wrong key detection.
As it is based on a resistor-ladder 5V divider, either the Select key and no key pressed were leading to a 1023 analog input value.

NOTE : this post is intended to help the next people who will try to reach this info based on keywords like mine. :wink:

6

Another issue – still for my 5V-resistor-divider analog input based keypad, that might apply to other sensors – is that the 3.3V reference of the A/D converter does not follow the 5V power’s fluctuations, right?

Because depending on whether the board is powered externaly or through USB, 5V means 4.3V to 5V.

That means that the analog levels related to the 5 buttons (keys), when pressed, depend on the powering conditions :naughty:.

Then better than hard-setting the A/D reference to 5V, should I give a little surgery to the LCD-KeyPad shield in order to wire the top of the resistor-divider to 3.3V instead of 5V.

7

[quote]hat means that the analog levels related to the 5 buttons (keys), when pressed, depend on the powering conditions .
[/quote]
Yes this would be teh right way to do it

Mose 3.3V micros I have seen do not accept 5V reference so this is not an option. But moving the resistor from 5V to 3.3V will surly cover all you need.