A/D Negative voltages

[jack12ko]

First time using this web site, so please forgive if this is not the way to post a message here.
I am using the Solarbotics 28x1 development board, which has four (4) A/D inputs. I have a negative voltage I would like to read, positive voltages work great, Is this possible, if so can anyone direct me on how to get this accomplished. looking at the data sheet for the pic16F866 this is very possible, however can this be done with the development board I have.

Kindly thanks in advance.

Jack

 

[Andrew Cowan]

Does the negative supply have a seperate ground?

If so, just connect the ground to the ADC pin, and the negative voltage to the PICAXE ground.

What's the application?

A

 

[fernando_g]

Otherwise, you need:
a) an opamp with a gain of -1...
b) a regulated negative supply for the opamp.
c) a schottky diode to protect the Picaxe input from going negative (during power up, for instance).

 

[jack12ko]

Thanks for the input

Thanks for the response,


No the grounds are the same. I have 3 voltage outputs from the same supply where one is negative, positive works just fine as you know, app is I am monitoring all 3 inputs so reversing the ground could be an issue here.

Kind Regards,

Jack K

 

[SilentScreamer]

Look at this thread - http://www.picaxeforum.co.uk/showthread.php?t=12179

 

[BeanieBots]

Welcome to the forum.

looking at the data sheet for the pic16F866 this is very possible

To the best of my knowledge, no PIC can tollerate a negative voltage on any of its pins. I'd be intrigued to know what/where makes you think it's possible.

However, it's actually very easy to measure a negative voltage.
If for example you want to measure -10v, simply make a potential divider using two 10k resistors with the 'bottom' of the divider connected to +5v instead of 0v.
-10v will then return a reading of zero.
0v will return a value of FSD/2.