ADC input question

Started by Vince Pearman July 17, 2002
Hi all, a question:

I'm using three ADC inputs on a BX24 to measure temperature, power supply
voltage (thru voltage divider), and barometric pressure (via an MPX4115).

I've noticed that the readings on these pins are affected by the state in which
all the other pins are set. i.e., I'm using pins 13 thru 15 for these inputs, but if
I init all other pins to input with pullup, or input tristate, the ADC readings on
13-15 are changed by a few %.

The input pins in question are always set as input tristate, it's only the state of
the unused pins which are changed. So why would the state of the unused
pins affect the ADC readings on the other pins?



--- In basicx@y..., Patrick Murphy <luke631@m...> wrote:
> Vince,
> I ran into a similar problem last December and posted my question
here on
> the list. I received a helpful solution from Professor Anderson. You
> may want to go to BasicX archives and search for "PortA I/O and A/D
problem"
> in the subject line.
>
> If you would like more help, please let me know. Professor
Anderson's
> solution worked for me.

To be specific, the "final" answer came here:

http://groups.yahoo.com/group/basicx/message/8139

Eric


Vince,
I ran into a similar problem last December and posted my question here on
the list. I received a helpful solution from Professor Anderson. You
may want to go to BasicX archives and search for "PortA I/O and A/D problem"
in the subject line.

If you would like more help, please let me know. Professor Anderson's
solution worked for me.

Friendly regards,
Patrick Murphy
James Valley Colony
Elie, MB, Canada

Vince Pearman wrote:
>
> Hi all, a question:
>
> I'm using three ADC inputs on a BX24 to measure temperature, power supply
> voltage (thru voltage divider), and barometric pressure (via an MPX4115).
>
> I've noticed that the readings on these pins are affected by the state in which
> all the other pins are set. i.e., I'm using pins 13 thru 15 for these inputs, but if
> I init all other pins to input with pullup, or input tristate, the ADC readings on
> 13-15 are changed by a few %.
>
> The input pins in question are always set as input tristate, it's only the state of
> the unused pins which are changed. So why would the state of the unused
> pins affect the ADC readings on the other pins? >
>
> ">http://docs.yahoo.com/info/terms/





Thanks Patrick and Eric - these messages helped a lot.

I remedied the problem by setting all pins on Port A (13-20)
to input tristate; the settings of the other pins didn't affect the
ADC readings (as one would expect).