ADC and I/O Issue

Started by Mario November 22, 2004

I am currently using Pin 20 to read a voltage using the ADC. Pin 20
is initialized as a High-Z pin. Connected to Pin 20 is an output of
a op amp that is outputting about 7.6 volts. When I want to take a
reading I close a set of relays that connects the signal to be
measured which is between 0 and 4.5 volts. All this works fine.

However I decided that I wanted to control an Analog Switch using
Pins 18 and 19.

When I set Pins 18,19 using PutPin(18,1) intead of 5 volts, I am
getting 7.6 volts on High. I trace it back to voltage connection on
Pin 20 because it reverted back to 5 volts when I grounded signal on
Pin 20.

The circuit operates fine, however I am concerned that the 7.6 volts
on Pin 20 might eventually degrade the BX-24.

Any comments to remedy this would be appreciated.

Thanks, Mario




--- In , "Mario" <domovoi22@y...> wrote:
>
> [...]Connected to Pin 20 is an output of a op amp that is
> outputting about 7.6 volts. [...]

The maximum voltage that should be applied to (most) input pins is
0.5V above Vcc. That means 5.5 volts in this case. This is a
typical input limit for digital parts.

You should either clip the input at 5 volts (using a series resistor
and a diode [anode to the input pin, cathode to Vcc]), adjust the op
amp gain for 5V output maximum, or use a voltage divider to scale the
op amp output to not exceed 5 volts.