Analog Input resistors dimensioning

Hello,

I like to measure analog voltages between 0…10 Volts with the Maestro18.
Since the board will only be able to support up to 5 volts, I want to use a simple voltage divider with 2 resitors.

In order to reduce the current as much as possible, I want to use high resistor values (<100k Ohm). But I need to consider the internal input resitor-grade of the Maestro in order to avoid a tilt in the voltage levels.

Can you tell me how big the internal input resistor value for a analog input is?

Thanks
-Michael

Hello, Michael.

When you configure a Maestro channel to be an input, the I/O line on the microcontroller becomes a floating input. There is a 220 Ohm resistor in series with the I/O line, on the board. There are no pull-ups or pull-downs enabled (unless you explicitly enable them). You can learn more details about the inputs by looking in the PIC18F4550 datasheet.

I think the biggest problem you will face is that left-over charge in the ADC from the previous reading will affect the reading of your input. The higher the resistance of your voltage divider is, the more of a problem this will be. Since the Maestro reads the channels in order, you can try setting the preceding pin as an input and tying it directly to GND to minimize that problem. Let us know if that works for you!

Once you have built your system you’ll probably want to use a multimeter to calibrate it.

–David

Hi David,

Thank you very much for your response.
Actually, I already got some strange behavior. The measured values were not stable in regards to the given voltages.
I use a regulated power source to provide the input voltages and measure it via a multimeter.

In the first attempt I used 100k Ohm Resistors, which I learned is far from a useful value.
I will retry with lower resistor values and also with your zero-voltage tip on channel zero and let you know.

Best regards.
-Michael

OK, here is what happended.

I don’t know why I did not do this before, but I tried to measure the current drawn from the Maestro when attaching a regulated generator (0…5V) directly.
My 20 Euro - low budget - multimeter was not able to measure any current. Also resitor measurement showed “infinite” as value.

Also, from the suggested document it seem clear that the maestreo has an input resistor value in the MOhm area.
In output mode, a channel has the internal 220 Ohm resistor. This is measureable as well, but seem not to apply in input mode.

As you indicated, I really get jumping / instable values on open channels that were not pulled down. The pull-down as suggested helped me out.

Thanks,
-Michael

Hello.

The 220 Ohm resistor is on the Maestro between the microcontroller’s pin and the pin labeled signal on the board. It is there regardless of the input mode of the channel.

- Ryan