Use analogue inputs with 5 or 10 V signal

[mike.ma...@gmail.com]

Hi,

this is somewhat a noob-question, but my skills in analogue hardware are limited.

The BeagleBones (and variants) come with six analogue inputs which each have a separate ground line GNDA and also a separate power supply VADC.

My questions here: how can I connect an external signal with 0..5V or 0..10V to these inputs in order to read their analogue value?

And how about GNDA, has it to be separated from normal GND?

Thanks!

Mike

[Adam Saenz]

You will need to use a voltage divider to lower the detectable voltage of your 0-5V or 0-10V inputs to a range of 0-1.8V or less.  Google voltage divider to see how this is done; sparkfun provides a pretty nice explanation of voltage dividers.  If you look at the schematic, an example of a voltage divider is done for AIN7 to read the 3.3V supply voltage.  AIN7 will see is 1.65V when VDD_3V3B is 3.3V.

ADCs are most accurate when the voltage they are reading is stable and free from noise or ripple voltages.  Digital circuits (e.g. the processor cor) are sources of digital noise so a separate VADC and GNDA are provided to minimize the influence of noisy digital sources.   If you look on the schematic VDD_ADC is actually connected to VDD_1V8 through a ferrite bead (inductor). The ferrite bead is there to block high frequency noise caused by the digital circuits, while allowing DC voltages to pass; so VDD_ADC is actually connected to 1.8V.  The AM3358 datasheet lists the max voltage input of VDD_ADC as 2.1V so you cannot connect a 5V or 10V reference directly to it.  It's best to use the existing 1.8V reference and scale the analog input voltages you want to measure so they are less than 1.8V.

Adam

[Adam Saenz]

Forgot to add that you should use the GNDA as the ground reference to your external analog input sources.

Adam

[mike.ma...@gmail.com]

So it could look like this!? External ground is directly connected to GNDA and the relation between the resistors is equal to the relation of the voltages to apply at each!? How about the exact resistor values - 18 kOhm and 10 kOhm would be the same from relation, but which one should I use really? Thanks :-)

[Dennis Lee Bieber]

On Tue, 26 Dec 2017 10:40:54 -0800 (PST),
mike.ma...@gmail.com declaimed the
following:

>
>
>So it could look like this!? External ground is directly connected to GNDA
>and the relation between the resistors is equal to the relation of the
>voltages to apply at each!? How about the exact resistor values - 18 kOhm
>and 10 kOhm would be the same from relation, but which one should I use
>really? Thanks :-)
>

How much current flow can you accept? Consider, without the resistors
you basically have a direct short from the input voltage source to ground.
Small value resistors will let a lot more current pass. You likely need to
balance between not having too much current flowing through the divider and
having so little current flowing that the ADC can't sense the voltage...

--
Wulfraed Dennis Lee Bieber AF6VN
wlf...@ix.netcom.com HTTP://wlfraed.home.netcom.com/

[Adam Saenz]

That circuit will work.  Yes on connecting external ground to GNDA, that will give you a common reference.  Those resistor values will allow the ADC to see about 1.78V when the input voltage is 5V, looks like a good place to start.  As far as resistor values (1k, 1.8k vs 10k, 18k), I'd go with the 1k, 1.8k pair as long as your source circuit can handle sourcing a 2.8k load (1.7mA max).  If it can't then I'd consider something that gives you a total resistance of 5-6k.  The 10k/18k pair gives you 28k total, should be ok but I've had ADCs have trouble working when resistances approach 100k.

Someone mentioned using op amps which is a good choice, as is using an anti-aliasing filter but since this is a noob type question I figured starting simpler is a better choice.  You can improve you analog circuit if you discover your application needs to minimize noise, or is sensitive to load, etc.  Start simple though.

Adam

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[Mike Maikaefer]

Great, thanks to all :-)

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