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On the Beaglebone Black, what is the most current that the 1.8V analog rail (pin 32 on header P9) can supply?

I can't find it in the System Reference Manual. I would've expected to find it in section 6.1.9, "Power Rails".

I want to use it with a voltage divider to get a 0.9V source. I plan to use it like this:

With two 10k resistors and the op-amp buffer, the current should be 90 uA -- very small.

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  • \$\begingroup\$ I would be extremely surprised if you were unable to draw 90uA from it. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 15 '16 at 16:09
  • \$\begingroup\$ Why not use a standard linear voltage regulator, or a voltage reference if your 0.9V supply doesn't need to provide much current? \$\endgroup\$ – helloworld922 Jul 15 '16 at 16:32
  • \$\begingroup\$ @helloworld922, I suppose I could do that. Are you suggesting using (say) a 5V voltage regulator and voltage-dividing it to 0.9V, similar to the picture above? The appeal of using the BB is that I'm using this 0.9V signal to level-shift a (possibly negative voltage) signal to the 0-to-1.8V range required by the BB's analog inputs. So I was thinking it would be nice to have the 0.9V also generated by the BB. Also, what is a "voltage reference"? Sorry, I'm new at this. \$\endgroup\$ – ConvexMartian Jul 15 '16 at 17:09
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To find the current capability of that pin, you need to look in the Sitara datasheet. The pin is called VDDA_ADC, and is specified for 10 milliamps.

However, I would not recommend putting a voltage divider on that signal without a buffer. Although that pin is often referred to as a "rail", it is actually the reference voltage to the ADC, so using it directly will add a path for noise into all eight ADC channels.

I would recommend adding an additional op-amp to buffer it prior to using it.

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