Yes, I like the method shown in the
schematic. Once the ADC voltages are known for a
certain analog input voltage (equal on each
channel) the D/A voltages can be compared and the math done.
A few milliseconds to determine the voltage ratio
is fast enough. If I can somehow shorten that
with a PRU, that is even better. I would rather not add outboard A/Ds.
Some of my problem is I don't know how fast some
of the other tasks will take. Those include
three stepping motors, a frequency counter, and
Ethenet. Possibly, too, a web page server.
At 11:41 AM 10/30/2015, evilwulfie wrote:
A resistor divider is not going to protect the
BBB. if you want true protection you will need to feed the BBB ADC
from a 1.8v op amp that way the BBB input cannot
go over the rails of the op amp.
OR see attached jpg for another method
A zener doide will also not be 100% protective
as the zener may not be fast enough to react to
a high voltage
On 10/30/2015 9:00 AM, Przemek Klosowski wrote:
You can't feed more than 1.8V to ADC without
damaging it. You can, however, scale it with a
simple resistor divider (resistor R1 connected
between the ground and second resistor R2,
which is then connected to your signal), and
hook the ADC to the middle of the
divider where it will be scaled down by the
ratio R1/(R1+R2). How quickly do you want the
result? If it's miliseconds, Linux is perfectly
capable of that; if microseconds, then PRUs can
do it; otherwise, you'd have to come with an analog multiplier circuit.
On Fri, Oct 30, 2015 at 7:23 AM, John
<<mailto:ra...@mediacombb.net>ra...@mediacombb.net> wrote:
I'm a hobbyist with a recent BBB and I want to
use a couple of ADC channels in real time
(alternate). As nearly as I can tell, this is
not supported by the two risk units.
Have I missed something?
BBB and Linux are new to me. One of the
things I would like to do is compute a ratio
between two voltages, each of which might
exceed 1.8 VDC. One solution might be to use
DACs to sent feedback voltages to operational
amplifiers that are supplied by 1.8 VDC. The
goal is to determine the ratio as quickly as
possible and without waiting for Linux to get
around to it (waiting for the ADC to happen).
John
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