Analog pin voltage tolerances

[curufinn9]

Hello Pinoccio and all,
I was curious about the voltage ranges on the analog pins. In the tutorials, it says that they can accept a maximum input voltage of 1.6V, but can read data at 0.3-2.0V. Could someone elaborate on the difference between input voltage and data voltage?

Thanks

[Matthijs]

Hmm, could you add a link to the tutorial you referring to?

I recently looked up some stuff from the datasheet:

In the 256rfr2 datasheet, page 553 under maximum ratings It says maximum
voltage on all pins is 3.6V and minumum is -0.3V Analog pins, while
doing analog reads, range from -0.3 - 2.0V

So that means that you can normally put 3.6V on all pins (though
sticking to 3.3V is probably better). However, analog pins should stay
below 2.0V while doing reads (which happen periodically when you
configure a pin as an input).

Does that help to clarify things?

Gr.

Matthijs

[curufinn9]

Matthijs,
Here is the link -
http://support.pinocc.io/hc/en-us/articles/200675645-Pins

Yeah, your explanation does make sense. It could be this is a safety rating, rather than a maximum rating. Give it a look and let me know what you think.

Thanks,
-Aaron

[curufinn9]

To give some reference as to why this matters - I'm looking to use Pinoccios as a transmitters for sensor readings in an industrial environment. However, we deal in voltages above even the 3.3V, so some treatment will need to be done to the signal prior to hitting the 'Noccios. Which is where my question about the difference in data voltages vs. input voltages. Hope this adds some structure to the question.

Thanks,
-Aaron

[wminarik]

In the old Forum (not the old old Forum) Jake indicated that AREF (the analog reference voltage) was set to 1.8V. Hence the ADC scales 0-1.8V to 0-1023. The "Pins' Hardware entry for the analog pins is a bit confusing.
-Bill

[cisco]

Alright, I just saw this conversation and I had to make sure myself of what is the allowable range for the ADCs.
So I hooked up my regulated power supply to the a0 input and a multimeter to monitor the voltage.
Here is a table that compares ADC input voltage to what the pinoccio ADC is reading:
http://tinyurl.com/mkku5j6

For each step, I executed the following command on my scout:

i=0; while (++i<40) print pin.read("a0"); delay(200);

and averaged the 40 samples.

From my observation, the "good" ADC range is 0 to 1.71V.
At 1.72V, I am already observing some jumps from the 9xx values to 1023.
It seems like the ADC input is clamping to the max. input if the voltage exceeds some value around 1.72V.
For 1.73 and above, ADC will just report 1021 or 1023 values, averaged everytime to 1023.

Hope that helps.

[Matthijs]

Hmm, this surprises me, since IIRC the default voltage reference is the voltage from the AVDD pin (on the MCU), which should be 1.8V (meaning 1.8V should read 1023). Alternatives are internal 1.6V and 1.5V references. I'm wondering what your AVDD voltage is (you'd have to check the schematic or 256rfr2 datasheet to see where you can measure it, though)...

[alexglow]

Thanks for the comments on the docs! We've been updating that Hardware/Pins page as part of an ongoing conversation. I'll work with Matthijs to make sure it's as clear and accurate as possible.

[cisco]

I checked the AVDD voltage on both my scout and it is close to 1.78V. And there is no ripple leak from the VCC, which is good news.

I did the test again and it is possible that 1.72V would be read correctly but once it gets to 1.725V, here is the output of the read loop (notice the jumps to 1023):

i=0; while (++i<40) print pin.read("a0"); delay(200);
988
991
992
990
990
991
1023
1023
990
990
990
992
1023
1023
1023
1023
1023
990
992
990
990
1023
1023
1023
989
1023
991
991
990
1023
990
988
991
1023
990
990
1023
989
1023

Not sure, where that noise / clamping comes from. AREF is not connected on my board and is at a still ground level.

[Andrea]

s curious about the voltage ranges on the analog pins. In the tutorials, it says that they can accept a maximum input voltage of 1.6V, but can read data at 0.3

Me too, I'm not getting the pins doc about max input voltage and AREF. The default is 1.6V, but can I change the AREF to 3.3V?

[Matthijs]

Me too, I'm not getting the pins doc about max input voltage and AREF. The default is 1.6V, but can I change the AREF to 3.3V?

Unfortunately, the ADC that reads analog voltages cannot handle voltages over 2.0V at all (though I recently learned the ADC has some protection circuits that should prevent from actually breaking when you connect a higher voltage, which is good).

The biggest internal AREF available is 1.8V, so you can only meaningfully read values <= 1.8V. If you really need it, you could connect an external reference voltage, but only up to 2.0V.

Does this answer your questions?

On a related note, I recently thought of a way to "abuse" differential analog reading to double the effective analog range (to 3.6V). In differential measurement, voltages can go from -AREF to +AREF, so if you subtract your input signal from 1.8V/AREF, you double the range. This requires shorting AREF to a second analog pin and needs support in the code. It might also not work at all - I haven't tried this yet.

[Andrea]

Thx Matthijs, it totally answer my question. I think that the documentation should retain that limit.
Thank you so much,
Andrea

[Kurdman]

I'm looking to use Pinoccios as a transmitters for sensor readings in an industrial environment

Aaron, could you share the progress you've made with this. I am exploring doing something very similar and any input or advice will be most welcome.

[curufinn9]

Hi Kurdman,
With the large voltages I'm dealing with on my transducer (7.5V), I had to treat the signal before going to the Pinoccio. Nice part is that the output differential is about 100 mV. Sad part is that is still too low to use the default AREF and get something appreciable. (I was getting a swing of 20 to 60/70 when reading the sensor from 0 to max output). To treat the signal, I built a in-amp from 3 op-amps and resistors. I used no gain resistor as it wouldn't take much to get over 1.6V, and I didn't want to exacerbate error. This put it at unity, which is fine. I then fed the output of the in-amp setup to the ADC, and was able to see some results that correlate with the on-board readings from the sensor (unsure how good the sensitivity is, but it reads about where I would expect).

Next step is I am getting in some voltage references and will put a voltage divider on the end of to get it down to 100mV and feed that to the AREF and see if I can get the swing of the transducer over the full range of 0-1024. Still waiting on those.

I also purchased an on-chip in-amp and am testing against the home-made version. Dunno if it is worth it to use the on-chip in-amp or if the 3 op-amps will be sufficient, but I did notice some difference between them, and haven't been able to verify which is more sensitive/accurate/etc. I will post here and let you guys know what I find out.

[Matthijs]

Be careful when feeding a voltage into AREF - if you ever (accidentally) start an analog read using an internal reference voltage, both voltage sources will short-circuit and probably be damaged.

Right now, there is no support in the scoutscript API yet for switching voltage source, so you'll have to use a custom Arduino sketch instead.

[curufinn9]

Thanks for the heads up Matthijs. Would it be fair to say then, before starting any analog read that you intend to use an external volt. reference, one should first initiate the external AREF before starting the analog read? Im just trying to ensure I understand you correctly.

[Matthijs]

I'm not sure what you're saying exactly, but what you need to do is call analogReference(EXTERNAL) before the first call to analogRead().

While looking around the library-pinoccio code to see if there are any places that reset the analog reference, I found Pinoccio.enableExternalAref() which calls analogReference(EXTERNAL) for you. Additionally, it disables the temperature readings, which also use the ADC.

[curufinn9]

Perfect, thanks Matthijs. Hopefully these references show up soon!