Thank you for your help, i think i will split the signal in two different
analog inputs, one for the range of 0-10 A(for more accuracy) and the other
one for 10-30 A with +/-0.1A.
On the other hand, aside the accuracy, i would like to implement the
on-board filter to make simplier my Android app, i already have found on
these forums how to change the sampling rate and reading the PIC's
datasheet i found i could find the average value of 16 adc values with
something like this:
for (count = 0; count < 16; count++) // average the 16 ADC value
{ ADCValue = ADCValue + *ADC16Ptr++; }
ADCValue = ADCValue >> 4;
But i'm struggling to know where to put that in the firmware code.
El jueves, 16 de julio de 2015, 11:43:15 (UTC-5), Ytai escribió:
>
> I don't see how sampling faster is going to improve your accuracy. The
> +/-100mA that you're seeing is about you might expect of your current
> signal properties and a 10-bit ADC, regardless of sampling rate. In other
> words, if the problem you're trying to solve is that of increasing
> precision, you're barking at the wrong tree.
> Of course, if you insist, you can fairly easily change the sampling rate
> and/or introduce any on-board filtering you please.
>
> On Thu, Jul 16, 2015 at 9:34 AM, Julio Castellanos <[email protected]
> <javascript:>> wrote:
>
>> For example if instead of sending all the values from the ADC, i
>> calculate in the firmware the max value of the signal and send it
>> periodically, i could increase the adc conversion rate and send the max at
>> the same rate that now i'm sending the values, so my app would be faster
>> because i wouldn't need to calculate anything, just multiply by 0.707 to
>> get the rms value. It's just what i thought, please correct me if i'm wrong.
>>
>> El miércoles, 15 de julio de 2015, 16:57:00 (UTC-5), Ytai escribió:
>>>
>>> I don't see how custom firmware is going to give you higher precision.
>>> The ADC on the PIC has 10-bit resolution, which translates into 3.3mV. With
>>> your full-range scaling of 50mV/A this would translate into about 66mA
>>> resolution. If that's insufficient you can either:
>>>
>>> 1. Use an external ADC with higher resolution.
>>> 2. Sacrifice your 30A range for higher precision in the low end by
>>> increasing the gain (be sure to clamp at 3.3V or else you'll be damaging
>>> the IOIO).
>>> 3. Dynamically change the gain under software control (e.g. by using
>>> a digital output to change a configuration resistor value on the op-amp).
>>>
>>>
>>> On Wed, Jul 15, 2015 at 2:10 PM, Julio Castellanos <[email protected]>
>>> wrote:
>>>
>>>> Yes indeed i need to conditionate the signal, i though there were pins
>>>> tolerant to 5 V but i just read that is only for logic input. Until now i
>>>> was measuring currents lower than 10 A so i didn't remarked the need of
>>>> the
>>>> conditioning. Thank you for your support.
>>>>
>>>> Also i was already using the buffered version of AnalogInput, i did
>>>> just like you said, i measured the peak-to-peak voltage and converted it
>>>> to
>>>> rms, then i did a simple conversion to current, that's when i got the
>>>> accuracy of +-0.1A, but i need it to be less than that, that's why i'm
>>>> looking for the option of doing it in the firmware. I wonder if that is
>>>> possible?
>>>>
>>>>
>>>>
>>>> El miércoles, 15 de julio de 2015, 12:09:16 (UTC-5), Ytai escribió:
>>>>>
>>>>> I think I understand now. So:
>>>>>
>>>>> 1. If you're really intending to measure a signal as high as 30A,
>>>>> the output voltage will be around 4.5V, which exceeds the range the
>>>>> ADC is
>>>>> willing to accept (0-3.3V).
>>>>> 2. Unless the sensor has a built-in amplifier, it is likely that
>>>>> it has a fairly high output impedance.
>>>>> 3. To address both of the above problems, I recommend conditioning
>>>>> the signal using one of more opamp stages, so that it has about 1.5V
>>>>> bias
>>>>> and about 1.5V amplitude @ 30A (or 50mV/A).
>>>>> 4. The IOIO samples at 1kHz. There should be no problem sampling
>>>>> of a 60Hz signal. Moreover, since you know it is a sine wave, all you
>>>>> really need is measure the peak-to-peak voltage which you can then
>>>>> trivially convert to RMS. I suggest that you look at the buffered
>>>>> version
>>>>> of AnalogInput, so that you don't miss or depend on your application
>>>>> thread
>>>>> loop frequency.
>>>>>
>>>>>
>>>>> On Wed, Jul 15, 2015 at 7:15 AM, Julio Castellanos <[email protected]>
>>>>> wrote:
>>>>>
>>>>>> Hi Ytai, thank you for your help. The signal is the output of an A.C.
>>>>>> current sensor, the frequency is the same as the network, 60 hz, and the
>>>>>> sensor can measure up to 30 A with a sensitivity of 66 mV/A. The signal
>>>>>> has
>>>>>> always a D.C. value of 2.5 V so if i measure 1 A the signal will have a
>>>>>> max
>>>>>> of 2.566 V and a min of 2.434 V. The absolute maximum with 30 A is 4.48
>>>>>> V
>>>>>> and absolute minimum of 0.52 V, so the signal is always in the range of
>>>>>> the
>>>>>> analog input of the ioio. I want to know the value rms of the current in
>>>>>> my
>>>>>> smartphone, and i tried obtaining the rms of the voltage by calculating
>>>>>> the
>>>>>> max value of a buffer of 1000 values and the multiplying it by 0.707. I
>>>>>> get
>>>>>> the value rms of the current with an accuracy of +-0.1 A, but i need it
>>>>>> to
>>>>>> be less than that, that's why i though it would be better to compute the
>>>>>> rms value of the signal in hardware so i dont have to send the whole
>>>>>> buffer
>>>>>> of the signals.
>>>>>>
>>>>>> El martes, 14 de julio de 2015, 18:44:30 (UTC-5), Ytai escribió:
>>>>>>>
>>>>>>> Before we begin, can you please specify the signal you're trying to
>>>>>>> measure better?
>>>>>>> Specifically, its min and max (instantaneous) voltage, and its
>>>>>>> bandwidth would be important.
>>>>>>> Based on that I can recommend a way of measuring it.
>>>>>>>
>>>>>>> On Tue, Jul 14, 2015 at 3:06 PM, Julio Castellanos <
>>>>>>> [email protected]> wrote:
>>>>>>>
>>>>>>>> Hello, i'm currently working with the IOIO analog input with a
>>>>>>>> sensor signal that comes out as an A.C. signal in the range of 2,5 V
>>>>>>>> as
>>>>>>>> mean value. I've tried to get the voltage through the analog input and
>>>>>>>> get
>>>>>>>> the rms value in software, but the value changes to much because of
>>>>>>>> the
>>>>>>>> bandwith limit of the IOIO:Android communication. I'm now looking to
>>>>>>>> get
>>>>>>>> the RMS directly in hardware and just send that value to the software
>>>>>>>> but i
>>>>>>>> gave a look at the hardware code for adc.c and i don't understand
>>>>>>>> almost
>>>>>>>> nothing, can someone point out to me a source where i can learn how
>>>>>>>> does
>>>>>>>> this code works?
>>>>>>>>
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>
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