On 06/01/2015 04:44 PM, Guenter Roeck wrote:
On 06/01/2015 02:21 PM, Chris Lesiak wrote:
On 06/01/2015 02:54 PM, Guenter Roeck wrote:
On 06/01/2015 09:27 AM, Chris Lesiak wrote:
The function ntc_adc_iio_read was assuming both a 12 bit ADC and that
pullup_uv is the same as the ADC reference voltage. If either
assumption is false, then the result is incorrect.
Attempt to use iio_convert_raw_to_processed to convert the raw
value to
microvolts. It will fail for iio channels that don't support support
But we need millivolts ?
No. I think we really do want microvolts.
Don't let the old comment "/* unit: mV */ " lead you astray. It is
incorrect.
Ok.
The value eventually makes its way to the function
get_ohm_of_thermistor.
That function is expecting the value to be in microvolts.
It is true that get_ohm_of_thermistor promptly converts both uv and
pullup_uv
to millivolts by dividing by 1000. But I don't think that conversion to
millivolts is necessary. In fact it is hurting the accuracy a little.
For example, take the ncpXXwb473 connected to 5000 mV and pulled down
through
a 47000 ohm resistor. At 25 C, the resistance of the thermistor is
47000 ohms.
The measured voltage will be 2500 mV. If we measure instead 2501 mV,
then
the calculated resistance will be 46962 ohms - a difference of 38 ohms.
So the precision of the resistance estimate could be increased by 38X by
doing the calculations in microvolts.
But how does that resolution affect the temperature estimate?
At room temperature, the sensitivity of the thermistor is -5.3x10^-4
C/ohm.
So, the resolution of the temperature measurement is 0.02 C. That
probably
is good enough for most purposes.
The result will be worse at lower temperatures, but if you wanted to
measure
low temperatures, you'd probably connect the thermistor to ground and
pull it
up through the resistor.
I think things could be improved in one of two ways:
1. Accept the precision that you get by doing the calculation
millivolts.
Simplify things by using millivolts throughout, although pullup_uv
needs to remain becaus it is part of the interface.
2. Create an additional patch doing the calculations with microvolts
instead of millivolts.
What are your thoughts?
You explain above that doing the calculations in mV looses precision.
Given that, what would be the point of the first proposal, especially
since the uV are already available (and may still be needed in
ntc_adc_iio_read if conversion through iio is not available) ?
Thanks,
Guenter
I do think that option 2 is the better choice. If you'd like, I'll
create a patch.
But if 1 were implemented, ntc_adc_iio_read would give a value in millivolts
even when iio_convert_raw_to_processed failed. Something like:
ret = iio_convert_raw_to_processed(channel, raw, &mv, 1);
if (ret < 0) {
/* Assume 12 bit ADC with vref at pullup_uv*/
mv = (pdata->pullup_uv * (s64)raw / 1000) >> 12;
}
--
Chris Lesiak
Principal Design Engineer, Software
LI-COR, Inc.
chris.lesiak@xxxxxxxxx
Any opinions expressed are those of the author and
do not necessarily represent those of his employer.
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