Hi Stefan, Sorry for a super late reply. I took a long vacation. On Wed, Nov 21, 2018 at 10:16:09PM +0000, Brüns, Stefan wrote: > > > Another concern may be voltage drop over the shunt, but for this case you > > > have a nominal voltage of 1.8V, so 30uV are 0.001%. > > > > > > > When measuring a 1.8v voltage running a small current (e.g. 33 mA), > > > > the power value (that's supposed to be 59.4 mW) becomes inaccurate > > > > due to the larger scale (25mA for method A; 62.5 mA for method B). > > > > Just found out that I have typos here: 25mW and 62.5mW. > > > > > Another look into the datasheet reveals, even at full gain (PGA=1), the > > > LSB is 40mV / 2^12 = 40mV / 4096 ~ 10uV. So when the current ADC reads > > > out as 3*LSB, this anything between 25mA and 35mA. This is the best case > > > figure. > > Current read doesn't get affected a lot actually, since hwmon ABI > > also reports current value in unit mA. However, the power read is > > the matter here. With a 62.5mW power_lsb, power results are kinda > > useless on my system. > > The reported current does not matter here, actually. Internally, the ADC value > will have an uncertainty of 10mA (at PGA=1). At 1.8V, your uncertainty is > 18mW. And thats *only* the quantization noise. It wont get better than that. The fact is that I do get better power results after setting the calibration value to 0x7ff. That's the necessity of this change. > Also note, you are apparently using the ina2xx hwmon driver - I strongly > advise against it, you should either use the ina2xx driver from the IIO > subsystem directly, or use the IIO driver via iio-hwmon. The IIO version is also using the minimum calibration value. It will not solve my problem here. > There is also always the possibility to read the bus and shunt voltage > registers and calculate the power manually. Won't that be a waste since the hardware could have provided a better accuracy? It would need more I2C bus reads and cpu cycles for calculation. I don't get why you're against a setting for calibration value. This is how the hardware got designed to cover different cases. Since we do have such a case that needs some accuracy, it'd be fair to add it into the driver. Plus, the feature won't change the minimum calibration value at all -- everyone would be happy. Thanks Nicolin
