On Mittwoch, 21. November 2018 20:40:52 CET Nicolin Chen wrote: > (Removing "m.purski@xxxxxxxxxxx" since it's not reachable any more) > > Hi Stefan, > > Thank you for the comments. > > On Wed, Nov 21, 2018 at 04:13:01PM +0000, Brüns, Stefan wrote: > > > === Problem === > > > Both methods simplify software routine by fixing one factor, which > > > sacrifices the precision of the hardware measurement results. > > > > > > Using ina226 for example, with method A, the current scale was 1mA > > > and the power scale was 25mA. > > > > > > With method B, calibration value is fixed at 2048 so the precision > > > is decided by shunt resistor value. It sounds reasonable since the > > > hardware engineers can use a larger shunt resistor when they need > > > higher resolution. However, they often concern power burning across > > > the resistor as well, so the resistor usually won't be so large: a > > > typical value 1000 micro-ohms, which results in a current scale at > > > 2.5 mA and a power sacle at 62.5 mW. > > > > Power loss surely is a concern, but figures should be kept reasonable. > > > > 1. You mention 1.8V bus voltage, and currents in the 30mA range. Using the > > 1mOhm current shunt: > > > > U_S = R_S * I_S 1e-3 Ohm * 30e-3 A = 30e-6 V (30uV) > > P_S = U_S * I_S = 30e-6V * 30e-3 A = 900e-9W = 0.9 uW > > > > INA219 Power Supply (Datasheet) > > Min operating Voltage: 3V > > Quiescent Current: 0.7mA > > -> Min power: 2.1mW > > > > So the INA219 alone uses 2.1mW, 1000 times more than the shunt. > > Chip can enter power-down or one-shot mode. Though this upstream > driver doesn't have these two mode supports yet, I am working on > it so they'll be added. The power-down current is 6uA, so even if you never leave power-down mode, you are down to 18uW. But on top of that, you need power for the conversion, and you need power for communication. > > 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. 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. 1. INA219 is not properly supported by the hwmon driver, see the changes in the IIO driver. 2. The IIO driver has many more features: - setting the PGA (INA219) - setting the bus voltage range (INA219) - selecting the conversion time (INA219/226) There is also always the possibility to read the bus and shunt voltage registers and calculate the power manually. Regards, Stefan
