On Saturday, November 25, 2017 7:50:16 PM CET Guenter Roeck wrote: > On 11/22/2017 07:32 AM, Maciej Purski wrote: > > Calibration register is used for calculating current register in > > hardware according to datasheet: > > current = shunt_volt * calib_register / 2048 (ina 226) > > current = shunt_volt * calib_register / 4096 (ina 219) > > > > Fix calib_register value to 2048 for ina226 and 4096 for ina 219 in > > order to avoid truncation error and provide best precision allowed > > by shunt_voltage measurement. Make current scale value follow changes > > of shunt_resistor from sysfs as calib_register value is now fixed. > > > > Power_lsb value should also follow shunt_resistor changes as stated in > > datasheet: > > power_lsb = 25 * current_lsb (ina 226) > > power_lsb = 20 * current_lsb (ina 219) > > > > Signed-off-by: Maciej Purski <m.purski@xxxxxxxxxxx> > > Setting the calibration register to a specific value may optimize precision, > but limits the supported value range, which is the whole point of providing > a calibration register. What am I missing here ? For the current register, any different calibration register value is completely useless - smaller values just truncate the register value, larger values adds noise in the lsbs. Both registers (current/shunt voltage) are 16bit, and nothing is going to change that. There is a *very* small allowed power operating area where scaling down would be of any use: 1. Bus voltage exceeds 25 (20) Volts 2. Shunt voltage is about 82 (320) mV In this case the power register overflows. More specifically, if: (shunt_voltage [mv] / 81.92) * (bus_voltage [V] / 25) > 1 Maximum allowed bus voltage is 36 V resp 26 V. (ina226/219). Kind regards, Stefan -- Stefan Brüns / Bergstraße 21 / 52062 Aachen home: +49 241 53809034 mobile: +49 151 50412019
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