4.6.  Slowly-varying inputs

Note: The functions described here use an old feature that is no longer implemented by the Comedi kernel layer. THEY WILL NOT WORK!

Sometimes, your input channels change slowly enough that you are able to average many successive input values to get a more accurate measurement of the actual value. In general, the more samples you average, the better your estimate gets, roughly by a factor of sqrt(number_of_samples). Obviously, there are limitations to this:

As you might have guessed, the Comedi library has functions to help you in your quest to accurately measure slowly varying inputs:

int comedi_sv_init(comedi_sv_t *sv,
 comedi_t *device,
 unsigned int subdevice,
 unsigned int channel);

The above function comedi_sv_init initializes the comedi_sv_t data structure, used to do the averaging acquisition:

typedef struct comedi_sv_struct {
  comedi_t *dev;
  unsigned int subdevice;
  unsigned int chan;

  /* range policy */
  int range;
  int aref;

  /* number of measurements to average (for analog inputs) */
  int n;

  lsampl_t maxdata;
} comedi_sv_t;

The actual acquisition is done with the function comedi_sv_measure:

int comedi_sv_measure(comedi_sv_t *sv,
 double *data);

The number of samples over which the function comedi_sv_measure averages is limited by the implementation (currently the limit is 100 samples).

One typical use for this function is the measurement of thermocouple voltages. And the Comedi self-calibration utility also uses these functions. On some hardware, it is possible to tell it to measure an internal stable voltage reference, which is typically going to be very slowly varying; on the kilosecond time scale or more. So, it is reasonable to measure millions of samples, to get a very accurate measurement of the A/D converter output value that corresponds to the voltage reference. Sometimes, however, this is overkill, since there is no need to perform a part-per-million calibration to a standard that is only accurate to a part-per-thousand.