From 3642cd3552cdc17e55603f218c46780da1d0c747 Mon Sep 17 00:00:00 2001 From: David Schleef Date: Wed, 12 Jun 2002 22:44:44 +0000 Subject: [PATCH] notes about the new calibration functions --- comedi_calibrate/README | 31 +++++++++++++++++++++++++++++++ 1 file changed, 31 insertions(+) diff --git a/comedi_calibrate/README b/comedi_calibrate/README index 7066421..445bd88 100644 --- a/comedi_calibrate/README +++ b/comedi_calibrate/README @@ -133,6 +133,11 @@ This is one way that STATUS_SOME is useful, because you can calibrate the zero offset, then get a much better idea which other channels are likely to be for the voltage reference. +Note that we haven't done anything with caldac[3]. It clearly +does something useful, but until we attempt a coarse calibration, +it's not certain what it does. It turns out to be a fine postgain +adjustment. + In this example, there doesn't appear to be a caldac that affects unipolar zero offset, so it will not be used in the final function: @@ -144,3 +149,29 @@ unipolar zero offset, so it will not be used in the final function: cal1(ni_reference_low, 1); } +There are a number of functions that are useful for optimizing a +given caldac, each optimized for different cases. The inconsistently +named postgain_cal() and cal1() measure the observable(s) at a +number of points throughout the entire caldac range, and then do a +linear fit to determine the optimum value for caldac. These functions +are good if the caldac dependence is strictly linear. They are also +useful if the target value for the observable is at the endpoint of +the measurable range, as when measuring unipolar zero offset, since +the functions automatically compensate for bad input values. + +The function cal_fine() is useful for fine-tuning of the results of +cal1(), especially if the dependence is close, but not quite linear. +The goodness of the linear fit is quantified by the S_min value in the +log -- an S_min value that is approximately the same (within a factor +of 2 or 3) as dof (degrees of freedom) indicates a good fit. An S_min +value that is about 10 times dof indicates that fine tuning is probably +necessary. An S_min value that is many orders of magnitude larger than +dof indicates that linear fitting should not be used. + +The functions cal_binary() and cal_postgain_binary() are used when +the caldac dependence is highly non-linear. It does a binary search +in the range of the caldac to find a decent value. Once a decent +value is found, cal_fine() should be used, since the caldac dependence +should be relatively linear in a small range around that value. + + -- 2.26.2