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I always seem to forget how to convert the calibration
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dump information into code for doing a calibration, so
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I'm writing this mostly for myself.
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Boards may have one of 4 calibrations statuses, depending
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on how well the calibration code is trusted. These are:
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STATUS_UNKNOWN, the default for no information; STATUS_SOME,
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meaning that a dump has been converted to initial code,
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but not tested; STATUS_DONE means that the output of a
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STATUS_SOME dump has been checked, and is correct;
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STATUS_GUESS is a marker that code has been converted
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from a previous version of the code, but not checked.
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The NI E series boards have several internal voltages that
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can be measured, and also several calibration DACs that function
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similar to adjustable resistors on old data acquisition boards.
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The information we need is which DACs affect which measurable
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voltages; then we can write calibration code that adjusts those
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DACs until the voltages are within spec.
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Usually, there are DACs (or multiple DACs) that are added to
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an analog input signal: 1) before the variable gain amplifier
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("pre-gain"), 2) after the variable gain amplifier ("post-gain"),
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3) between the board's stable voltage reference and the
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reference input to the ADC ("gain offset"), and 4) before a
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unipolar-to-bipolar adjuster ("unipolar offset"), or other
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In addition there are DACs that adjust the output voltages and/or
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reference voltage inputs to a D/A converter. These are pretty
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intuitive once analog input is understood, and is dependent on
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correct analog input calibration.
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The measurable quantities are 0 volts and an internal voltage
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reference near 5 volts, and can be measured at any gain. The
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interesting combinations are:
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ai, bipolar zero offset, low gain
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ai, bipolar zero offset, high gain
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ai, bipolar voltage reference, low gain
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ai, unipolar zero offset, low gain
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The unipolar zero offset may not be available on some boards.
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In a STATUS_UNKNOWN dump, for each measurable quantity and each
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calibration DAC, the DAC is varied throughout its entire range
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and the quantity measured. The data is linearly fit, and if
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the slope is statistically non-zero, a line is printed:
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caldac[0] gain=1.26(11)e-7 V/bit S_min=235.659 dof=254
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The information given is caldac index, slope (gain) and slope
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error (in parenthesis, modifying the last two digits of the
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slope), and two statistical parameters S_min and degrees
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of freedom. S_min and dof will be roughly similar for a
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good fit. If S_min is more than a factor of 4 greater than
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dof, this is probably not a good fit. Typically this means
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that the DAC doesn't affect the measureable strictly linearly,
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or there is systematic noise. The latter seems to common in
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E series boards, so I'm not too worried about the following
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dump where there are S_min/dof ratios above 4.
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Here's an example dump, generated by a STATUS_UNKNOWN dump for
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a pci-mio-16xe-10, with the analog output section removed:
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Warning: device not fully calibrated due to insufficient information
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Please send this output to <ds@schleef.org>
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Id: comedi_calibrate.c,v 1.21 2001/10/10 22:07:53 ds Exp
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Driver name: ni_pcimio
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Device name: pci-mio-16xe-10
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Comedi version: 0.7.61
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ai, bipolar zero offset, low gain
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offset -6.795(14)e-3, target 0
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caldac[0] gain=1.26(11)e-7 V/bit S_min=235.659 dof=254
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caldac[2] gain=3.96840(14)e-4 V/bit S_min=1390.18 dof=254
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caldac[3] gain=4.348(11)e-6 V/bit S_min=258.75 dof=254
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caldac[8] gain=5.4659(69)e-7 V/bit S_min=386.361 dof=254
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ai, bipolar zero offset, high gain
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offset -2.4224(55)e-4, target 0
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caldac[0] gain=3.61(45)e-9 V/bit S_min=247.26 dof=254
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caldac[2] gain=3.96644(48)e-6 V/bit S_min=351.927 dof=254
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caldac[3] gain=4.063(46)e-8 V/bit S_min=272.024 dof=254
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caldac[8] gain=5.46305(30)e-7 V/bit S_min=314.035 dof=254
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ai, bipolar voltage reference, low gain
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offset 4.992959(13), target 5
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caldac[0] gain=-4.4928(11)e-5 V/bit S_min=1111.4 dof=254
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caldac[1] gain=-2.792(11)e-6 V/bit S_min=248.971 dof=254
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caldac[2] gain=3.96488(14)e-4 V/bit S_min=1059.18 dof=254
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caldac[3] gain=4.318(11)e-6 V/bit S_min=437.441 dof=254
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caldac[8] gain=5.4810(70)e-7 V/bit S_min=404.213 dof=254
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ai, unipolar zero offset, low gain
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caldac[2] gain=3.96773(39)e-4 V/bit S_min=158.236 dof=107
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[The explanation gets a little fuzzy here]
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The resulting function for calibration will look something like:
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void cal_ni_pci_mio_16xe_10(void)
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postgain_cal(ni_zero_offset_low, ni_zero_offset_high, XXX);
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cal1(ni_zero_offset_high, XXX);
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cal1(ni_reference_low, XXX);
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cal1(ni_unip_offset_low, XXX);
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You get to fill in the XXX's. The post-gain calibration DAC will
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be the one for which the ratio of caldac slopes for the low and
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high gain measurables is similar to the ratio of input ranges for
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low and high gain. This ratio is typically 100 or 200, and really
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should be printed by the program. Thus, for this dump, we choose
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caldac[2], since the ratio is very nearly 100. We don't choose
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caldac[0] or caldac[3], because the gains are smaller, and the
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ratio isn't exactly 100 or 200.
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Next is the pre-gain calibration. Adding a voltage before the
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amplifier will affect every input range selection equally, so the
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pre-gain cadac slope will be nearly equal for both bipolar zero
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offset at low and high gain. In this example, it would be caldac[8].
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Next is the voltage reference calibration. The caldac controlling
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the voltage reference adjustment is proportional to the offset,
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so the correct caldac will typically be the one that has a large
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slope for the bipolar voltage reference measurement, but a small
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slope (by a factor of 2e4, here) for the zero offset measurements.
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It could be any of caldac[0], caldac[1], or caldac[3], or possibly
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all of them. We'll choose the caldac with the largest slope for
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rough calibration, then use the one with the smallest slope for
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fine calibration, namely caldac[0] and caldac[1].
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This is one way that STATUS_SOME is useful, because you can calibrate
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the zero offset, then get a much better idea which other channels
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are likely to be for the voltage reference.
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Note that we haven't done anything with caldac[3]. It clearly
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does something useful, but until we attempt a coarse calibration,
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it's not certain what it does. It turns out to be a fine postgain
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In this example, there doesn't appear to be a caldac that affects
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unipolar zero offset, so it will not be used in the final function:
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void cal_ni_pci_mio_16xe_10(void)
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postgain_cal(ni_zero_offset_low, ni_zero_offset_high, 2);
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cal1(ni_zero_offset_high, 8);
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cal1(ni_reference_low, 0);
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cal1(ni_reference_low, 1);
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There are a number of functions that are useful for optimizing a
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given caldac, each optimized for different cases. The inconsistently
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named postgain_cal() and cal1() measure the observable(s) at a
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number of points throughout the entire caldac range, and then do a
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linear fit to determine the optimum value for caldac. These functions
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are good if the caldac dependence is strictly linear. They are also
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useful if the target value for the observable is at the endpoint of
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the measurable range, as when measuring unipolar zero offset, since
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the functions automatically compensate for bad input values.
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The function cal_fine() is useful for fine-tuning of the results of
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cal1(), especially if the dependence is close, but not quite linear.
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The goodness of the linear fit is quantified by the S_min value in the
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log -- an S_min value that is approximately the same (within a factor
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of 2 or 3) as dof (degrees of freedom) indicates a good fit. An S_min
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value that is about 10 times dof indicates that fine tuning is probably
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necessary. An S_min value that is many orders of magnitude larger than
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dof indicates that linear fitting should not be used.
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The functions cal_binary() and cal_postgain_binary() are used when
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the caldac dependence is highly non-linear. It does a binary search
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in the range of the caldac to find a decent value.
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Notes by fmhess******************************************************
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I would use cal_binary() always, as opposed to cal1() or cal1_fine(),
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since it is the best algorithm.
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cal_relative_binary() is the same as cal_postgain_binary(). I prefer
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the more general name because the function is useful for more than
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just postgain offsets. It adjusts a caldac so the difference between
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two observables is correct (although their absolute values may still
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be offset), which works for postgain offsets, but
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is also good for gain calibrations when the gain adjustment
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couples with the offset.
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cal_linearity_binary() was added for convenient calibration of
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analog output linearity on NI boards. It should be fed 3
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observables that are well separated from each other. It adjusts
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a caldac so that the ratio (obs3 - obs2)/(obs2 - obs1) is
added
removed
comedi_calibrate/README
