VXI VT1422A - Manual (Page 492)

490 Generating User Defined Functions Appendix F
Table F-1. 'C' Sin(x) Vs. VT1422A Haversine Function
Limitations
As stated earlier, there are limitations to using this custom function
technique. These limitations are directly proportional to the non-linearity
of the desired waveform. For example, suppose the function X*X*X over a
range of ±1000 is to represented. The resulting binary range would be ±1024
and the segments would be partitioned at 1024/64 intervals.
This means that every 16 units would yield an Mx+B calculation over that
segment. As long as the numbers input are VERY close to those cardinal
points, good results will be attained. Strictly speaking, "perfect" results will
only be attained if calculated at the cardinal points, which may be reasonable
for an application if the input values are limited to those exact 128 points.
The waveform may also be shifted anywhere along the X-axis and
Build_table() will provide the necessary offset calculations to generate the
proper table. Be aware too that shifting the table out to greater magnitudes
of X may also impact the precision of the results dependent upon the
linearity of the waveform used. Suffice it to say that the best results will be
attained and it will be easiest to grasp what is being done if the center stays
near the X=0 point since most of the results of the measurements will have
1e-6..16 values for volts.
Truncation errors may be seen in the fourth digit of the results. This is
because only 15 bits of the input value is sent to the function. This occurs
because the same technique used for Custom EU conversion is used here and
the method assumes input values are from the 16 bit A/D (15 bits = sign bit).
This is evident in Table F-1 where the first and last entries return
±0.9999
rather than ±1. For most applications, this accuracy should be more
than adequate.
'C' sin(-1.570798) -1.000000 'VT1422A' sin(-1.570798) -0.999905
'C' sin(-1.256639) -0.951057 'VT1422A' sin(-1.256639) -0.950965
'C' sin(-0.942479) -0.809018 'VT1422A' sin(-0.942479) -0.808944
'C' sin(-0.628319) -0.587786 'VT1422A' sin(-0.628319) -0.587740
'C' sin(-0.314160) -0.309017 'VT1422A' sin(-0.314160) -0.308998
'C' sin(0.000000) 0.000000 'VT1422A' sin(0.000000) 0.000000
'C' sin(0.314160) 0.309017 'VT1422A' sin(0.314160) 0.308998
'C' sin(0.628319) 0.587786 'VT1422A' sin(0.628319) 0.587740
'C' sin(0.942479) 0.809018 'VT1422A' sin(0.942479) 0.808944
'C' sin(1.256639) 0.951057 'VT1422A' sin(1.256639) 0.950965
'C' sin(1.570798) 1.000000 'VT1422A' sin(1.570798) 0.999905