Specifications

Section 3. CR3000 Measurement Details
synchronized with real time and restarted. The CR3000 actually measures the
elapsed time since the last time the counters were read when determining
frequency so in the case of an overrun, the correct frequency will still be
output.
The resolution of the pulse counters is one count. The resolution of the
calculated frequency depends on the scan interval: frequency resolution =
1/scan interval (e.g., a pulse count in a 1 second scan has a frequency
resolution of 1 Hz, a 0.5 second scan gives a resolution of 2 Hz, and a 10 ms
scan gives a resolution of 100 Hz). The resultant measurement will bounce
around by the resolution. For example, if you are scanning a 2.5 Hz input once
a second, in some intervals there will be 2 counts and in some 3 as shown in
figure 3.7-1. If the pulse measurement is averaged, the correct value will be
the result.
3 2 3 2
FIGURE 3.7-1. Varying counts within Pulse interval.
The resolution gets much worse with the shorter intervals used with higher
speed measurements. As an example, assume that engine RPM is being
measured from a signal that outputs 30 pulses per revolution. At 2000 RPM,
the signal has a frequency of 1000 Hz (2000 RPMx(1 min/60 s)x30=1000).
The multiplier to convert from frequency to RPM is 2 RPM/Hz (1 RPM/(30
pulses/60s) = 2). At a 1 second scan interval, the resolution is 2 RPM.
However, if the scan interval were 10 ms, the resolution would be 200 RPM.
At the 10 ms scan, if every thing was perfect, each interval there would be 10
counts. However, a slight variation in the frequency might cause 9 counts
within one interval and 11 in the next, causing the result to vary between 1800
and 2200 RPM!
3.8 Self Calibration
The CR3000 performs a self-calibration of the analog voltage measurements
and excitation voltages. The range gains and offsets and the excitation voltage
output will vary with temperature. The self calibration allows the CR3000 to
maintain its specifications over the temperature range.
Rather than make all of the measurements required to calibrate all
range/integration type combinations possible in the CR3000, the calibration
only measures the range/integration type combinations that occur in the
running CR3000 program. The calibration may occur in three different modes.
1. Compile time calibration. This occurs prior to running the program and
calibrates all integration/range combinations needed. For the 250 usc
integration multiple measurements are made and averaged to come up
with gain values to use in the measurement. Five measurements for the
250 usec integrations. When this calibration is performed the values in
the calibration table are completely replaced (i.e., no filtering is used).
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