Operation
40
F-FEM-CNT — User’s Guide
4.2 Parameterization
4.3 Specifications
4.3.1
Accuracy Specification
The measuring time influences the accuracy of the speed acquisition in the
following ways:
With a measuring time of 1 ms and a quartz frequency of 20 MHz (accuracy
25 ppm), the quantization error is 50 ns. This corresponds to a quantization error
of 50ns/1ms = 50 ppm or an accuracy of 14.29 bit. In this case, the counter
counts up to 20000, which requires a 15-bit width (log
2
[20000] = 14.29 bit).
With a measuring time of 2 ms, the quantization error is also 50 ns. However,
this corresponds to a quantization error of 50ns/2ms = 25 ppm or an accuracy of
15.29 bit, etc.
This means that the quantization error decreases as measuring times are
increased.
The measurement error that is caused by inaccuracies of the reference
frequency remains constant. The total error is determined by the reference error
plus the quantization error. The total error tends to come closer to the reference
error if measuring times are very long.
Information
For further information please refer to the F-FEM-Parameterization Manual, see
Secondary literature on page 11.
Information
From PUMA Open version 1.5 onward, it is checked, whether the input values
are within the specified range. You find the valid ranges in the F-FEM Parameter-
ization manual.
Make sure that the adjustment- and calibration points are within the valid range!
NOTICE
All specifications are for an ambient temperature between 0 and 60 °C and a
supply voltage of 24 V +
20 % unless otherwise noted.
Quantity Value Comment
Basic accuracy of reference clock ± 25 ppm at temperatures from 0 to 60 °C
Resolution (reference clock) 50 ns at 20 MHz
Counter width 16 bit hardware extended to 32 bit by software
Result width 32 bit floating point according to IEEE notation
Tab. 4
Total Error = Reference Error + Quantization Error
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