AN3322 datasheet(25/38 Pages) STMICROELECTRONICS

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AN3322

Computational issues and compensation algorithms

Figure 13. Error of URMS vs. the timer error in timer ticks

Figure 14. Error of URMS(1)

1. Detail of Figure 13 (zoom in x-axis).

Table 4 shows that the precision of the timer resolution is enough to make coherent

sampling in order to reach the requirements for precision. There is no need for additional

correction or approximation of the samples, unless the future requirements for the higher

accuracy class of the meter appear. The system is always able to synchronize to the

network frequency with the error of the three steps of the timer constants. This leads to the

error of URMS three times 0.004% (according to Table 4) (that means an error URMS value of

0.012%). This is still ten times lower than other influences to the system that can affect

overall precision.

2.3

Timer setting constraints for coherent sampling triggering

The automatic system that is implemented to synchronize sampling with the network

(negative feedback introduced in Section 2.4) sets one parameter. This parameter is the

reload constant of the timer for sampling triggering. Allowed values of this constant should

be limited to an interval avoiding incorrect system behavior in case it is set to a very different

value from the ideal one.

The limits of the timer reload constant are computed from variation of the STM32

microcontroller main system clock and from allowed variation of the network frequency

prescribed by international standards.

Sample time precision (steps of 16-bit timer at the 36 MHz STM32 device)

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