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MSP430F6779: EMDC Metering Accuracy and Sampling Trade-Offs

Part Number: MSP430F6779
Other Parts Discussed in Thread: MSP430I2041

When setting up projects in EMDC, you must pick the "Sampling Frequency (Hz)" and "Oversampling Rate (OSR)".

In my case, I needed to reduce the sampling frequency because I needed more CPU time for my application.

What is the impact of reducing the sampling frequency in EMDC?

Can TI provide any literature on this topic, regarding the trade-offs between the Sampling Frequency, Oversampling Rate, and metering accuracy/precision?

  • Hello,

    Reducing the sampling frequency is usually done by increasing the oversampling rate (OSR). The MSP430F6779A devices do allow both the modulation frequency and the OSR to be adjusted whereas some devices like MSP430i2041 have a fixed modulation frequency. Increasing the OSR typically increases the resolution of the Sigma-Delta ADC which should translate into better metering accuracy. This resolution is commonly referred to as the effective number of bits (ENOB).

    One caveat of reducing the sampling frequency is that it lowers the maximum input frequency that can be measured without aliasing. For example, if you need to capture a 2kHz signal, you'd need a sampling frequency of at least 4 kHz. If you lower that to 2 kHz, you can only capture signals up to 1kHz.

    I would recommend two resources. First, check out Figure 5-17 in the MSP430F6779A datasheet shows the relationship between SINAD (another way to represent ENOB) and OSR. Second, check out the Answers to Common Sigma-Delta ADC Questions on MSP MCUs app note.

    Regards,

    James