I should measure the maximum amplitude from a 2.5MHz signal - is this possible with a KM-Family µC?
(max. modulator frequency = 6,5MHz, but conversion time approx. 1.5µs?)
What is the max. analog bandwidth?
The sigma delta conversion rate depends mainly of two factors, the Modulator Frequency and the OSR value (Reference Manual Chapter 25.5 Analog Front End -> OSR Select). The sample rate equals to Modulator Frequency / OSR. There are six OSR options 64, 128, 256, 512, 1024 and 2048. The fastest sample rate is 6.5MHz (Highest modulator freq) / 64 (Min OSR) and equals to 101.562 kSPS.
In your case you have a frequency of 2.5MHz which means that you will need at least a 5 GSPS ADC (according with nyquist) to sample your signal. Maybe using some technique (hardware voltage average for example) might help.
thank you for your answer!
What do you think, is it possible to undersampling the signal?
Is the analog bandwidth = 1/conversion_time = 1/1.5µs = 660kHz?
In this case it isn't possible to undersampling, is this correct?
The conversion time also depends on the modulator clock and the OSR value. For single conversion mode or the first sample in continuous conversion mode the conversion time is: AFE_STARTUP + 3 x OSR x MOD_CLK
For the subsequent conversions in continuous conversion mode it is:
OSR x MOD_CLK
AFE_STARTUP = the time required by the sigma delta ADC AFE to initialize (at-least 20 uS)
OSR = Over sampling ratio (64, 128, 256, 512, 1024 or 2048)
MOD_CLK = Modulator clock cycle time (1/MOD_FREQ)
So, considering a MOD_FREQ of 6.5 MHz (max frequency) and the lowest OSR (64) we have a minimum conversion time of 9.8 uS in continuous mode.
The feasibility of the undersampling method will depend on your signal bandwidth. For example, considering a sampling frequency of our sigma delta ADC of 99.01 kSPS, the nyquist frequency will be FS/2 = 49.505 kSPS. The signal you are trying to capture is 2.5 MHz, so it will fit in the middle of the 51th nyquist zone with a bandwidth of +- 49.505 kHz. It means that your signal must be between 2,450.495 kHz and 2,549.505 kHz ideally and considering that you have a very good band pass filter implemented.
Better conversion rates (and hence, bandwidth) can be achieved using the SAR ADC. The maximum conversion rate is 818.33 kSPS (which means a nyquist freq of 409.165 kSPS), but again, it will mainly depend on the bandwidth allowed for your application.
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