Kinetis MicrocontrollersのトピックAFE (Analog Front End) KM-Family - Questions

AFE (Analog Front End) KM-Family - Questions

Roland — Fri, 05 Dec 2014 13:11:58 GMT

Hello,

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?

Thank you!

Roland

Re: AFE (Analog Front End) KM-Family - Questions

santiago_lopez — Fri, 05 Dec 2014 23:08:23 GMT

Hi Roland,

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.

Saludos

Re: AFE (Analog Front End) KM-Family - Questions

Roland — Sat, 06 Dec 2014 10:00:38 GMT

Hello Santiago,

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?

Thank you!

Roland

Re: AFE (Analog Front End) KM-Family - Questions

santiago_lopez — Mon, 08 Dec 2014 19:33:49 GMT

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

Where:

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.

Saludos