MC56F84xxx ADCs Input interface and Random Sampling

[zaidhamed]

Hi,

I want to buy the evaluation board of the microcontroller in the subject matter, but there is some information I cannot infer from the documentation. I am interested in using the simultaneous sampling feature of the two 12-bit cyclic ADCs in a random sampling scheme. As I read from the datasheet it is doable via software trigger or synchronization with another peripheral, I do not think there is a true random number generator on board, is there ? What is the best way to achieve random sampling up to 1 MSPS using this controller?

Secondly, I will be reading voltage signals differentially and I could see that the ADC input supports this, do you recommend that I use an external instrumentation or difference amplifier circuit before inputting the signal to the ADC or the integrated PGAs are good enough ? How fast are the PGAs, can they handle 25 MHz input signals (I intend to use very low sampling rate for fast signals), in other words what is there GBW product at each mentioned gain factor?

Appreciate your help.

Zaid

[xiangjun_rong]

Hi, Zaid,

Regarding your first question that you want to sample the parallel analog channels with a random sampling cycle up on to 1uS, I think it is okay. Of course, the MC56F84xxx does not have hardware random number generator, but you can use firmware to generate a random value. You can use the quadTimer to generate the triggering signal to trigger ADC to sample via crossbar, for example, you can use TA0 to generate the ADC triggering signal.

Pls refer to the section 3.3.3.3 XBAR Interconnections in the MC56F847xxRM.pdf, the XBAR_OUT12 is ADCA_TRIG ADCA (Cyclic ADC) Trigger signal. The TA0_OUT is XBAR_IN36. If you write the code: XBARA_SEL6=36, it is okay, TA) will trigger ADC..

You can use the ADC end of scan interrupt, in the ISR, you can read the two value from ADC_Resultx register, clear ADC flag, generate random value, and update the TMRA_0CMPLD1 register with the random value.

Regarding your second question, the 12-bit cyclic ADC module supports differential input mode and single-ended input mode, regarding the PGA of the ADC, it is a simple PGA, we do not provide the curve between the gain and frequency, but I suppose that it can not amplify the 25MHz signal, 25MHz frequency is too high, in other words, the gain will reduce greatly from the programmed gain.

BR

XiangJun Rong