ADS131M08: ADS131M08: Synchronising multiple ADC

Hi Pradeep,

Pradeep Raaj said:

1. In datasheet the clock frequency is mentioned as 8.192Mhz (nominal). Is it okay to clock in the XTAL1 pin with 8MHz crystal? As I read in TI forum that the Clock frequency has to be in phase and synchronous with the SCLK. Considering using of any microcontroller the CPU clock frequency will be 2,8,16,32 or 64MHz and it was possible to create master clock using microcontroller in the fraction of same.

For a small clarification, SCLK in phase with CLKIN will give the best performance but if CLKIN is not synchronous with SCLK, you won't get bad performance, just not the best.

However, because of the way our synchronization algorithm works, the fact that you're using multiple devices, and the recommendations in the datasheet (below), I highly recommend, as you said, getting a MCU with a local oscillator that can fan out to all the devices so everything is synchronized. The short of it, is if you choose to have a local crystal oscillator at every separate ADC, the jitter, drift, and exact frequency is unknown so my previous "synchronized within 1 t_CLKIN period", will not hold true and you'll have to do some tolerance calculations on the crystal and determine the typical error between conversions for your system.

Also, to answer your question, yes 8MHz (as opposed to 8.192MHz) is fine to use and within the clock specifications for each of the modes of the device, as shown below. The problem is that the output data-rate, bandwidth, and f_MOD will scales with the CLKIN so you'll just have to recalculate what these important values will be. I've already sent the SAR vs. Delta-sigma TIPL video on the other post so you can use that as a reference if you need a refresher. 8.192MHz is an industry standard and helps with other base 2 type calculations on a MCU so keep that in mind.

Pradeep Raaj said:

2.Considering a XTAL1 is clocked in using a clock source using external oscillator and not using microcontroller ( based on previous answer, 8MHz or 8.192MHz ) , what are the possible ways to achieve this ?Two oscillators for two ADC or one oscillator for two ADC. In this case how to synchronise with the microcontroller?

I think I've already answered this above but feel free to clarify if I did not.

Pradeep Raaj said:

3. Suppose in case a single DAQ board has to be synchronised with other DAQ board. How can it be achieved making sure that all are synchronised? Kindly share some references for it.

I looked around and I haven't found any specifically for DAQ or anything like that. I did find an app note, which is for a different part number, but really talks about the tips and tricks of sharing clocks : https://www.ti.com/lit/pdf/sbaa520 

Something tells me a clock buffer is going to be your answer. I don't know how your boards are stacked up but the way I would do it, is MCU board: crystal oscillator -> PLL of MCU -> MCU CLKOUT to clock buffers if they're far enough away -> feed to all ADC and DAQ boards and systems.

Best,

-Cole