Tying SPI inputs together to run several ADCs at once

[Ajaho]

Dear Community,

I'm working on STM32H743ZIT6 based project.

Here I want to set up 4 ADCs which have SPI interface.

On developer's forum is said "If all ADCs will be sampled at the same point in time with the same settings, then the SCLK, CS, and SDI can be tied together".

Also there mentioned: "Make sure the digital interface is able to drive all of the ADCs at once".

Is STM32H743ZIT6 able to drive all of the ADCs at once?

According to electrical characteristics:

Digital high input voltage logic level - 0.7V x DVDD (min) / DVDD + 0.3V (max);

Digital low input voltage logic level - -0.3 (min) / 0.2 x DVDD (max).

DVDD = 3.3V

Thanks!

[AScha.3]

Hi,

>Is STM32H743ZIT6 able to drive all of the ADCs at once?

Which ADC ? + SPI clk at ... ?

+

And where going the 4 SDO with data to ?

[Ajaho]

Hi,

https://www.ti.com/lit/ds/symlink/ads8685.pdf

https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/612190/ads8685-shared-spi-interface

"separate SDO lines from each ADC"

[Michal Dudka]

It is OK to drive multiple ADS8685 by one set of SCK/SDI/CS lines... but what is your question ?

[AScha.3]

Hi,

as Michael said, driving 4 ADS8685 is no problem,

so >Is STM32H743ZIT6 able to drive all of the ADCs at once? < yes,

but :

- where / how you get the 4 SDO lines ? 4x SPI using for this ?

- and why using 4 x 16bit ADC chips, the H743ZIT has 16bit ADCs with 28 possible inputs...?

[Ajaho]

Yes, 4x SPI.

In my current configuration I used 4 SPI already, but all ADCs are driven traditionally - separately.

But this solution is very helpful and needed.

Internal ADCs are not ok for my project, not only because I need 4, but MCU offers 3, but also by architecture specifics.

[AScha.3]

Ok, if you think, the 4 ADCs performing better than internal ADC ...

you could drive 1 SPI as master , send+receive, and 3 SPI as slaves, receive only,

connecting the spi clk from master spi. Should work.

[Michal Dudka]

---

@AScha.3 wrote:

- and why using 4 x 16bit ADC chips, the H743ZIT has 16bit ADCs with 28 possible inputs...?

---

Technical note. 16bits on H743 is "virtual" (most parameters according to about 12 or 13bit ADC). While ADS8685 is true 16bit with pretty features (wide true bipolar input range, selectable gain, stable reference, overvoltage protection...).

[AScha.3]

Ok, i see. But no free lunch ... 2 of them about same price as the H743 . 🙂

[Ajaho]

Thank you, I did it. Should slave SCK, which still showed as active be unconnected/floating?