Electronics > Microcontrollers

Should one gate SPI signals to an ADC or DAC, to reduce noise?

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peter-h:
If you have a product where an SPI is used to drive multiple peripherals, the data and clock wires go to every device, and then a /CS selects the device being spoken to.

This distribution of the two signals has the potential to create noise all over the place. Normally, with an ADC or DAC, the SPI transaction has no edges during the conversion (unless one is polling the ADC for EOC, which is sometimes necessary) and that should minimise noise, but you can't do it if driving multiple devices - unless you gate the two signals with the /CS.

Siwastaja:
Gating them is indeed done in some high-precision reference designs, have seen it. I think the easiest way to know what difference it makes is to test with prototypes. Estimating or simulating it would be pretty complicated and it's hard to guess if the added disturbance is at -50dB or -100dB.

Also don't forget the obvious thing of slowing the edges down with series resistance at driver. SPI signals being unidirectional, it's obvious where to put source termination.

thm_w:
Needs context, if its 8 or 12 bit ADC, unless very poorly laid out, and lacks any filtering, I can't see it making much difference.
24-bit ADC that is used to its limits? Sure.

For 24-bit ADC, datasheet will usually tell you to route digital signals away from the analog inputs, and recommend various filter components.

uer166:

--- Quote from: thm_w on April 24, 2025, 11:19:37 pm ---For 24-bit ADC, datasheet will usually tell you to route digital signals away from the analog inputs, and recommend various filter components.

--- End quote ---

A lot of them will also recommend splitting the ground plane into "analog" and "digital", and connect them underneath the ADC. (*ahem* Microchip).


Feel free to completely ignore this nonsense and use actual proven techniques to reduce crosstalk (physical separation, defined return paths, solid low impedance ground plane without slots, PSRR accountability, etc).

Bud:
I had a 24-bit ADC design and found no detectible difference by adding series resistors to the control lines.

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