Should I Add Capacitors Per Sensor or Just One Global Filter

[muhammed12]

Hi
In my project, I have a board that includes an ESP32, 4 ADS1115 chips, a SEN55 sensor, and 9 Alphasense gas sensors. All components are powered by a 5V supply coming from another external electronic board. However, there is some noticeable noise on the 5V line."

I know that Alphasense recommends placing capacitors on the ISB line, and ADS1115 recommends adding bypass capacitors on VDD. Is it enough to place a capacitor or filter at the main 5V input only, or can noise also come back from the ESP32 and affect the other components?

Would it be better to isolate the power lines for the ESP32, ADS1115 chips, and the sensors from each other in some way?

I have pull-up resistors on the SDA/SCL lines connected to the ESP32’s 3.3V. Should these be powered from a separate 3.3V regulator, or is it okay to use the same 3.3V from the ESP32 itself?

Should I place individual capacitors (like 100nF or 10µF) for each ADS1115 chip and for each ISB line of the Alphasense sensors? Or is a single filter at the 5V input sufficient?

Lastly, can someone guide me on how to choose the most effective filtering method for power noise reduction? The circuit is very sensitive and reads precise analog signals, so minimizing noise is critical.

[MicroController]

Should I place individual capacitors (like 100nF or 10µF) for each ADS1115 chip and for each ISB line of the Alphasense sensors? Or is a single filter at the 5V input sufficient?

Manufacturers usually recommend/mandate one 100nF decoupling cap for each chip, "as close as possible" to each chip's supply pins. Not following that recommendation is certainly not going to help with reducing noise.

or can noise also come back from the ESP32 and affect the other components?

Yes, definitely. Every (esp. digital) chip in your design can generate noise on its supply lines when operating. If you're about analog signal integrity you should definitely add (at least) the recommended decoupling caps to each chip.

If your power supply isn't 'clean', you may indeed want to add some 'global' filtering to the power line, in addition to the individual chips' caps.

[eriksl]

My $0.02: if you need additional decoupling capacitors may depend on various things.
- the manufacturer will always strongly recommend them
- a part that has no analogue parts may not need a strong decoupling, unless it has to supply some "real" current at times, like a mosfet gate driver
- on the other hand, ADCs and DACs need LOTS of decoupling. I'd add both an electrolytic and a ceramic capactor to each device. You may even need additional measures, like all of them having a decoupling coil or all of them having their own LDO regulator.

As a rule of thumb, if I have a lot of sensors or other chips on a PCB, I add one capacitor every 2-5 devices, depending on their needs. 100 nF ceramic capacitor are next to free and also very small, so really no need to not use them gratitiously.