how to damp sim800 gsm noise on adc

[m3r]

hi
i use lm35 temperature sensor and stm32 f030k6 for reading sensor output
when i call sim800 , lm35 output is noisy and incorrect value and When I open the antenna, the noise disappears.
How can I remove this noise?
thanks

 

[KlausST]

Hi,

Iassume it's not the problem of the modem, bt your LM35_to_STM32 circuit problem.
It is called EMC.
Besides the LM35 circuit, there may be other parts of your circuit that can cause the measurement errors:
* non EMC compliant ADC_reference circuit
* non EMC compliant power supply circuit
* maybe even other parts..

You need a proper EMC compliant PCB layout and wiring, maybe you need to install EMC filters = low pass filters.

--> Show us your PCB layout and wiring
--> Show us your schematic

Klaus

 

[m3r]

it's my schematic and pcb

U6-> LM35

i put 10uf and 100nf on lm35 output to gnd

 

[KlausST]

Hi,

You need to read the LM35 datasheet about "heavy capacitive output loading"

Besides this your PCB layout has no GND plane. It is copper pour, which is not much better than single GND wires.
--> use one layer just for GND. If you are not experienced then I recommend to no cut the GND plane and put no other wires into this layer.

In other threads I've used similar PCB layouts of members to show how to analyze GND currents and how to see the copper pour pieces as receiving HF antennas (and transmitting, causing EMI).

Mind: a sinal isn't a single wire from A to B, it always includes the return path in same magnitude.
This means: even if the wire from LM35 is short, you have to take care about the return path. In your case I wouldn't be surprised if the effective return path is 5-10 times longer... and incircles a lot of "antenna area".

Klaus

Added: ... additionally there may be current flow across the STM32 die from one GND pin to the other - difficult to analyze.

 

[FvM]

If this ever works on a two layer PCB, you need to take more care about achieving a continuous ground.

 

[m3r]

If this ever works on a two layer PCB, you need to take more care about achieving a continuous ground.

How can i do this?

 

[KlausST]

Hi,

How can i do this?

Spend more time for the PCB layout.
A nowadays layout is not just connecting signals from A to B.

Make the whole bottom layer GROUND.
Move parts so that wiring becomes more easy.
Place decoupling capacitors next to the IC´s supply pins.
Then place via next to the part pins to connect GND.
Then route the most important signals first, then the less important signals.

Check your layout.

***

In your current layout you several times unnecessarily split the GND plane. Why?
See this:

The red area is GND and the red wire is GND, but the red wire splits the GND plane into pieces.

***
A GND plane is essential with high frequency signals and with high (pulsed) currents. Now in your case the device with the highest power consumption is the SIM800C and it is the device with the highest frequency...but you completely refused to use a GND plane there. A no-go in my eyes.

Don´t confuse yourself with irritating signal names: signal LED1 goes to LED3; signal LED2 goes to LED4.

LM2576 datasheet includes a section: "Layout guidelines". Don´t ignore it. There´s a good reason why they wrote this chapter.


Klaus

 

[FvM]

How can i do this?

Possibly by placing a number of vias and combining top and bottom ground pours intelligently.

 

[m3r]

Hi,


Spend more time for the PCB layout.
A nowadays layout is not just connecting signals from A to B.

Make the whole bottom layer GROUND.
Move parts so that wiring becomes more easy.
Place decoupling capacitors next to the IC´s supply pins.
Then place via next to the part pins to connect GND.
Then route the most important signals first, then the less important signals.

Check your layout.

***

In your current layout you several times unnecessarily split the GND plane. Why?
See this:
View attachment 151435
The red area is GND and the red wire is GND, but the red wire splits the GND plane into pieces.

***
A GND plane is essential with high frequency signals and with high (pulsed) currents. Now in your case the device with the highest power consumption is the SIM800C and it is the device with the highest frequency...but you completely refused to use a GND plane there. A no-go in my eyes.

Don´t confuse yourself with irritating signal names: signal LED1 goes to LED3; signal LED2 goes to LED4.

LM2576 datasheet includes a section: "Layout guidelines". Don´t ignore it. There´s a good reason why they wrote this chapter.


Klaus

thanks a lot
I make these changes and announce the final result.