Noise on PCB during 2.4GHz RF transmission

[MinhDuc]

Hello friends, 

I am using nrf52840-CKAA for my customize PCB. 

I am facing the noise during RF transmission on my ADC. The peak noise happen when I send a packet of data via RF antenna. 

Power_Tx = 4dBm

 

When I decrease the Tx Power to -12dBm, the peak noise was gone.

I wonder that MCU takes a lot of power during RF transmission so it affect to the stability of power line. 

In nrf52840-CKAA, currently I tie all VSS pin together instead of tie each of them directly to the GND plane. Is it the problem? 

I appreciate all your suggestions. 

Minhduc

[Simonr]

Hi

After discussing this with one of our HW experts, this is normal behavior, as the radio pulls the voltage lower when used which thus affects the ADC.

Best regards,

Simon

[hmolesworth]

Common problem, the nRF VDD sags a lot during BLE transmission which (worse) is often asynchronous to the ADC readings. One fix is to feed the supply for the external ADC via an ideal diode, thus when the nRF VDD sags the external ADC VDD holds up with a more-or-less linear droop for the duration of the nRF VDD sag; this droop is orders of magnitude less than nRF VDD sag so does not have a big impact on ADC readings. A separate regulator which blocks reverse-drain can also be used instead of the ideal diode.

[MinhDuc]

Hello hmolesworth,

Yes I think it makes sense. 

In the case of ideal diode, should I use each diode for each VDD pin of external ADC? 

Best,

[hmolesworth]

I would just common the ADC VDD pins and drive from a single Ideal Diode; it is not clear which VDD pin supplies which subsystem within the ADC die, often one is used for io and others for analogue but it doesn't matter here I don't think. Capacitor should be (say) 47uF 6.3V or (better) 10V or 16V rating. Normally connect EN to Vdd (ie nRF VDD) or leave floating (EN has internal pull-up). Note "It is not necessary (or recommended) to use separate “analog” and “digital” power lines."

VSTIM would perhaps be treated separately if also coming from nRF VDD.

[MinhDuc]

It was very clear. I don't have experience using ideal diode for this purpose. Can you please recommend which one is better for my case? Ideal diode or separate LDO? 

Appreciate your help.