Dear friends, I am working with stm32f103RBT6 MCU. I have used AMS1117 3.3V regulator both for MCU VDD and also for the VDDA pin. due to noise from GPRS device I need a way to reduce this noise. ADC value is affected about 2% of input value when GPRS is sending or connecting to server. I have a separate power switching regulator for GPRS and have used small signal capacitors as well as ferret beads. I think I should isolate MCU by a DC/DC from power source or use a reference voltage IC for AVDD. Is there any point other than these?
It's possible to get much, much better results with just a band-cap voltage reference and a bit of filtering. But even if you don't use a separate voltage reference you should manage much better than 2% since you do have a separate regulator for the radio module.
Also remember that you need to keep an eye on the ground plane too.
Thanks for your guidance, but what is a "band-cap voltage reference"? Also using scope, I see some small ripples on the VDD pin of MCU in transmitting mode of radio modem, maybe as you said it need a good ground plane.
Oops - spelling error by me. Should be band-gap.
It's a special type of zener diode with a very flat curve and it is commonly used as voltage reference. So you might have a processor with 3.3V supply and combine it with a 2.048V voltage reference making a 10-bit ADC measure 2mV/step and a 12-bit ADC measure 0.5mV/step. But there are many possible voltages available.
It's quite common to try to make a cutout in the ground plane so you basically gets two ground planes joined at a single point. One ground plane for the digital functionality and the other for the analog functionality. Then the analog side will be less affected by high currents and high-frequency spikes on the digital section.
And an advantage if you can make use of a ADC voltage reference lower than the supply voltage is that ripple on the supply voltage will get actively filtered by the line regulation of the voltage reference. Add a simple LCR filter and you can get a very stable voltage reference magnitudes better than what you are seeing now.
Many thanks.
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