Hello again. I am currently trying to build my first shield for the stm32f103c8t6.
I am planning on implementing an external ADC to allow improved data acquisition (MCP3903).
However, I have a few questions that I need some help with and would appreciate some input.
I am trying to reduce the noise picked up by the external ADC. Are there specific things that
I need to look out for? For example:
a)The ADC MCP3903 has 6 differential signal inputs which I would like to pair with 1x3.3V ,1x5V and
GND each via a micro usb interface. Would the power lines influence the signal lines?
b)To my understanding Analogue and Digital ground should be separated. However, I am unsure
how to do this correctly. From what I have read on forums, many only use a Capacitor connecting
the two. Is that sufficient or is there a better method. It would be nice to maybe get a solution via
an example 
c)The power supply usually carries noise which will be used as external reference used by the ADC.
what would be the best & simplest solution to suppress the noise?
Thank you for your time.
a) I'm not sure I understand the question. There's no 3.3V on USB and it seems a bad idea to use a USB connector for something that isn't within the USB specification for data and/or power.
b) This is not a trivial topic, but there are good application notes from the various vendors for mixed signal layout. e.g. Successful PCB Grounding with Mixed-Signal Chips - Follow the Path of Least Impedance | Analog Devices
c) Depending upon the amount of isolation necessary, some combination of low pass filtering between analog and digital power and perhaps separate regulators may be necessary.
Some ICs, including the MCP3903, have a evaluation board available from the manufacturer and the documentation for these typically include schematics and often circuit board layout details that can be incorporated design. The manufacturer after all has a keen interest in helping designers use their products.
Klagemauer:
Hello again. I am currently trying to build my first shield for the stm32f103c8t6.
I am planning on implementing an external ADC to allow improved data acquisition (MCP3903).
However, I have a few questions that I need some help with and would appreciate some input.
I am trying to reduce the noise picked up by the external ADC. Are there specific things that
I need to look out for? For example:
a)The ADC MCP3903 has 6 differential signal inputs which I would like to pair with 1x3.3V ,1x5V and
GND each via a micro usb interface. Would the power lines influence the signal lines?
b)To my understanding Analogue and Digital ground should be separated. However, I am unsure
how to do this correctly. From what I have read on forums, many only use a Capacitor connecting
the two. Is that sufficient or is there a better method. It would be nice to maybe get a solution via
an example
c)The power supply usually carries noise which will be used as external reference used by the ADC.
what would be the best & simplest solution to suppress the noise?
Thank you for your time.
Sorry for the late reply. You have been a huge help.