Noise in ADC

[ronandouguet]

Posted on July 08, 2013 at 15:03

Hello,

I work on a project based on Core407V board (STM32F407) and on the DP83848 module (Ethernet). I have developed my own daughter board to plug these two boards and others sensors. The purpose of my project is to acquire analog measurements by using 12-bit ADC in order to send these data at others devices throught Ethernet module (UDP protocol) But I noticed that I have a problem with the DP83848 module: when I plug this module on my board, there is a noisy which is added on my analog measurements. My schematic is the same that the “Open407V-C Standard� board (wvshare). To get an idea about the noise : without DP83848 module : the mean error is 0.003 V with DP83848 module : the mean error is 0.100 V Thank you in advance for your assistance.

[frankmeyer9]

Posted on July 08, 2013 at 18:23

I don't know your schematics, but you could start with decoupling the power supplies.

At about 100mA for a DP83848, I would probably use a separate 5V -> 3.3V regulator.

[ronandouguet]

Posted on July 08, 2013 at 22:02

Thank you for your response.

So, on my daughter board, the DP83848 module is supplied by the 3.3V of the Core407V board. But I tried to power the DP83848 module with an external supply and the problem is the same. In fact, I think that I have no problem with the supply since when I just plug the Vcc and Gnd pins of the DP83848, the ADC is not disturbed. The error occurs when I plug the others pins and more specificaly the RMII_REF_CLK (50 MHz). That is why, I think that is the communication with the clock that disrupts my analog digital converters, but I 'm not sure ?

[frankmeyer9]

Posted on July 09, 2013 at 16:09

I am a software person, with rather mediocre knowledge in advanced circuit design, PCB routing and EMI compatibility.

In fact, I think that I have no problem with the supply since when I just plug the Vcc and Gnd pins of the DP83848, the ADC is not disturbed. More specifically, I mean you could have a problem with the routing & PCB layout of the power supply, especially in regard to AVdd and AGnd. A digital MCU with switched currents will certainly interfere with analog parts of your circuitry - the less optimal the design, the higher the distortions. Look at websites of companies in the analog business, like TI, NatSemi, LT, and others, for recommendations on PCB layout. I've picked up enough knowledge in this field, to leave such things to professionals when professional results are required.