6 Replies Latest reply on Sep 13, 2012 3:04 PM by DaKn_263916

    Mixed signal PCB layout


       I am developing a PCB for an analogue sensing application. It uses the 12-bit Delta Sigma ADC on a PSoC3. As usual, the application is very space constrained (11mm x 21mm), so I have had to make some compromises in the PCB layout which I would not have done on a larger PCB.


      The board is supplied by regulated 6v, and contains two 5v linear regulators. An MCP1702 for the digital supply, and an MIC5205 for the analogue supply. The board is sensing five A1324 Hall effect sensors. Each Hall effect output signal is filtered by a 100nF + 1k RC filter. One sensor is on the PCB itself (bottom right). The other 4 plug into the right hand 6-pin connector.

      The chip is acting as an SPI slave, but ADC samples are always taken between SPI transactions, so the SPI should not interfere with the analogue signals.

      Sadly, I am still seeing some noise (about 1.5 LSB at 12-bits) on the analogue signals, and I wonder if there is anything I could have done differently (in the layout or otherwise) to improve it.


      Other PCB designs I have done using the MCP2308, and the same dual 5v supplies, same sensors, and same RC filters have achieved no noticeable noise at 12 bits.

      I would really like to push it towards 12 ENOB. The reason is not accuracy, but velocity measurement. Currently this level of noise is making it impossible to do accurate position and velocity control on a robot.