Multiplexer Ground Analog or Digital?

Discussion in 'General Electronics Chat' started by pra_kod, Jul 27, 2009.

  1. pra_kod

    Thread Starter New Member

    Jul 27, 2009
    Hi all,
    I am using Aduc848 Microcontroller for Data Logging application for logging data of 20 series connected 20V batteries. In this system I am facing problems of ADC accuracy, offsets,noise in readings and Controller Reset problem which occurs only when batteries are discharged. I have designed a circuit in which I used a common single +5v supply for pins AVDD & DVDD of Aduc848. These are analog & digital power supply pins of Aduc848 respectively.I made separate ground for analog section (AGND) and digital section(DGND). Both the grounds are shorted at only one point near +5v power supply.
    So signal conditioning circuit,buffer,AGND pin of Aduc848 will have analog ground connected. I want to use an analog Multiplexer (MUX) to increase channels. Multiplexer has analog i/p channels having AGND & channel selection lines A0,A1,A2 which are connected to digital o/p ports of Aduc848 having DGND. MUX IC has V=,V- & GND pins.
    So circuit is like this:
    Voltage divider circuit using Resistors (Attenuator)-->Buffer and filter--->Multiplexer(MUX)--->Buffer and filter-->AIN1(Analog i/p channel of Aduc848.

    So in this link I am sure that all except MUX will have AGND. I want suggestion about MUX GND.

    MUX channels are selected by Aduc848 port pins P2.0,P2.1,P2.3 which are configured as Digital o/p ports.

    1)Which ground should I connect to MUX GND? AGND or DGND?
    2) Shall I use separate +5v supply for AVDD and DVDD pins of Aduc848?
    3) How to prevent Resetting of Aduc848 when batteries are in Discharge mode?

  2. AdrianN

    Active Member

    Apr 27, 2009
    The MUX has to be on the Analog Ground. This is what I would do. The GND pin of the MUX is connected to the Analog Ground. There is no problem with the digital signals A0, A1 an A2 crossing boundaries, because they are static signals. You only set them once in a while (I would assume). If they are dynamic, changing the MUX address at high speed then, yes, there is a problem. But I don't think this is the case.

    I would split the 5V supply in two as well. Have a lot of filtering (ferrite beads, inductors and ceramic caps and tantalum caps) on the analog +5V supply. I advise you to do this because you want to keep the currents for analog and digital circuits separated. Separating just the ground would not do it.

    The Analog and Digital Grounds need to meet at the ADC level. Shape the ground plane and the power plane so that they split the ADC signals and there is no crossing between the two realms.