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msp430f6723 analog and digital module lifts the common ground voltage from 0 to upto 10mvolt

jimmy shah
Prodigy 190 points
Other Parts Discussed in Thread: LM4132, LM334

Hi all I have designed a rtd temperature scanner with two rtd pt 100 sensor which is calibrated using ti lm334 constant current source with lm4132 as Voltage referance to controller  sd24_b Vref (3.3v) and in output side there is a LCD, 4 realys and 1 buzzer.

Problem I am facing is that when I read ADC data it giving good reading but now when i switch on realys or lcd back light it is lifting my whole circuit ground by few milli volts which can spoil my ADC reading.

I have 12v 2A power supply as my main source.

Can any one tell me what causing this problem?

Thanks in Advance

  • 0
    Guru 23000 points
    Then you have a very bad schematic and/or hardware (PCB) routing (layout). The analog ground is not sufficiently isolated from the digital ground and the analog ground running high (digital) current flows.
  • 0 in reply to Leo Bosch
    Prodigy 190 points

    Ya I thought at first same thing but  I am unable to find its real problem which can solve my problem.rtd.pdfyou can check the attached filesrtd._loutpdf.pdf

  • 0 in reply to jimmy shah
    Guru 23000 points
    As already suspected a 'bad' schematic and PCB routing.

    Every wire or PCB trace is an inductor as well as resistor. A 0.25 mm (10 mil) wide PCB trace equates to ~1.9 mOhm/mm.
    Therefore, you should avoid flow through the analog ground lines by keeping the analog ground separately from other consumers. Inside the processor this (mostly) done by AVss and DVSS to separate. Create a Star point between Ground-in and AVss and DVss and other consumers (ground-out), preferably close to the MCU (ADC part) AVss and DVss connections.

    Decoupling capacitors (0.1uF) should preferably be placed on top of the device (MCU) pins, but at least very close and direct to the appropriate Vcc and Vss pad. Prevent connections "through" the Vcc and Vss pad.

    You can save your PCB likely, by the addition of a separate, solid (0.5-1qmm), wire connections.
    Print your PCB layout where you have the ground net highlighted.

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