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DCR010505: Isolated ground | Should it be left floating?

BOOM
Prodigy 155 points
Part Number: DCR010505

Tool/software:

I am using DCR010505U for isolated 5V output, for isolated transceivers.

There is a lot of ripple on all the digital output lines and 5_ISO line.

Below is the output from an optocoupler output.

This GND_ISO (connected to isolated ground of interfaces) is not connected to any other reference ground in the circuit and is left floating. If not connected, this noise is seen on 5V ISO line. If I connect this GND_ISO to Power ground or to the chassis, the ripples are no more seen. Shouldn't such isolated grounds be left floating?

I have a power ground (GND_P), isolated DC/DC converter that steps down this input to 5V (GND_D). After this I am using DCR module for Isolated power (GND_ISO) and it is picking up noise from ambient. This above snip is when I have just connected the DC jack of power supply to my device input, and the supply is not turned ON. Signal is from the output of an optocoupler When in application, this may get terminated at the other end depending on the application and end user. But for EMC tests and general testing of units, where should I connect this GND_ISO?

  • 0
    TI__Mastermind 33115 points

    I find it not easy to follow your overall system grounding. You mention "power ground" and "chassis" and "DC jack" but none of these points of reference are shown by these names in your block diagram. A few things I do see are that your EMI filter has GND_C that is separate from GND_P and this indicates there is some component in the VIN- return...maybe a CMC? However you show GND_P on both sides of the EMI Filter and this essentially shorts the return leg of the EMI filter. If you are using a CMC in the EMI filter, the return potion is shorted, the common-mode filtering capability is lost.

    Second, I see two stages of isolated DC/DC but I'm not sure why you need double isolation - trying to increase isolation voltage rating from GND_P to GND_ISO might be one reason but then why is GND_ISO floating? is there a load between VP_5_ISO and GND_ISO? 

    Steve

  • 0 in reply to Steven Mappus1
    Prodigy 155 points

    Hii Steve  

    GND_C on EMI filter was just to show the chassis connections on an EMI filter module

    EMI filter is in VIN and GND_P line. GND_P is VIN- return.

    Isolated section of all transceivers is the load between VP_5_ISO and GND_ISO (as shown in the right - opto, RS232)

    DC jack : was an observation on how the noise was observed when the power supply jack is just simply connected to the power cable of device, the supply power is turned ON but output is not ON. Still trying to conclude on what the actual reason behind this behaviour is.

  • 0 in reply to BOOM
    TI__Mastermind 33115 points

    Grounding still; doesn't make sense but if you want to review waveforms or can provide a better schematic, please post?

    Steve