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  1. #1
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    Measuring voltage from different sources Common ground

    Hi All,
    This is my second attempt to post my question on the forum. My first post did not show up after pressing the post button, so I'm not sure what exactly happen there... It appeared that the posting went through, but did not show up, so I thought it might take some time if it needs approval from the moderator. After waiting more than a day and the posting still not appearing, I will try to reformulate my question.

    I have a design with resistor dividers and low pass RC filters dropping down 0-32V to ADC range of 0-1.8V. With multiple channels, I attempt to measure voltages from different voltage sources on a boat - battery voltage, voltage over the engine temperature sensor, voltage over the oil pressure sensor etc. All sources share common ground, but the problem on fibreglass boats is that all devices and grounded with a wire, not a chassis therefore it's easy to get ground level difference. Prime example is when someone flushes the toilet, the voltage measure on the engine temperature sensor drops slightly down, thus the calibrated temperature value goes up (the higher voltage on the sensor represents lower temperature) and leads to triggering false alarms.
    Here is my current solution:
    Click image for larger version. 

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    How do I resolve the issue? I have been thinking of the following new implementation:
    Click image for larger version. 

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    The Op Amps I have selected are NCV20074DTBR2G https://www.onsemi.com/pub/Collateral/NCS20071-D.PDF
    I'm using 6 ADC channels of STM32 Wroom32U https://www.espressif.com/sites/defa...tasheet_en.pdf

    The requirements I'm trying to meet are:
    • Measure 0-32V DC voltage of 6 to 8 independent channels and display the value 10per second on a separate gauges (my SW already does the display portion).


    • Resolution 0.05V


    • Avoid ground loop measuring issues. Tolerate about +/- 0.5V ground differences.


    • When wires on the inout are reversed by mistake, do not cause short circuit (this is a potential problem with my current solution as it has ground connection on the one of the input connectors)


    • Ideally offset the output of the op amp to avoid the non linear portion of the ADC (downfall of using cheap uC). on the Op Amp the input should be 0-32VDC and the output should be 0.15V to 1.8V to use the almost linear portion of the ADC, otherwise my lower voltage measurement (below 5V) will be questionable and this range is where the most sensor output voltages are.



    I have some doubts of my new differential measurement solution as I'm not sure if using isolated PS for powering the ADC and the OpAmp and having floating ground with regards to the ground I'm measuring is or could be an issue.
    What happens if the wires on the input are reversed and I feed negative to the positive input of the Op Amp and vice a versa.
    How do I protect the input for EMI and other transient Voltages? do I add caps to ground on each input after the input resistors?

    Thank you in advance for your time reading my post!
    Cheers!

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    Re: Measuring voltage from different sources Common ground

    Hi,

    Read about GND star wiring.
    And for sure you need proper wire gauge for lowvoltage drop.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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    Re: Measuring voltage from different sources Common ground

    Hi KlausST,
    Thanks for helping me with that hint. I do understand the basics of the star principal, but in real life is hard to accomplish that. I do have extensive ground plate on my PCB with many vias, to help having the same ground potential at different points on my PCB. I doth think my problem lies there. My issue arrises as different systems on the boat are connected with grounding wire and if large load is applied somewhere, that ground caries current that might create difference in the potential. I have to supply my device with DC power from the bus. That implies that I will have connection to one of these grounds that may float a bit comparing to other grounds, depending on what loads are connected at any particular moment. In order to avoid that and be able to independently measure the voltage on each channel, I have used isolated DC-DC converter to have independent ground and +3.3V providing power to the op amps and the the uC with the ADC.
    Click image for larger version. 

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    Will that work for what I'm trying to achieve?



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    Re: Measuring voltage from different sources Common ground

    Consider improving stability of supply voltage for each low-power circuit (sensor-related). They each need some degree of isolation from noise (voltage droop) on ship-wide supply wires.

    The easiest way is to install resistor-and-zener regulation close to each sensing circuit. The resistor value should not be so high that it causes too great voltage drop at the sensor circuit. A reservoir capacitor is a good idea, with a large enough value to maintain voltage through brief droops.



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    Re: Measuring voltage from different sources Common ground

    thank you for your reply,
    I don't really understand what you are trying to tell me. I have very solid 3.3V and decoupling capacitors on every IC in my circuit. I know that power supplied to the opAmps and the uC is not the cause of the issue.
    I'm piggy-backing from older analog gauges. there is a battery voltage going to the analog gauge and from the other end of the gauge coil to the sensor and them to ground. I'm measuring the signal between ground and sensors gauge connection(usually on the back of the Analog gauge). The voltage varies usually between 1-4V. So if I measure with regular voltmeter the value is stable when loads are turned on/off on the boat. with my setup, I see the value moves with few hundreds millivolts, thus generating jump in the coolant temperature I'm measuring or respectively in the oil pressure.
    What do you mean by " install resistor-and-zener regulation close to each sensing circuit"?



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    Re: Measuring voltage from different sources Common ground

    Quote Originally Posted by JordanS View Post
    What do you mean by " install resistor-and-zener regulation close to each sensing circuit"?
    I pictured that the ship's system voltage drops whenever a pump turns on, etc (when someone flushes the toilet). Voltage drop sounds like a prime cause to throw off sensor readings.

    Since you speak of the sensors having ADC, it implies they send a digital signal to the central unit (not an analog voltage). A digital signal has an advantage of being almost immune to noise. So if the sensor sends an incorrect value, we look for the cause in an analog reading.

    I would not eliminate ground imbalance as a cause. And you state your voltage regulation is good. So I suggest zener regulation as just another possibility to consider. It creates a pocket of stable supply voltage for a device which is disrupted by noise or sags.

    Click image for larger version. 

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    Re: Measuring voltage from different sources Common ground

    A diff amp with a gain of 1.8 / 32 will condition the inputs to be inside the CM range of a good rail to rail op -amp, 1k8 and ( 12K,10k,10k = 32k ) all 1% or 0.2%

    and give you 1v8 out for 32V in,

    - - - Updated - - -

    the diff amps will get rid of the common gnd problem too ...



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