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ADS1115: ads1115

Part Number: ADS1115
Other Parts Discussed in Thread: INA139

Hi,

I've two question, its connect eachother. Firstly; I'm tried to use ads1115 module to obtain of geophysical data from the ground. The data  we want to measure is the ground voltage. Level of ground voltage is about to milivolts.Because of that we should have precision measurement. And  our measurements with ads1115 is not stable. we cant measure the real values of ground voltages. Normally ads1115 is a very very stable to measure voltages which is milivolts  but if you want to measure ground voltages,it doesnt work correctly. Why we can not  measure  ground voltages with ads1115 correctly? 

And the second one is; While shunt resistors measurement, ads1115 differantial voltage measurement results not stable. If current over 100 mA, ads1115 raw data values is not stable and the all of measurement goes to false. what is the reason of that ?

And how wwe can fix the problems? Like using capacitors or resistors to connect ads1115 analog pins?

Thank you.

  • Metin,



    I need more information about your circuit to help you. In general, I think that the ADS1115 is capable of making good measurements near ground, and should easily make shunt resistor measurements.

    Are you able to share a schematic? Even a simplified schematic would help me understand your setup. Also, if you are having problems with noisy data or unstable data, adding some sort of RC filter may help. However, can you post your output data? The raw data from the ADC may give clues to what the problem is.

    Regardless, more information about your system, what you are measuring, and the results you are getting may help in debugging the circuit.



    Joseph Wu

  • Hi Joseph, thank you for interest.

    We use one arduino mega and two different ads1115 module. first ads1115 use to measure ground voltages and the second one for the measure shunt resistor. two of ads1115, use to differantial measurement mode with analog read pins ao and a1. first ads1115's analog pins which ao and a1 is directly connect to the ground with two electrodes. we try to measure directly ground voltages.

    And the second ads1115's analog pins which a0 and a1, connect to shunt resıstors and try to measure voltage reduction of shunt resistors.

    Actually, our circuit is very simple; one of ads1115 is try to measure ground voltages and it has no other different circuit. i guess, the ground voltages has a noise and this noise try to disrupt our datas. how can i fix that problem?

    And the second ads1115 is try to measure shunt resistor which connect to dc step up converter's exit. the dc step up converter exit can from 1.5 volts to 30 volts. we can set to exit voltages with potentiometer. and we measure current of the first ten volts with no problems with ads1115. but if we want to raise voltages from ten volts to thirty volts ads1115's raw data goes not stable actually raw data goes to very very high values and its not reasonable values. and calculated current goes hundreds of amps. its impossible. the voltage values arround to 30 volts, ads1115's raw datas is goes broken and no stable datas comes from ads's.
    i can't understant what is happening to ads1115 raw datas? because ads1115 working below the ten volts and measured very stable with shunt resistor, but if over the ten volts measurement goes crazy..

    thank you for now
  • Metin,


    I'm having a bit of a hard time understanding how you have your system set up. I think it would be easier if you put together a basic schematic and showed the connection between the devices and the ADCs with the arduino controllers. When you go to post a response, in the bottom right hand corner, there's a link to "Insert Code, Attach Files and more". Using that link you can get a better interface where you can insert pictures to show the schematic.

    First, I'm a bit unsure about what you mean by measuring the ground. With respect to what voltage are you measuring the ground? Normally, the ADC would have a common ground to the measurement board. Also, the input voltage should always be within the VDD and GND of the ADC. The input voltage should not be floating with respect to the ADC ground potential. A diagram of your connection to the ADC would be very helpful in describing this.

    Additionally, your measurement of the shunt voltage is also a concern. If this is a high side shunt measurement, measuring the source of the step-up converter, then this might be a problem. If the ADC and the board have a common ground, then the ADC input cannot be at 30V. This is well beyond the absolute input range of the ADC. Again, the analog inputs must be between VDD and GND of the ADC. The only way to make a shunt measurement would be to use a low-side shunt measurment, with a common ground between the ADC and the board you want to measure.


    Joseph Wu
  • Joseph,

    I've add the circuit drawings. It seems complicated because of my drawing skill but they are simple.
    By the way i told you that we measure ground voltages actually it's soil voltages. we try to measure of soil resistvity. like this, we use two current electrodes to give current to soil, and we use another two electorodes for measuring soil voltages. if you give current to soil, you create a voltage in the soil and you can measure soil voltages. So, we need value of current what we give to soil and we need value of soil voltages too.

    if we Go back to the circuit,


    First device is for shunt measurement and it supply energy from dc-dc step down converter and the converter exit voltage is 5 volt. one of ground connections of the converter is connect to arduino's ground. we use ao and a1 pins for measuring shunt resistors. of course it's a differantial measuring modes of ads1115. This is how i measure of shunt.

    We use the second device for measure soil voltage. this device supply energy from arduino 5 volts vcc pins. And device's gorund connect to arduino's gorund pins. We use ao and a1 pins for differantial measurement of soil voltages.

    By the way the arduino's supply voltages come from 12 v battery just like dc-dc step down converter's.
  • Metin,



    I'm sorry - I thought I had sent a response for your last post but I guess it didn't post.

    I mentioned this in the last post, but my concern about your schematic is that there's no common ground potential through all of your devices. Also, because the the driving voltage from the 1.3V to 30V, your input voltage could be far out of the input range of the ADC, possibly damaging the device.

    For the first point, the ADC and the input circuit should have some common ground. Right now, the ADCs and the 12V battery (to the step-down 5V converter) have the same ground. However, the other 12V battery for the step-up converter tries to measure a voltage that is as high as 30V based on the high side shunt measurement. In that you should probabaly have a common ground between the supplies.

    For the second point, you have voltages that are as high as 30V. If either input voltage is near 30V, you may damage the device. For the current shunt measurement, I would use some sort of high-side current shunt measurement, where the input voltage can be high, but the output voltage would be within the ADC input range. Maybe you could use an INA139 as a high-side measurement or use a low side measurement.

    You may also need to add some series resistance to the the analog inputs in case of an overvoltage event. This would reduce any input current going into the device and possibly protect it from damage. I would go as high as a 10kΩ resistor.

    I don't really have any specific insights into your application. You'll have to test this configuration just to make sure there aren't any other things that affect the accuracy of this system.



    Joseph Wu
  • Joseph,

    I can't understand why they should have common ground.

    Becausei If i want to measure 1.5 Volt battery, i use a0 and a1 pins with differantial measuring mode. there is no common ground but i can measure perfectly. i mean, if firstly remove items and (12 v battery and dc step up conveter and shunt resistor) after that connect 1.5 volt battery to a0 and a1 pins; you can measure 1.5 volt battery and no common ground. i have added
    the figures of circuits. (figure 1, belong to 1.5 volt battery measuring)

    Or i want to measure this circuit' shunt, the circuit have 12 v battery, 1.3 to 30 v step up converter, 47 ohm resistor and the shunt. (figure 2 belong this circuit) Ads1115's a0 and a1 pins perfactly measuring the voltage over the shunt. Again there is no common ground my ads1115's and the circuit. But ads1115 measure perfectly.

    Bu if i want to measure circuit and the circuit connect to soil just like in my old post, i can't measure of shunt voltages.

    So what is the diffence? Why i can measure one of them and why can't measure other of them?

  • Metin,



    The ADC can only measure inputs that are within the ADC supplies. For example, if the supplies are 0V to 5V, then any analog input cannot be outside this range (like 6V or -1V).

    The reason that you can measure the inputs while floating is that with these inputs, the voltage source can inherit the ADC's common-mode voltage to make a measurement. There is no common voltage between the input source and the ADC power and it doesn't require any power to force the input and the ADC to share a common input voltage.

    However, the problem comes because when you try to measure the shunt resistor used for driving the source current electrode which is at a high potential, and then the soil electrodes which may be at a low potential. Because those two input voltages do have a common voltage which may be separated by many voltages, there will be current flowing between these input voltage.

    For the two ADCs, they must be at the same ground. If you are running two Arduinos, then they have a common ground voltage that connect to the USB ground. Because of this, the inputs must also be within the 0V (which is the USB ground) to +5V (which is supplied by the Arduino). If the inputs are outside of this input range, the input multiplexer won't be able to transfer input charge, and there is risk of damaging the inputs.

    In your setup, the soil electrode measurements could be as low as 0V with respect to the driving voltage, and the shunt current measurement could be as high as 30V with the step-up converter. The ADC won't be able to make a measurement through this entire range.

    As I mentioned previously, you may need to use some sort of input conditioning so that you can measure the input through the entire range. For the INA139, you could measure that high a voltage using a low voltage supply. This could be used for the shunt current measurement. For the soil electrode voltage, I'm not sure what those voltages will be, but they will certainly be somewhere between 0V and 30V. If those voltages are below 5V, then you could measure them directly.



    Joseph Wu