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Original question:

ADS1115: Setup and questions about negative voltage reading and buffering

ADS1115: Measuring negative voltages with ADS1115 in differential mode

Venkatramayya Yalamati
Prodigy 20 points
Part Number: ADS1115
Other Parts Discussed in Thread: OPA333

I want to measure battery set cell voltages using Arduino & ADS1115 in differential measurement mode. The user may put the probes across the battery cells either in positive/negative polarities hence, can I use the following circuit for interfacing with ADS1115? Plz suggest me if any better ideas & also the best op-amp model... Thanks in advance

  • 0
    TI__Expert 4800 points

    Hello,

    Another solution might be to use diodes to ensure the current can only flow one direction through the circuit. This might introduce some errors due to the voltage drop across the diodes:

    A drawback to the solution you posted is that the op amp will be driving the output of the probe, potentially creating issues if the probe was carelessly shorted to the circuit.

    You may also wish to add a buffer for the input of the ADS1115 after the voltage divider depending on how accurate your measurements are.

    We recommend using the OPA333 for some applications in the datasheet, but I would recommend posting a question to the op-amps forum as you will get better recommendations for your specific application.

    Let me know if you have any more questions for this device!

    Levi DeVries

  • 0 in reply to Levi Devries
    Prodigy 20 points

    Hi Levi DeVries... Thanks for the attention to my query. In my application, I actually wanted to measure voltage across battery cells in our station which do NOT share common ground with my circuit of ADS1115.. So, without using all this op-amp setup, can I directly measure the floating battery cell voltages (whose voltage is in the order of 1.2V to 1.7V) across the differential inputs of ADS1115 in both polarities? My circuit (of ADS1115) is supplied from 18650 battery of 4.1V to 3.0V

  • 0 in reply to Venkatramayya Yalamati
    TI__Expert 4800 points

    Hi,

    Venkatramayya Yalamati said:
    can I directly measure the floating battery cell voltages

    The voltages you can measure depend on the input voltage into the device and the programmed full-scale range. In your case, the voltages on each pin must remain below 3V and above 0V with respect to the device's ground reference. You couldn't measure this voltage directly without using diodes to ensure the polarity of the cell voltages like I have demonstrated above. Since the voltages you are trying to measure are less than the full-scale range of the device, you could also clamp the voltage inputs of the device so that the floating voltage cells are kept within range for the device.

    You can't directly measure the voltage bidirectionally because the highest voltage you are trying to measure is larger than half the lowest value of your supply voltage (3V/2 + 1.7V > 3V).

    Let me know if you have any more questions,

    Levi DeVries

  • +1
    Prodigy 20 points

    Below circuit (from TI forum, link: ADS1115: Voltage Divider for differential voltage - Data converters forum - Data converters - TI E2E support forums ) worked very well with ADS1115 + Arduino for bi-directional voltage measurement !! (Vcc can be 3 to 5V) It is successfully measuring as low as +/- 2V range voltages (with the same 1:200 voltage divider) with ads.setGain(GAIN_SIXTEEN) and also I able to measure higher voltages above +/- 500 V by keeping the programmable gain of ADS1115 at ads.setGain(GAIN_TWOTHIRDS)) or ads.setGain(GAIN_ONE))