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ADS1115: what is the FSR of the ADS1115

Kadri Koc
Prodigy 25 points
Part Number: ADS1115

I am fairly new to this concept so bear with me if I made a mistake or am unclear about something.

I am designing a DC kWh meter for my internship. The voltage which has to be measured is going to be 5VDC after a voltage divider and the current that is going to be measured is 20A using a LPSR 15-NP Hall effect sensor.

I was wondering if the ADS1115 is a good choice for my application or if I should use another ADC. I have studied the datasheet but it is unclear to me what the FSR is. How do I determine what my FSR is. My VDD is 5V so do I use the 6.144V or the 4.096V. I need to know this because I want to know what the effect of the Zcm is on the voltage divider. That way I can calculate the voltage that the ADC sees.

I thank you in advance and hope I have informed well.

 

 

  • 0
    TI__Guru* 97755 points
    Kadri,


    I've never used a Hall Effect Sensor, so I'm not entirely sure how this measurement works. However, I presume that the LPSR 15-NP puts out a voltage and you want to measure this voltage with respect to ground.

    The ADS1115 is an ADC that can make both differential and single ended measurements. In the configuration register, you would set the value of MUX[2:0] to select the inputs. and then set PGA[2:0] to set the full scale range. This is basically the gain setting of the ADC. If you expect to measure voltages that are as large as 5V, then you would select the ±6.144V. If the input voltage is less than 4.096V, then you would select ±4.096V as the FSR.

    Note that regardless of the the inputs, the measurement is always output as AINP-AINN. If the AINN is higher than AINP, then this is a negative value. If you select a single-ended measurement, the measurement will still be reported in 2's complement notation, so you only use 15 bits (with only positive values). This is listed on page 26 of the datasheet.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 25 points
    Joseph,

    Thank you for your clear explanation. You are correct regarding the Hall Effect sensor. In the meantime I have written down some specs for my ADC. And was wondering about the amount of ADCs. I need 2 inputs for the voltage divider and the transducer. Do I need 2 ADCs for this or is it possible to use one ADC which has a MUX like the ADS1115.

    Also I wanted to ask if I may give a few specs and if you can tell me if the ADS1115 is good enough or if I should consider another ADC.
    I want an ADC with:
    16 Bit-resolution
    Power supply of 5V
    Digital Interface preference I2C
    Atleast 2 input channels (U,I)
    Architecture preference Delta Sigma, if not then SAR
    Single input voltage 5V
    input single type single-ended

    Thank you in advance for your reply.

    Kadri Koc
  • 0 in reply to Kadri Koc
    TI__Guru* 97755 points

    Kadri,


    In general, I think the ADS1115 is a good part for you. However there are a few concerns that you'll need to consider in selecting the device.

    First, the ADS1115 has a max data rate of 860SPS. In many applications, that's good enough and it's likely fast enough for your Hall effect sensor. Because it has a digital filter, there is a frequency response. The shape of the frequency response is basically a sinc filter as shown in Figure 21. The first zero is at the data rate frequency. Figure 21 shows this for 8SPS.

    Lastly, because the measurement is single-ended, you only use positive values, which means that you won't see negative codes (presuming there's no offset error). This means you're really at a 15-bit measurement instead of a 16-bit measurement. Also, if you have a 5V input, this means that you'll require the ±6.144V input range, which means that you'll be losing a small amount of resolution (any codes representing an input from 5V to 6.144V would be unused if your input doesn't go that high).

    Everything else seems to fit your needs, however it's likely this is still the best device for you. If you have any questions, feel free to post back.


    Joseph Wu