• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 47 subscribers
  • Views 819 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1118: Single-ended input FSR

Shridhar Ambady
Prodigy 20 points

Part Number: ADS1118
Other Parts Discussed in Thread: ADS1113

The MCU we're using has very poor gain drift with temperature, and we're evaluating the ADS1118 for use in bringing in 4-20mA and 1-5V signals. I want to make sure I understand correctly how the single-ended operation and FSR works. Can someone please verify that these three statements are accurate?

1) If I have a single-ended signal going into AIN0, and the signal goes from 0-1V, I would need to set the FSR to 2.048V to actually get a 1.024V range

2) If I have a Vdd of 3.3V, and I set the FSR to 6.144V, I will be able to read single-ended signals on AIN0 up to 3.072V.

3) If I have a single-ended signal going into AIN0, and the signal goes from 0-1V, I can tie AIN3 to GND and set the FSR to 1.024V to get the full range.

Is this all right, am I reading the datasheet correctly, or am I mistaken in my thinking?

  • 0
    TI__Guru 58921 points

    Hi Shridhar,

    Here are the answers to your questions:

    1. The absolute input voltage on each ADS1118 analog input pin (AIN0 through AIN3) can extend from GND to VDD. This is shown in Table 7.3 in the ADS1113 datasheet. So if you are measuring between AIN0 and GND, and the input you apply to AIN0 ranges from GND to 1V, then you can certainly use FSR = 1.024V for this specific set of input voltages
    2. As stated in #1, the absolute input voltage on each ADS1118 analog input pin (AIN0 through AIN3) can extend from GND to VDD, so if VDD = 3.3V then the maximum input voltage you can apply to AIN0 is also 3.3V
    3. I am not sure I understand why this is different from #1, or why the FSR would be different.

    Also, note that the FSR is related to differential input voltage read by the ADC (that is V(diff) = AINP - AINN). So if the voltage you apply to AIN0 = 4V, the voltage you apply to AIN1 = 3V, and you are measuring between AIN0 and AIN1, then V(diff) = 4V - 3V = 1V and FSR can be +/-1.024V.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 20 points

    Oh I see, I think I misunderstood the datasheet. The 16-bit resolution is for +-FSR, in two's complement, so a 1V signal can still be fully read by setting FSR to 1.024V, but because I'm only using the positive range, I'm really only getting 15-bit resolution.

    If I wanted to get the full 16-bit resolution for a 0-1V signal, could I feed a stable 500mV into AIN3, configure the ADC to set AIN3 as the reference, and then use an FSR of +-512mV? That way AIN0 is 500mV above or below AIN3 and thus we're able to achieve 16-bit resolution, right?

  • +1 in reply to Shridhar Ambady
    TI__Guru 58921 points

    Hi Shridhar

    Shridhar Ambady said:
    Oh I see, I think I misunderstood the datasheet. The 16-bit resolution is for +-FSR, in two's complement, so a 1V signal can still be fully read by setting FSR to 1.024V, but because I'm only using the positive range, I'm really only getting 15-bit resolution.

    Correct

    Shridhar Ambady said:
    If I wanted to get the full 16-bit resolution for a 0-1V signal, could I feed a stable 500mV into AIN3, configure the ADC to set AIN3 as the reference, and then use an FSR of +-512mV? That way AIN0 is 500mV above or below AIN3 and thus we're able to achieve 16-bit resolution, right?

    Correct as well!

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 20 points

    That answers everything I needed, thank you so much!