• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 2 answers
  • Subscribers 46 subscribers
  • Views 1193 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.

Original question:

ADS1115: Measuring negative voltages with ADS1115 in differential mode

ADS1115: Plz suggest differential amplifier IC or op-amp IC models to buffer the input stage of ADS1115

Venkatramayya Yalamati
Prodigy 20 points
Part Number: ADS1115
Other Parts Discussed in Thread: OPA862, ADS1120, ADS112C04

This is my setup for bi-directional measurement of DC battery cell voltages battery set voltages ranging from 0.1V to 600V using ADS1115 (changing gain with help of PGA feature) with ATMEGA328 programmed by Arduino. This circuit is working well with good accuracy but, I want to put a differential amplifier (or) voltage buffer circuit at the input stage of ADS1115 to increase it's input impedence.. plz suggest me suitable differential amplifier IC model(s) or precesion op amps for making voltage buffers to drive the ADS1115 inputs. Thanks in advance!!!

  • 0
    TI__Expert 7635 points

    Hello,

    Venkatramayya Yalamati said:
    I want to put a differential amplifier (or) voltage buffer circuit at the input stage of ADS1115 to increase it's input impedence

    The ADS1115 already has an input impedance of 6MΩ for the common mode and 15MΩ differentially for the 4.096V input range, how high are you trying to increase the input impedance to? This is a question for the Amplifiers forum, so I would ask a new question using the OPA862 as a starting point. 

    With the circuit you are using, you will need to minimize errors as much as possible to get the 0.1V resolution you are aiming for, so keep this in mind when you are determining the resistor tolerances. You should also consider an input filter to limit the noise that reaches the ADS1115.

    Please also keep in mind that the ADS1115 cannot have the voltages on it's input pins drop below the ground level without damaging the part.

    Let me know if you have any more questions,

    Levi DeVries

  • 0 in reply to Levi DV
    Prodigy 20 points

    Hi Levi DeVries.. thanks for your attention to my query. I am changing the ADS1115 gain to 16x (where the input impedence will be 710k ohms only) using PGA setting feature of ADS1115 for measurement of individual cell voltages of battery set. So, I want to increase the input impedence & try to eliminate the effect of voltage divider resistance (2M ohm : 9.4k ohm) on the ADC input... and as you suggested, I have measured the resistance values, before using them in circuit. I calculated the voltage division factor based on the measured resistances & using that factor in program for multiplication to get the final voltage. Also i am going to use capacitors of 0.1nf each at AIN0 & AIN1 to Gnd, and a 1uf capacitor across AIN0 & AIN1. 

  • +1 in reply to Venkatramayya Yalamati
    TI__Expert 7635 points

    Hello,

    I was under the impression you wanted to measure up to 600V, so I believe you will need to use the ±4.096V input range to capture the full scale of your input voltage. This is why I assumed you would be using the 15MΩ input impedance.

    The ADS1120 has an integrated PGA and is a similar part to the ADS1115, so it may meet your needs here. Alternately, you could ask a question on the amplifiers forum as they will be the experts on discrete solutions.

    Keep in mind that an external input amplifier will increase your system's noise.

    Levi DeVries

  • 0 in reply to Levi DV
    Prodigy 20 points

    Thanks Levi DeVries... I will ask in amplifier forum... anyways, I am changing the gain of ADS1115 dynamically using program commands, based on the measured voltage to give best resolutions in both cases of tiny cell voltage as well as total set voltage measurements.. voltage divider is same for both cases.

  • 0 in reply to Venkatramayya Yalamati
    TI__Expert 7635 points

    You could also try the ADS112C04, which combines a programmable amplifier with an ADC. This part has a typical input current of 1nA, so it will cause something on the order of 1mV of error in your system, well below the 100mV you would like to sense. It also has a higher-gain internal amplifier for better detection of the voltage levels you are trying to sense.

    Let me know if you have any more questions,

    Levi DeVries