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Original question:

ADS1115: what is the FSR of the ADS1115

ADS1115: FSR and maximum ratings in datasheet difference

srikant n
Prodigy 20 points
Part Number: ADS1115
Other Parts Discussed in Thread: ADS1120

I need to to read a sensor output which gives the analog output +/- 2.5V. Do I need to use it in differential mode to read it into digital?Please sugget me if any input circuitry is required to interface it with ADC.

I read FSR +/- 2.048. WHat is the difference between absolute maximum ratings and FSR. thanks

  • 0
    TI__Guru**** 168485 points

    Hello Poster!

    Our support staff is taking time with their families for the Independence Day holiday here in the US.  It may be July 8th before we can get back to you with help on your query.  We apologize in advance for the delay and will get back to you as soon as possible.

  • 0
    TI__Guru* 92655 points

    Srikant,


    Let me answer the basic questions from your post, and then we can discuss the question about input circuitry that's needed.

    First, the absolute maximum ratings for the device are the maximum inputs that the device can tolerate without damaging it. For this device the voltage any analog input must be between -0.3V and VDD+0.3V. This is to prevent the ESD diodes on the analog input pins from forward biasing.

    Second, the device is a fully differential ADC. For this device the ADC measurement measures the voltage from a positive input AINP and subtracts the voltage from the negative input AINN. The ADS1115 has an input multiplexer and you can select which of the analog inputs are AINP and AINN. Basically, the ADC measurement is a representation of AINP-AINN, where both inputs should be in the range from 0V to AVDD. If AINN is 3V and AINP is 2V, then the input measurement is -1V. The input may be a negative measurement, but neither input is a negative voltage.

    Third, the FSR refers to the full-scale range. This determines the largest signal that the ADC is able to measure. For the standard FSR of ±2.048V, the maximum signal that can be measured (between AINN and AINP) is 2.048V.

    If your system measures a differential input signal and both inputs are between 0V and VDD, then you probably don't need any input circuitry between the sensor and the input. If your sensor output impedance is high, then you may need to add a buffer between sensor and ADC.

    If you need to measure an input signal that goes from -2.5V to +2.5V, then you may need to add some level shifting circuitry. This would set the inputs so that the voltages are between GND to VDD. This could be done with a difference amplifier type INA and setting the reference of that INA to mid-supply.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 20 points

    Hi Mr.Joseph,

    Thank you so much for the answer.

    I have a sensor that outputs +2.5 to -2.5 V.One of the specs of the sensor mentioned is Frequency range : 3dB @ > 500 Hz. Does it mean I need to chose sampling rate atleast 1kSPS?

    in that case AD1115,data rate specifeid is 8 SPS to 860 SPS. Do I need to find another ADC for my sensor? or am I going in wrongway in analysing of the sensor. Please suggest me.

  • 0 in reply to srikant n
    TI__Guru* 92655 points

    Srikant,


    If the sensor lists the frequency range as 3dB @ > 500 Hz, it just means that the sensor will be band limited to frequencies under 500Hz or so (depending on the specifics of the sensor).

    I would note that ADS1115 uses a digital filter to get the data from the modulator of the ADC. This digital filter also has a frequency response that limits the frequency of the input that the ADC is able to capture. This frequency response is shown in Figure 21 of the datasheet. This figure shows the frequency response for the ADC running at 8SPS. This figure scales with the data rate, so using the fastest data rate of 860SPS, the figure is similar, but the first notch is at 860Hz instead of the 8Hz shown in the figure.

    If this bandwidth is too small, then you may want to look at a device that has a higher data rate. One device that may be suitable is the ADS1120. The highest data rate is 2kSPS. It is a similar device, but uses SPI for communication. One other benefit of this device is that the analog supply can be run from ±2.5V (while the digital supply can be run from a single 3.3V supply. This way, you don't need a buffer stage that shifts the input range of the sensor.


    Joseph Wu