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ADS1115: Protection circuit for ADC input

DARSHAN salian
Prodigy 100 points
Part Number: ADS1115
Other Parts Discussed in Thread: LM2904

I am using ADS1115 for measuring DC analog voltage in range of 0-10V.(Sensor_output_range)

I am using IC in single ended input configuration and continuous conversion mode with FSR= 6.144 V and output data rate of 860sps. 

i am using a pull down resistor R14 of 1Mohm to avoid floating condition when the input pin is not connected ,i have selected 1Mohm resistor because the sensor is of resistive  type that's why the 1Mohm doesn't interfere with the sensor output voltage and given to the input of first stage buffer which acts as a voltage follower ,The output of the buffer is given to the voltage divider for scaling the input voltage from (0-10V) to  ,As the voltage is scaled it is given as the input to the second stage buffer ,The buffer added so that the voltage divider resistor(R!5=R16) doesn't interfere with the RC circuit and change the cutoff frequency of the RC circuit which is 8.6 kHz  .RC circuit is use as a low pass filter. the diode D5 is a schottky diode which is used for overvoltage protection as the ic can only handle the input voltage  of  VDD+0..3(VDD=5V).I used R18 just to model the input impedance of the ADC1115 as 10Mohm(is i have assumed by taking the average of common mode impedance and differential mode impedance).

I have simulated this circuit  and i am getting correct output .

Any minimization i can do in the circuit or any suggestions? 

  • 0
    TI__Guru* 94285 points

    Darshan,


    At this point, do you really need the second buffer of U1? I don't know if there is any benefit to it.

    Also, each of these opamps will contribute some amount of offset. For the LM2904, these devices have a max offset of 7mV, which is quite large. Depending on the source voltage's output impedance, you could just use the voltage divider and measure it directly with the ADC.

    What is the smallest voltage that you expect to measure on the input? Note that the output of the opamp may only drive to 5mV to ground. 

    If you think that you might see high voltage on the input (which you might, with the 12V supplies), you may need the input schottky diode for protection as you've drawn it.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 94285 points

    Darshan,

    I haven't heard from you in a while, so I thought I'd check in with your progress. In my last post I made some suggestions for your circuit using the ADS1115. 

    I'll close this post for now, but if you have any other questions about your circuit, post back and we can continue to work on it.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 100 points

    thanks for your concern Joseph Wu your suggestions really helped me to optimize my design