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

I am using thr IC in single ended input configuration and continuous conversion mode with FSR= 6.144 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.

The sensor output voltage is given to the input of the 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 0V to 10V. After the voltage is scaled it is given as the input to the second stage buffer.The buffer was added so that the voltage divider resistor (R15=R16) doesn't interfere with the RC circuit and change the cutoff frequency of the RC circuit which is 8.6 kHz. The 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 am getting correct output.

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

ADS1115 datasheet.


1 Answer 1


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

First, minimize the circuit so I can read it on my laptop: -

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Second, the LMx58 might generate anything up to 5 mV input offset voltage and this means immediately that your error budget (if you had one) is being eaten into by 10 mV (two op-amps). This would concern me.

Third, the resistor potential divider (R15/R16) might introduce a 2% gain error if you use 1% resistors so bear this in mind.

Fourthly, the LM358 won't be guaranteed to swing its output any closer to the 12 volt rail than 9 volts so it's a showstopper if you want to produce a 10 volt output from U3.

Fifthly, the LM358 is right on the limit with 10 volts at its input - look up "Input common-mode voltage range" in the data sheet. This applies to U3 and not U1.

Recommendations: -

  • U3 needs to be an op-amp that is guaranteed to produce at least 10 volts on its output from the 12 volt rail.

  • I'd use a 5 volt rail-to-rail op-amp for U1 and get rid of D5 and V9 - running from 5 volts means it can't produce excessive voltage into the ADS1115 ADC. Of course you might lose the top 20 mV of voltage span but your first stage buffer loses the equivalent of 0.5 volts due to its output not being guaranteed to rise higher than 9 volts.

  • Use power supply decoupling capacitors on the op-amps.

  • Choose op-amps with much, much better input offset voltage specifications and make sure that R15/R16 are precision resistors.

  • I'd probably consider making the 2nd stage buffer a unity gain sallen key filter to provide better anti-aliasing.

  • \$\begingroup\$ input common-mode voltage range of LM358 is v+-1.5V,so the output voltage can go till 10.5 V \$\endgroup\$ Commented Sep 1, 2020 at 13:18
  • \$\begingroup\$ thanks for your recommendations @Andy aka \$\endgroup\$ Commented Sep 1, 2020 at 13:19
  • \$\begingroup\$ i am measuring DC signal with ADS1115 like should i Select the cutoff for RC filter as 10x output data rate or some where around 100 to 200 HZ \$\endgroup\$ Commented Sep 5, 2020 at 15:45
  • \$\begingroup\$ You need to select it to avoid aliasing, preferably as low in frequency as you can do so. \$\endgroup\$
    – Andy aka
    Commented Sep 5, 2020 at 15:49
  • \$\begingroup\$ aliasing also affects dc signal?@Andy aka \$\endgroup\$ Commented Sep 5, 2020 at 15:53

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