Using ADS1115 (or any other external ADC), how its possible to read zero instead of random floating values while no any signal at the ADC's input pin?

The ADS1115 is driven by an op-amp buffer as follows. enter image description here

The op-amp's + input is sometimes connected to a pH sensor, sometimes left unconnected.

  • Signal is op-amp buffered pH sensor signal so its impedance is near by zero I think. – user30878 Jun 29 '17 at 7:35
  • Ok. But what will going on when input is connected? I'm asking because I want to understand before trying. – user30878 Jun 29 '17 at 7:37
  • You get the output impedance and the 10k in parallel, so if the output impedance is considerably lower than 10k then the 10k will have virtually no effect on the signal. – Majenko Jun 29 '17 at 9:53
  • That's an interesting point - I assumed the poster was asking about when nothing was connected. @user30878, could you clarify please? Are you asking how to get 0-readings when nothing is connected to the ADC, or are you saying you get fluctuating readings when you have the opamp connected but nothing connected to the op-amp inputs? (Or something else?) – Mark Smith Jun 29 '17 at 12:28
  • Op-amp's output is always connected to ADC input but nothing connected to op-amp's input in my case. By fluctuate I mean randomly floating values. – user30878 Jun 29 '17 at 12:31

The behaviour you are seeing is what happens when you have high-impedance inputs. Basically, if you don't drive them, you get random values.

Unfortunately in your case, where the sensor you are using has no output current to speak of, I don't think there's anything you can do about this electronically.

You'll have to solve the problem in some other way if you really need to be able to disconnect the sensor. A mechanical solution would be a connector which shorts when disconnected. Switched headphone connectors do this, for example - when you pull out the jack, they make a connection so that the speakers can be switched on.

  • You mean 100 times larger resistor than sensor? It would be 100 mega Ohm then. – user30878 Jun 29 '17 at 12:55
  • In fact I don't need to get zero. I just want to get rid off random values. – user30878 Jun 29 '17 at 12:55
  • What is the ouput impedance of the pH sensor? I have no idea how they work... – Mark Smith Jun 29 '17 at 12:56
  • It can be go to 1 tera ohm. – user30878 Jun 29 '17 at 12:56
  • Yes. There is nothing added to this circuit. i395.photobucket.com/albums/pp34/teriyakithunder/pHprobe.jpg . Just the output goes to ADC input. – user30878 Jun 29 '17 at 13:04

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