1

I have designed a circuit for an SGX EC410 oxygen sensor by using these application note -PAGE 4, FIGURE 4 - (http://www.sgxsensortech.com/content/uploads/2014/08/A1A-EC_SENSORS_AN2-Design-of-Electronics-for-EC-Sensors-V3.pdf). When I try to measure the biased voltage between working (SENS) and reference (REF) electrode, the value of -600 mV is held for about one second and goes down to few milli volts rapidly. The same happens when I connect the sensor without voltmeter. Am I doing something totally wrong, or the circuit is not quite right? enter image description here

Viktor Scherf
  • 21
  • 3
  • What OpAmps are you using? – user1582568 Feb 16 '16 at 15:13
  • @user1582568 There are two TLC2272cd dual operational amplifiers from Texas Instruments (IC1+IC2 and IC3+IC4). – Viktor Scherf Feb 16 '16 at 15:22
  • OK, those should be fine, I was wondering about bias current. Could it be that you are simply over range? Have you measured the voltage at the count electrode when the bias has disappeared? I will not be possible to measure the voltage at REF with a meter because as soon as you draw current from that node the voltage will be affected. – user1582568 Feb 16 '16 at 15:31
  • Maybe you can explain what should happen? – Andy aka Feb 16 '16 at 15:36
  • @user1582568 I have just measured the voltage at the count electrode with no bias (it is actually not zero, but something about 250 mV after long swing period) and that was the full supply voltage, e.g. +5V. The OpAmp is overloaded, right? – Viktor Scherf Feb 16 '16 at 16:21
  • The OpAmp is trying to maintain 3.1V at the inverting input by increasing its output. You could do a quick measurement just to confirm that there is 3.1V at the non-inverting input (pin3). If this is OK then it looks like there is no voltage at the REF pin. Either there is not enough/too much gas (whichever way it goes) or the sensor is not working I would suggest. – user1582568 Feb 16 '16 at 16:32
  • @Andyaka The circuit should simply provide some micro amperes at ca. 20% O2 (ambient conditions) and transform (and amplify) these current into an appropriate voltage that can be measured with Arduino. The bias voltage is needed for chemical redaction of oxygen. In these case the sens electrode draws current. – Viktor Scherf Feb 16 '16 at 16:37
  • IC3 - how does it get feedback? – Andy aka Feb 16 '16 at 17:37
  • @user1582568 It works!!! The problem was that the circuit was not properly grounded. After I connected the correct ground (and not GND of Arduino) it has displayed the correct values. Thank you very match!! Thanks to Andyaka too! – Viktor Scherf Feb 17 '16 at 14:38

0 Answers0