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I'm designing a circuit to read out the C2H4-CA10 sensor.

Following the typical applications document, I designed the following:

KiCad schematic

To verify the schematic, I created a new TINA simulation:

TINA simulation

When probing VM1, I get a output value of 999,952953mV with IG1 @ 350nA (which equals the starting current for the sensor.)

When the sensor is at 10ppm, the current value should be 1550nA, and the voltage over VM1 rises to 1,000000V.

I've got a few questions.

  • Do both OP1 and OP2 need to have a power supply connected? It's not part of the schematic in the MEM1 document.
  • If yes, how do I need to connect them? I've searched around on Google and didn't find a single potentiostatic circuit with an external power supply connected. If I remove the power supply from OP2, the output value drops to 20uV, so I suppose OP2 does need it.
  • Why is the output voltage limited to 1V@1550nA? I connected it to a 5V supply.

When realising the circuit once the design process has been completed, I want to make Rgain disconnectable from C2 to allow for better tweaking, is the circuitry affected if I use two headers with jumpers to connect/disconnect Rgain?

EDIT:

I just noticed that changing the schematic to this one:

TINA Simulation v2

The output voltage is generated according to I = Isensor * Rgain. OP1 and OP2 both receive a supply voltage of -1V at it's negative pin and 1V at it's positive pin

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  • \$\begingroup\$ Why is T1 shorted out? Op-amps need power supplies. \$\endgroup\$ Commented Feb 17, 2021 at 12:35
  • \$\begingroup\$ Do you mean the short behind the JFET? That was a little mistake but it didn't the output much \$\endgroup\$ Commented Feb 17, 2021 at 12:46
  • \$\begingroup\$ @tim687: "Supply voltage" goes to pins 4 and 7 on your op amps, not the + and - pins (3 and 2.) Operational amplifiers require a power supply to operate. If you see potentiostatic circuits without a power supply then they aren't using ap-amps. \$\endgroup\$ Commented Feb 19, 2021 at 11:44
  • \$\begingroup\$ @JRE The power supply of the op-amps are connected to pins 4 and 7, the voltages and current inputs of the circuitry are connected to pins 2 and 3. Regarding the ap=amps, that could be it. Which amps are they using then? \$\endgroup\$ Commented Feb 19, 2021 at 11:53
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    \$\begingroup\$ The diagrams in the MEM1 application note shows conceptual diagrams, not completed circuits that you can copy and build. They leave out a lot of details so as not to clutter things up and obscure the concepts. One of the things they left out is the power supply to the op-amps drawing in the power supply would clutter the drawing. It is assumed that anyone capable of building such a circuit will know that op-amps require a power supply to operate. \$\endgroup\$ Commented Feb 19, 2021 at 12:10

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  1. All op-amps require a power supply. They do not work without external power.
  2. Where the power supply is connected to the op-amps depends on the specific op-amp you are using.
  3. The required operating voltage depends on the specific op-amp you are using. Typically, you would use +-3.3V or +-5V. The exact voltage depends on what your op-amp allows, how well your op-amp functions when its input or output voltages are close to the power supply voltages, and probably many other concerns.
  4. Which op-amp you choose depends on many things. Supply voltage is only the first of many things.
  5. The 1.5V "Maximum voltage in normal operation" mentioned on page 3 of MEM1 does not refer to supply voltage. That 1.5V is the maximum voltage that a sensor will produce. You don't supply power to it. It generates a (tiny) amount of power when exposed to the gas it is supposed to detect. The information on page three is in a section titled "Intrinsic Safety Considerations." It is telling you that the sensors meet the requirements to be considered intrinsically safe. This is important given that some of the sensors are used to detect explosive gases.
  6. Judging by the very small currents mentioned, building a functional and accurate circuit will require op-amps with low leakage and low offset. Given the slow reaction time of the sensors, drift will be a concern - you'll want an op-amp with low drift.
  7. The small currents will also require careful layout to keep external interence out of your measurements.
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  • \$\begingroup\$ Now this is the answer I needed. The current generator in the simulation, is that placed in the proper direction? \$\endgroup\$ Commented Feb 19, 2021 at 13:07
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    \$\begingroup\$ @tim687: I honestly do not know. I do this kind of thing as a hobby - I have very little formal training. The ideal current sources (and their arrows) used in simulators generally confuse me. They use a convention which I have never taken the time to work out and understand. \$\endgroup\$ Commented Feb 19, 2021 at 13:13
  • \$\begingroup\$ The LMP7721 seems like a good candidate since it has low leakage and low offset values. \$\endgroup\$ Commented Feb 21, 2021 at 9:39

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