TLC27L2: About Potentiostatic Circuit with TLC27L2 and OPA2244

Hello SS,

Please see my comments and questions along with your original text.

[Case 1 - TLC27L2]

1. When Bias Voltage is 600mV, TIA output is very unstable. Is this a noisy output, or does the output show oscillation? 

2. When Bias Voltage is 400mV, TIA output is a little unstable.

3. But Gas senser is required 600mV bias voltage.

[Case 2 - OPA2244]

1. When Bias Voltage is 600mV, TIA output is looks like stable.

[Questions]

1. What is difference between TLC27L2 with OPA2244?

The TLC27L2 is a very low power CMOS op amp with a typical operating current of 10 uA. The OPA2244 is a low power bipolar op amp with a typical operating current of 50 uA. Their electrical specifications are quite different from each other. 

2. Why this issue is occurred? 

If the instability issue is oscillation, then it is a matter of the differences in the phase margin that exists with the two different op amps in the circuit. Since they are quite different in their design their complex output impedance (Zo) characteristics and ability to drive capacitive loads are different. Comparing the OPA2244 SMALL SIGNAL OVERSHOOT vs LOAD CAPACITANCE graphs on datasheet Pg. 8, and the TLC27L2 PHASE MARGIN vs CAPACITIVE LOAD on datasheet Pg. 25 and the Figure 41. Effect of Capacitive Loads and Test Circuit, it is clear the TLC27L2 cannot drive nearly as large of a capacitive load as the OPA2244 and remain stable. 

You mention that the output of the TIA is where you observe the instability. What kind of cable or wiring do you have connected at that point of the circuit?

3. What is critical spec of OP-AMP in Potentiostatic Circuit?

The datasheet for the O2 Gas sensor doesn't provide much electrical information about the three internal elements. That doesn't help us understand what kind of load it presents to the op amps. We do know that some gas sensors such as a hydrogen sulfide detector presents a very high capacitive load to the op amps which can lead to an unstable condition. That maybe the case for this O2 Gas sensor.

4. If I use OPA2244, is any side effect? 

I don't see any downside to applying the OPA2244 in the application other than higher operating current. The OPA2244 provides higher dc precision, lower noise and wider bandwidth performance than the TLC27L2.

Regards, Thomas

PA - Linear Applications Engineering

Hi SS,

Would it be possible for you to provide the actual circuit diagram for your O2 sensor circuit? I need to see the component values and if there is anything in your circuit that differs from the sensor manufacturer's suggested circuit. 

Regards, Thomas

PA - Linear Applicaitons Engineering

Thanks Thomas.

I attached file include shematic and test result of TIA output.

3858.O2_M-100_Membrapor_bias_voltage_Review_Report.pdf

Check this file.

U1B is potentiostatic AMP and U1A is TIA.

TIA output is so slow and slow oscillation.

But OPA2244 is good operation.

Point is why this situation is occurred and Why TLC is not good?

Hello SS,

I received a direct inquiry today about this same issue from our TI Korea FAE, Ethan Kim. I am reviewing the report and will provide feedback once we have an understanding of produces the behavior cited in the report. He will be receiving more questions from me which he will surely pass on to you.

Regards, Thomas

PA - Linear Applications Engineering