TLC4502: TLC4502

Hi Gopi,

Can you clarify what you mean by "IC is not reacting with sensor"? I see you have tested various nodes in the circuit with independent meters, can you share these measurement values? What are the voltages at each pin of the TLC4502 during the failure mode?

After replacing the IC, is there anything else done to the PCB such as cleaning?

I see you posted a similar concern a few months ago, can you share what was the root issue and what was your solution? I see from that post that the amplifier circuit was functional with a voltage input source but not when connected to the pH sensor. Is that true in this case as well? If you replace pH sensor input with voltage source does the TLC4502 produce the correct output voltage?

Thanks,

Zach

Dear Zach -

a. Yes, amplifier circuit was function with a voltage input source (programable power supply), the output from the IC is measured using a fluke device and the output is as expected in the test case.

b. When a pH  sensor (Sensorex S2722) is connected to the amplifier - amplifier output is within the rage of pH buffers. Peculiarly, sometime the same amplifier becomes a brick, and there wouldn't be any measurement to record. However, in such a scenario if we replace the sensor with a PPS the circuit will work as expected. And the same time pH sensor will work with a bench top meter as expected.

To resolve the issue, if we replace the IC with a new IC everything becomes normal.

c. w.r.t my earlier post - we did overcome the issue with complete redesign of PCB following the high-impedance circuitry design/layout guidelines. The amplifier referred in this newer post and point a & b, the PCB is redesigned version.

What are the voltages at each pin of the TLC4502 during the failure mode?

Supply pin (8) 5V (CRO wave form is pure DC line)

Amplifier 1 ( pins 5,6,7) connected Voltage follower mode, Pin 7 - (Configured 833 mV, connected with sensor terminal 2)

Amplifier 2 (pins 1, 2, 3) connected in non-inverting amplifier, pin 3 - connected with sensor terminal 1

Pin 1 - output measurement - 300 mV (Expected 3V).

When an IC is replaced we do clean the surface area around the IC.

We observed this issue in  3 PCB outs of 13 PCB's we built. Always the issue occurred in running circuits. 

Thank you and let me know if you have any additional questions.

Hi Gopi,

I see that the failed circuits will operate properly with a low-impedance input voltage source but not with the high-impedance pH sensor probe. After replacing the IC and cleaning the board area around the amplifier the circuit operates properly with the high-impedance probe.

High impedance circuit nodes are highly sensitive to contamination, which will reduce the performance. It is possible that over time the PCB becomes contaminated by oil, dust, salts, etc. from the environment or from user's handing and touching the board itself. It is likely that when the IC is replaced and the PCB is cleaned, these contaminants are removed from the sensitive high-impedance nodes which restores the performance.

Can you attempt to clean one of the failed boards without replacing the IC? Specifically focus on areas around pin 3 which is the high-impedance circuit node. You could also try placing the original IC back on the board, apply the cleaning procedure, and test the performance again. If the performance is restored, this indicates that the PCB contamination is the cause of the error.

Regards,

Zach

Thank you Zach. I shall keep you posted with results. 

Hi Gopi,

Sounds good. I'm looking forward to the results.

Regards,

Zach