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I am investigating a production issue for a legacy design where a tlc2272acpwr opamp is configured in a buffer follower configuration with the output tied to - and the + input is driven by a another opamp at -2.0V in series with two 33 megohm resistors in series. The voltage at the + input is -0.29V. The voltage rails are at +5 and -5V. The output connects through 23.7k to an ADC input. On passing boards the ouput of the tlc2272 and the input of the ADC is -1.48V. The test limit requires ADC input is no lower than -1.65V. On failing boards the voltage is -1.66 to -1.69V. The connections between the components in the circuit have been verified and the components have been swapped but the failing boards continue to fail and no incorrect connections have been found. The circuit was simulated and voltage on the input matched the output over the range the circuit operates. When the input of the circuit goes high (3.6V) the output matches it and the simulation.
Are there conditions where the opamp won't behave like an opamp and a buffer follower output won't follow the input?
Justin,
First of all, your TLC2272A circuit works in the simulation as it should - in a buffer configuration output follows the input input until input common-mode voltage range limits it at +4V - see below.
However, something does not make sense in your description: how on a passing boards the output of the TLC2272 and the input of the ADC can be at -1.48V while you drive the input of a buffer to -2V - they all should be at -2V as shown below.
Thus, I wonder what supplies do you use for your ADC? Most ADC's do NOT work on +/-5V.
Your decription of the failing board would make me believe that you have a damaged ESD protection diode between the Vinp and positive rail because -.29V at Vinp and Vin at -2V would require 51nA input current to flow thru 33M input resistor - see below. But since even a "passing" board does not make sense, I can't be sure what else is going on? Please use the attached schematic instead of write up to show what is going on - a picture is worth thousand word.
Here is the circuits connected to the buffer (J29). The RLF signal is -0.289V on a passing board and RLB is -1.485V. On a failing board RLF is -0.292V and RLB is -1.69V. RLF is driven by VLOV, which is -2.013V on passing and failing boards. RLF can be driven by analog switch J21 but under the test condition it isn't.
