OPA4197: OPA4197 pin status for input....

Hi Peter,

Do realize that the block diagram circuit is a simplification and other components should be added for a practical implementation. For the circuit as shown the momentary open-circuit voltage that the OPA197 input sees is highly variable and will depend on the leakage current paths present during that time. The OPA197 needs an input bias current path to properly function and it will source, or sink its input bias current from the leakage paths.

The OPA197 input bias current is typically only +/-5 pA.(+/-20 pA max at 25 C), while the TLP3440 detector OFF-state current can be as high as 1 nA at 25 C. Since the detector has diodes to each supply rail one or the other will become the majority leakage current path and provide the OPA197 input bias current. Where the input voltage ends up is completely dependent on all the variables.

Practically, each OPA197 input that connects to a multiplexer channel output should have a bias current resistor to ground, or another potential. Most often that resistor connects from the non-inverting input to ground. The resistor provides a fixed input bias current path for the op amp input when the multiplexer switch goes to the open state. If the switch is open for much of the time the OPA197 input voltage will be very near the ground level and the output follows. Since there is capacitance associated with the multiplexer circuits and the op amp input there is some initial transient behavior during the multiplexer switching, but that will quickly settle out because of the current path to ground.

Regards, Thomas

Precision Amplifiers Applications Engineering