A customer of ours has asked questions regarding the Over-Voltage Protection Circuitry on the INA128. Here are their comments:
At no time are we exceeding input voltage limits. While testing channels that use these parts as buffer amplifiers (with a gain of 1), applying a solid -10V with respect to ground to the “+” input (of just this amplifier, in just this channel) through a 15K resistor, causes voltage to appear across this resistor, measured to be as high as 7V (about 4.7mA). The question for TI is whether there is input-voltage-limiting circuitry within the amplifier that could leak current to pins 4 or 5 of the DIP (Ref or V-) and cause this condition. Presumably, if this voltage-limiting circuitry is just a pair of diodes to V+ and V- respectively, this could NOT be the cause of our problem.
A third associate here mentioned knowledge of an FET incorporated into input circuitry of such an amplifier where it participated in the task of protecting the amplifier from ESD. If it wouldn’t compromise TI’s proprietary information to reveal further details, it would be helpful to know more about this input protection circuitry and what specific component may have created a leakage path either to ground or the negative rail. Considering the way these parts are carefully handled and protected from ESD from the receiving department through final fabrication, I’d imagine that emergence of this condition would be of interest to TI as well.
Can we offer any information on this circuitry? I have checked their schematic (I can send separately if needed) and there does not appear to be an issue. It seems to me that it is a damaged chip and they have not eliminated this as a possibility. I think they would like to know more about this circuit to help explain what they have seen. I understand (they do too) that this information might not be available, but I thought that I might ask.
Thanks for your help with this!
Richard Elmquist
