I am using an AD8253 as the primary gain stage in a data acquisition circuit with supply rails at +/-15V. During checkout of an evaluation board, I found that inputs that are overdriven but do not exceed the supply rails cause the gain amplifier to draw substantially more current. Reviewing the datasheet, Figure 31 "Fault Current Draw vs. Input Voltage" seems to describe this condition but I still have some questions.
- What causes the supply current to ramp up so dramatically?
- What causes the input bias current to increase? The input voltage is below the supply rail so the clamping diodes should not be conducting. Is the input impedance changing?
- Figure 31 shows the input and supply currents for a differential voltage up to 10V. What happens when the differential input voltage reaches Vs which in my case is +/-15V?
- I am considering adding input resistance or current limiting the supplies to reduce the effects of the overdrive condition since this could easily cause the part to reach its 140C max junction temperature. Are there other alternatives to preventing the overdrive condition?