ZHCSIQ4B September 2018 – December 2018 OPA828
The OPA828 is internally protected against ESD events which can occur during manufacturing, handling, or printed-circuit-board assembly. The internal ESD protection diodes are not intended to protect the OPA828 during normal operation when the device is operating under power. The ESD protection circuitry involves several current-steering diodes connected from the input and output pins and routed back to the internal power-supply lines, where the diodes meet at the power-supply ESD cell, an absorption device, internal to the operational amplifier. This protection circuitry is intended to remain inactive during normal circuit operation. In cases where the inputs or output can be driven above the positive power supply or below the negative power supply care must be taken to limit the current through the internal diodes to 10 mA or less. In harsh electrical environments external protection circuitry may be required and is dependant upon the application requirements and environmental conditions.
One example of protecting the OPA828 inputs against an input over-voltage condition is illustrated in Figure 44. In this example the non-inverting input to the OPA828 is protected with the addition of an external resistor. If the input voltage, VIN, exceeds either power supply voltage, the input ESD diodes become forward biased at approximately 0.5 V. It is recommended to limit the current through the forward-biased internal ESD diodes under such conditions; see Absolute Maximum Ratings. In this specific example illustrated in Figure 44, the addition of the input resistor provides the necessary current limiting and allows for input voltages at VIN up to ±25.5 V. Assuming a symmetrical, dual power supply configuration, the maximum input voltage for this circuit configuration can be determined from Equation 1:
Adding the series input protection resistor as described above will add an additional source of noise to the circuit. Resistance values below 250 Ω contribute less than 10% of additional noise. A resistance value of 1 kΩ increases the noise by approximately by 40%. The OPA828 has an equivalent input noise resistance of approximately 1 kΩ.