Key benefits of input over-voltage protected op amps in systems
The hostile environments found in many of today’s applications require integrated circuits to withstand high voltage and current. Systems designers need to select high-performance ICs to use in these hostile environments. IC manufacturers specify absolute maximum ratings for every integrated circuit; these ratings must be observed in order to maintain reliable operation and meet published specifications.
What if a large signal or electrostatic discharge (ESD) is accidentally applied to either input of an op amp that is used in the front of a signal chain interfacing sensors and real world signals? Applying a voltage to input pins beyond a power supply setting of a standard amplifier can be a way to damage or destroy an op amp.
Most op amps have internal ESD protection diodes on the input pins (figure 1) to allow the ICs to be handled during the PC board assembly phase. These diodes are small to minimize capacitance and leakage, and are not designed to handle sustained input currents greater than a few mA.
Internal p-n junctions to the device energize and permit current flow from the inputs to the supplies when the op amp input pin is roughly +-0.5 V above / below the supply rails. This fault current can quickly rise to damaging levels if not limited. Good, experienced board designers have learned to use protection circuitry to prevent permanent damage in these unforeseen situations. But adding external protection circuitry in front of high performance ICs is going to introduce errors and degrade the performance of the overall circuit.
Figure 1: Typical input pin protection of an op amp
Lack of a protection circuit can be damaging. As mentioned, these internal ESD diodes allow the inputs to go above and below the supplies by a slight amount (usually one VBE) before turning on. But this VBE drop is process and temperature dependent and can vary from part to part. Figure 2 shows the temperature dependency of internal diodes of a CMOS AD8646.
Figure 2: Current flow starts at different overvoltage levles according to temperature.
So, how should system designers protect their op amps in general purpose applications? The general advice to limit the amount of current flowing into a part when an ESD diode turns on is to add an external series resistor at the expense of additional thermal noise and other negatives which will be pointed out later. To ensure optimum dc and ac performance, it is also recommended that source impedance levels be balanced.
Additionally, if there are possibilities that the input voltage could exceed either of the supplies by more than a VBE, then the designers are advised to protect their circuits with Schottky diodes, MOV or channel protectors. Figure 3 below shows a traditional protection scheme against overvoltage and over current by using an external series resistor and Schottkys:
Figure 3: A typical recommended protection scheme for general purpose applications
Schottky diodes turn on faster than internal diodes, so the internal units never reach their threshold. Diverting fault currents externally eliminates potential stress, protecting the op amp. RS is chosen so that the maximum current is no more than 5 mA for the worst case VIN.
But the above recommended protection scheme works well only for general purpose applications but is not suitable for high precision signal conditioning. Externally added components introduce errors into the signal path and reduce the overall signal conditioning circuitry and degrade measurement accuracy.
For example the errors caused by introducing an external limiting series resistor, En,total, are thermal noise of the resistor, Voltage noise due to amplifier noise current flowing through the resistor. and voltage drop due to the amplifier bias current flowing through this resistor. Overall offset error, Vos (total), introduced as the result of this resistor is:
The issues of clamp diodes at the input of op amp pins to be concerned about are:
- Additional pole in the signal path due to diode junction cap and Rs
- Diode leakage currents that will double for every 10°C
- Change in forward voltage drop as a function of temperature.
- Generated current noise ( In )problem which might interfere with the measurement