[Guide]A key subsystem in the programmable logic controller is the analog input module, which provides a high-precision front end to measure various sensors. However, in many cases, the amplifier input stage is connected to a remote sensor through a long cable and is susceptible to overvoltage conditions. In this article, I will introduce the basic concepts of operational amplifier (op-amp) input overvoltage protection and discuss how to choose the correct clamp protection circuit for overvoltage faults.
The data sheet of the operational amplifier used in the input module should provide specifications for absolute maximum ratings under electrical overstress conditions. Electrical overstress conditions are divided into two categories: Electrostatic Discharge (ESD) and Input Electrical Overstress (EOS). An ESD event is the sudden transfer of electrostatic charges between two human bodies under different electrostatic potentials. Electrostatic potentials can usually be separated by thousands of volts, and charge transfer usually occurs within a fraction of a second. Conversely, EOS events occur when the circuit is exposed to overvoltage conditions (such as failures caused by accidental connections) for a considerable period of time. These EOS ratings indicate the maximum power supply voltage, input voltage, and input current that the device can withstand without damage.
Generally, operational amplifiers have internal ESD protection structures designed to protect the operational amplifiers during manufacturing and production testing. The three common structures used in ESD protection (shown in Figure 1) are series resistors, steering diodes, and absorption devices. The steering diode is turned on to guide the ESD pulse from the sensitive circuit element to the absorption device. The absorption device absorbs the energy of the ESD pulse and limits the voltage level to prevent damage.
Fig. 1 Generally, the operational amplifier contains three ESD protection structures.
The op amp’s maximum rating for EOS depends on the maximum voltage and continuous current that the internal ESD diode can withstand. However, these structures are not intended to protect the equipment from the longer EOS events that may occur during circuit failures. Instead, an external circuit clamp may be required to protect the op amp input circuit from EOS events. Schottky diodes and series resistors are one reason to help protect the input of the op amp from overvoltage faults.
Let us consider the ±10V analog input module circuit shown in Figure 2. In this circuit, the operational amplifier buffer provides high input impedance and can interface with various sensors. The THP210 fully differential amplifier (FDA) attenuates and level shifts the buffered signal to drive the analog-to-digital converter. FDA is a precision, low-noise, low-drift amplifier, configured as a second-order Butterworth low-pass filter with a corner frequency of 100 kHz.
Figure 2 The front end of the high-impedance ±10V analog input module uses Schottky diodes and other components to protect the op amp from EOS events.
Two types of protection circuits are shown in this example. The clamp circuit is designed to provide input protection for ±40 V continuous overvoltage faults. Transient Voltage Suppression (TVS) diodes are used to clamp the power rail and absorb current from the clamp circuit to keep the power supply below the absolute power supply rating of the op amp ±20V. TVS diodes are similar to Zener diodes, but are designed for fast, large transient power dissipation. The SMF12A shown is a unidirectional TVS with a reverse isolation voltage of 12 V, a breakdown voltage of 14.7 V, and a maximum clamping voltage of 19.9 V. When using 1.24-kΩ, the current limit during a ±40V fault is 20 mA. , 1/2 W RLIMIT resistance, as shown in Figure 3.
The Schottky diode used in the input of the op amp has a metal semiconductor junction, and its forward voltage is lower than that of a silicon junction diode (for example, a diode used for ESD protection in an op amp). Figure 3 details how this property of the external protection clamp circuit works with those internal ESD diodes.
In this example, BAS40 is a small signal Schottky diode with a forward voltage close to 380 mV at 1 mA. In contrast, the internal ESD structure has a forward voltage of approximately 550 mV at the same forward current. Therefore, the Schottky diode conducts before the internal ESD diode of the amplifier, and most of the inrush current flows through the external clamp circuit. The internal ESD structure can only withstand 10 mA of current, while the external Schottky diode can handle up to 200 mA of forward continuous current, providing strong protection.
Figure 3 This commonly used op amp input protection Schottky diode clamps to conduct before the internal diode, so that most of the surge current flows through the external diode.
Although the external Schottky diode clamp circuit provides strong overvoltage protection, the disadvantage of the clamp circuit is that it introduces signal errors. During normal operation, a reverse-biased Schottky diode will show a reverse leakage current, which will flow through the RLIMIT resistor, resulting in an undesirable offset. The BAS40 used in this example provides an extremely low leakage current of 200 nA, thereby keeping the offset error to a minimum. You can also choose to reduce the RLIMIT resistance to minimize these offset errors, but the trade-off is to increase the fault current. This increase in fault current will require a resistor with a higher power rating.
However, the diode leakage current may vary slightly with changes in the reverse voltage. Therefore, the mismatch of the reverse leakage current between the diodes will cause a small non-linear error, which is a function of the input voltage. In addition, the leakage current of the diode increases exponentially with temperature. For example, the typical leakage current of this Schottky diode is about ~20nA at 25⁰C. However, the leakage current can be increased to 2µA at 85⁰C, and can be increased to 10µA above 100⁰C.
Fortunately, some modern precision operational amplifiers provide integrated input overvoltage protection, thereby eliminating the need for such external clamp circuits. Figure 4 shows the integrated input protection of the OPA2206. Its input is protected and can withstand a voltage of ±40 V beyond any power supply, and can withstand a voltage of ±40 V if the power supply is turned off.
Figure 4 The integrated op amp input protection clamp will change the impedance when the input is overloaded, provide protection during EOS, and minimize the impact during normal operation.
OPA2206’s internal protection circuit can provide low series impedance under normal signal conditions, thereby maintaining the required operational amplifier accuracy. However, if the input is overloaded, the protection circuit will increase the series impedance and limit the input current to approximately ±5 mA. Therefore, the integrated input protection clamp allows you to obtain accurate results through reliable protection, while reducing costs and reducing solution size.
Overvoltage protection is a broad subject, and the methods shown are just some of the many different ways to protect the inputs of op amps. For more information, check out TI Precision Labs-Electrical Overstress Video Series. This series details EOS operational amplifier protection and how to design a suitable clamp circuit for your application.