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Signal Chain Basics #159: Provide robust input overvoltage protection for amplifier analog input modules

A key subsystem in a programmable logic controller is the analog input module, which provides a high-precision front end to measure a wide variety of sensors. In many cases, however, the amplifier input stage is connected through long cables to remote sensors and is susceptible to overvoltage conditions. In this article, I will introduce basic concepts of operational amplifier (op-amp) input overvoltage protection and discuss how to select the right clamp protection circuit for an overvoltage fault.

The data sheet for the op-amp used in the input module should provide a specification for the absolute maximum ratings on electrical overstress conditions. Electrical overstress conditions are classified into two groups: electrostatic discharge (ESD) and input electrical overstress (EOS). An ESD event is the sudden transfer of electrostatic charge between two bodies at different electrostatic potentials. The electrostatic potential can often be thousands of volts apart, and the transfer of charge typically occurs within a fraction of a second. In contrast, an EOS event occurs when the circuit is exposed to an overvoltage condition, such as a fault caused by an unintentional connection, for a relatively long period of time. These EOS ratings represent the maximum supply voltage, input voltage and input current that the device can withstand without damage. Table 1 shows the absolute maximum ratings for a representative precision op amp, the Texas Instruments OPA2205.

Table 1 Absolute maximum ratings for the Texas Instruments OPA2205 precision amplifier.

Typically, op amps have internal ESD protection structures designed to protect the op amp during manufacturing and production testing. Three common structures used in ESD protection, shown in Figure 1, are the series resistor, steering diodes and absorption device. The steering diodes turn on and direct the ESD pulse away from the sensitive circuit elements to the absorption device. The absorption device absorbs the energy of the ESD pulse and limits the voltage level to prevent damage.

Figure 1 Typically, three ESD protection structures are included inside an op amp.

An op amp’s maximum ratings for EOS are based on the maximum voltage and continuous current that the internal ESD diodes can sustain. However, these structures are not meant to protect the device for longer EOS events that may occur during a circuit fault condition. Instead, external circuit clamps may be needed to protect the op amp input circuits from EOS events. Schottky diodes and series resistors are one cay to help protect the op-amp inputs from overvoltage faults.

Let’s consider the ±10-V analog input module circuit shown in Figure 2. In this circuit, op-amp buffers provide a high input impedance to interface with a variety of sensors. The THP210 fully differential amplifier (FDA) attenuates and level-shifts the buffered signal to drive the analog-to-digital converter. The 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 This high-impedance ±10-V analog input module front end uses Schottky diodes and other elements to protect the op amp from EOS events.

Two types of protection circuit are shown in this example, with the clamp circuits designed to provide input protection for a ±40-V continuous overvoltage fault. The transient voltage suppression (TVS) diodes are used to clamp the power rails, sinking the clamp circuit current to keep the supplies below the op amp’s ±20-V absolute supply rating. TVS diodes are like Zener diodes, but designed for fast, large transient power dissipation. The SMF12A shown is a unidirectional TVS with a reverse standoff voltage of 12 V, a breakdown voltage of 14.7 V and a maximum clamping voltage of 19.9 V. The current during the ±40-V fault is limited to 20 mA using the 1.24-kΩ, 1/2-W RLIMIT resistor, as shown in Figure 3.

Schottky diodes, used here on the op amp inputs, have a metal semiconductor junction that offers a lower forward voltage drop than silicon junction diodes such as those used for ESD protection in the op amp. Figure 3 details how this attribute of the external protection clamp circuit works in conjunction with those internal ESD diodes.

In this example, the BAS40 is a small-signal Schottky diode with a forward voltage close to ~380 mV at 1 mA. In comparison, the internal ESD structure has a forward voltage of ~550 mV at the same forward current. Therefore, the Schottky diodes turn on before the amplifier’s internal ESD diodes, and most of the in-rush current flows through the external clamp. The internal ESD structure can only withstand 10 mA, while the external Schottky diode can handle forward continuous currents up to 200 mA, providing strong protection.

Figure 3 This commonly used op-amp input protection Schottky diode clamp turns on before the internal diodes, routing most of the in-rush current through the external diode.

While the external Schottky diode clamp provides robust overvoltage protection, however, a disadvantage of this clamp is that it introduces signal errors. During normal operation, the reverse-biased Schottky diodes exhibit a reverse leakage current that flows through the RLIMIT resistor, producing undesired offsets. The BAS40 used in the example offers a very low leakage current of 200 nA, keeping the offset error to a minimum. You can also choose to reduce the RLIMIT resistor to minimize these offset errors, with the trade-off being an increase in fault current. This increase in fault current will require a resistor with a higher power rating.

However, the diode leakage current can change slightly as a function of reverse voltage; therefore, the mismatch of reverse leakage current between the diodes causes small nonlinear errors as a function of input voltage. Furthermore, the leakage current of the diodes increases exponentially over temperature. For example, the typical leakage current for this type of Schottky is approximately ~20nA at 25⁰C; but, this leakage current can increase to 2µA at 85⁰C and to 10µA at temperatures above 100⁰C.

Fortunately, some modern precision op amps offer integrated input overvoltage protection, eliminating the need for this type of external clamp circuit. Figure 4 shows the integrated input protection of the OPA2206. Its inputs are protected for voltages up to ±40 V beyond either supply, or to ±40 V if the supplies are turned off.

Figure 4 This integrated op-amp input protection clamp changes impedance as the input gets overloaded, providing protection during EOS while minimizing impact during normal operation.

The OPA2206’s internal protection circuitry provides low series impedance under normal signal conditions, thus maintaining the desired op-amp precision. If the input is overloaded, however, the protection circuitry increases the series impedance and limits the input current to a value of approximately ±5 mA. Therefore, the integrated input protection clamp enables you to obtain accurate results with reliable protection, while reducing cost and shrinking solution size.

Overvoltage protection is an extensive subject, and the approaches shown are just some of many different ways to protect the inputs of an op amp. For more, check out the TI Precision Labs – Electrical Overstress video series. This series presents a detailed overview of EOS op-amp protection and how to design the proper clamp circuit for your application.

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