8.3.1 Phase-Reversal Protection

The OPA4188 device has an internal phase-reversal protection. Many op amps exhibit a phase reversal when the input is driven beyond its linear common-mode range. This condition is most often encountered in noninverting circuits when the input is driven beyond the specified common-mode voltage range, causing the output to reverse into the opposite rail. The OPA4188 input prevents phase reversal with excessive common-mode voltage. Instead, the output limits into the appropriate rail. This performance is shown in Figure 39.

Figure 39. No Phase Reversal

8.3.2 Capacitive Load and Stability

The OPA4188 dynamic characteristics have been optimized for a range of common operating conditions. The combination of low closed-loop gain and high capacitive loads decreases the amplifier phase margin and can lead to gain peaking or oscillations. As a result, heavier capacitive loads must be isolated from the output. The simplest way to achieve this isolation is to add a small resistor (for example, R_OUT equal to 50 Ω) in series with the output. Figure 40 and Figure 41 illustrate graphs of small-signal overshoot versus capacitive load for several values of R_OUT. For details of analysis techniques and application circuits, see Feedback Plots Define Op Amp AC Performance, available for download from www.ti.com.

Figure 40. Small-Signal Overshoot versus Capacitive Load (100-mV Output Step)

Figure 41. Small-Signal Overshoot vs Capacitive Load (100-mV Output Step)

8.3.3 Electrical Overstress

Designers often ask questions about the capability of an operational amplifier to withstand electrical overstress. These questions tend to focus on the device inputs, but may involve the supply voltage pins or even the output pin. Each of these different pin functions have electrical stress limits determined by the voltage breakdown characteristics of the particular semiconductor fabrication process and specific circuits connected to the pin. Additionally, internal electrostatic discharge (ESD) protection is built into these circuits to protect them from accidental ESD events both before and during product assembly.

These ESD protection diodes also provide in-circuit, input overdrive protection, as long as the current is limited to 10 mA as stated in the Absolute Maximum Ratings. Figure 42 shows how a series input resistor may be added to the driven input to limit the input current. The added resistor contributes thermal noise at the amplifier input and its value should be kept to a minimum in noise-sensitive applications.

Figure 42. Input Current Protection

An ESD event produces a short-duration, high-voltage pulse that is transformed into a short-duration, high-current pulse as it discharges through a semiconductor device. The ESD protection circuits are designed to provide a current path around the operational amplifier core to prevent it from being damaged. The energy absorbed by the protection circuitry is then dissipated as heat.

When the operational amplifier connects into a circuit, the ESD protection components are intended to remain inactive and not become involved in the application circuit operation. However, circumstances may arise where an applied voltage exceeds the operating voltage range of a given pin. Should this condition occur, there is a risk that some of the internal ESD protection circuits may be biased on, and conduct current. Any such current flow occurs through ESD cells and rarely involves the absorption device.

If there is an uncertainty about the ability of the supply to absorb this current, external Zener diodes may be added to the supply pins. The Zener voltage must be selected such that the diode does not turn on during normal operation. However, its Zener voltage should be low enough so that the Zener diode conducts if the supply pin begins to rise above the safe operating supply voltage level.

8.3.4 EMI Rejection

The OPA4188 device uses integrated electromagnetic interference (EMI) filtering to reduce the effects of EMI interference from sources such as wireless communications and densely-populated boards with a mix of analog signal chain and digital components. EMI immunity can be improved with circuit design techniques; the OPA4188 benefits from these design improvements. Texas Instruments has developed the ability to accurately measure and quantify the immunity of an operational amplifier over a broad frequency spectrum extending from 10 MHz to 6 GHz. Figure 43 shows the results of this testing on the OPA4188 device. Detailed information can also be found in EMI Rejection Ratio of Operational Amplifiers available for download from www.ti.com.

Figure 43. EMIRR Testing