Hello. And welcome to the next video in the EOS on ADC TI Precision Lab Series. In the last video, we saw that using a resistor and Schottky key diode was an effective method for protecting an ADC. But the additional components impacted the performance of the system. 

Furthermore, the current limiting resistor required a high power rating, which is inconvenient due to cost and a large PCB footprint. In this video, we will show an improved solution that uses a current limiting resistor with a lower power rating that also improves AC performance. 

As you can see on the slide, the improved solution places the protection resistor within the amplifier's feedback loop. Under normal operating conditions, the output resistance of the improved configuration is the current limiting resistor, Rp, divided by 1 plus beta times Aol. Since Aol is very large, the effective impedance is substantially reduced, and even large protection resistors do not impact the ADC settling behavior. 

Additionally, under fault conditions, the resistor will still limit the total current that the amplifier can drive. The capacitor, Cp, acts as an AC short across the protection resistor at higher frequencies to minimize the impact of the resistor as Aol rolls off with frequency. 

This slide shows the ADC settling TINA simulation for the circuit with the protection resistor inside the feedback loop. Notice that the settling error is less than one half of 1 LSB, which is within our error budget. Furthermore, note that the protection resistor is 1 kiloohm, whereas the protection resistor in the previous circuit was 249 ohms. 

The key point to take away here is that not only does the circuit have improved settling performance, but it also has better current limiting capabilities. On the next slide, we'll check the measured performance. 

Here is the measured performance for the improved solution, and it is actually better than the typical ADC specifications. Notice that the FFT shows relatively low noise and minimal harmonics. Additionally, see the oscilloscope wave form that shows the clamp signal when an over-voltage is applied. In general, you should use the improved solution as shown in this video. 

However, some amplifiers do not provide access to the feedback loop. For example, in the case of an instrumentation amplifier, the feedback resistors are inside the device, and you do not have access to them, so the original method must be used. The improved solution is a good way to improve system performance as well as the current limiting protection for fault conditions. 

It's important to note that adding output impedance inside the amplifier's feedback loop can affect the stability of the amplifier. Thus, when using this method, it is important to do a stability test for your solution. For a more detailed overview, please see the Precision Lab Series on op-amp stability. 

Shown on the left is the standard circuit used to test stability. It is essentially the amplifier in an open-loop configuration with an injected test signal. These stability curves are generated using post-processing on the Vfb, Vo, and Vout test probes in the circuit. The equations used for post-processing are shown below the circuit. They include Aol, 1 over beta, and Aol times beta. 

Shown on the right are the simulation results for the stability test. The key takeaway from the simulation is the point at which the Aol and 1 over beta curve intersect. At this point, we identified the phase margin on the Aol times beta curve. This design has 70 degrees of phase margin, which indicates that the circuit is very stable. 

Generally, if the circuit has more than 45 degrees of phase margin, it is considered to be stable. As mentioned previously, this slide shows a very brief summary of op-amp stability. And to learn more, please consult the Precision Lab Series. 

The last slide of this video shows the simulated open- and closed-loop output impedance for the improved circuit. For the open-loop simulation, the large 1 terahenry and 1 terafarad capacitor is used to break the feedback loop, which results in an open circuit. The open-loop circuit measured output impedance is the Rp and Cp parallel combination in series with the amplifier's output impedance. 

At low frequencies, the output impedance is dominated by the 1 kiloohm resistor. However, at higher frequencies, the 1 kiloohm resistor is shorted by the capacitor, which results in the amplifier's output impedance becoming dominant. 

The closed-loop output impedance curve show that closing the loop around the Rp and Cp parallel combination reduces the output impedance substantially. Recall from a previous slide that the open-loop output impedance is converted to a closed-loop output impedance by dividing 1 by 1 plus Aol times beta. 

At low frequencies, Aol times beta is very high, so the closed-loop output impedance is very low. This effect is very helpful as it allows us to effectively drive our load without seeing the large 1 kiloohm resistance. During a fault condition, however, the 1 kiloohm resistor will still limit the current. That concludes this video. Thank you for watching. 

Please try the quiz to check your understanding of this video's content. Question 1-- for the circuits below, what is the voltage delivered to the load? The correct answer is C. Vout1 is equal to 4 volts, and Vout2 is equal to 2 volts. For the circuit on the left, Rp is inside the feedback loop, so the voltage delivered to the load is 4 volts. 

To understand why this is true, remember that the virtual short between the amplifier's inputs will effectively short 4 volts to the output. For the circuit on the right, the amplifier's output is 4 volts. But it's divided across both resistors, so the load only gets half of the output signal. 

Question 2-- for the circuits below, what is the current delivered to the load? The correct answer is C. Iout1 is equal to 12 milliamps, and Iout2 is equal to 60 milliamps. In this case, the very low output resistance is effectively a short on the output. The amplifier on the left's output current is limited by Rp, and most of the 12 volt output will drop across the 1 kiloohm Rp. 12 volts over 1 kiloohms results in 12 milliamps. 

For the other amplifier, the output would ideally be 12 volts across the 100 ohm resistor, or 120 milliamps. However, the amplifier's output is limited by the short-circuit output protection. Looking at the curve on the right, the output is limited to be about 60 milliamps at 25 degrees Celsius.