VS+

VS

VIN+

VOUT

Power

Supply

ESD Cell

VIN

FB

Internal

Circuitry

Figure 7-3. Internal ESD Protection

7.3.2 Feedback Pin

For high-speed analog design, minimizing parasitic capacitance and inductance is critical to get the best

performance from a high-speed amplifier such as the OPA818. Parasitics are especially detrimental in the

feedback path and at the inverting input. Parasitics result in undesired poles and zeroes in the feedback that

potentially reduce phase margin or cause instability. Techniques used to correct for this phase margin reduction

often result in reduced application bandwidth. To keep system engineers from making these tradeoff choices and

to simplify the printed circuit board (PCB) layout, the OPA818 features a feedback pin (FB) pin on the same side

as the inverting input pin (IN–). Figure 7-4 shows how this configuration allows for a very short feedback resistor

(RF) connection between the FB and the IN– pins, thus minimizing parasitics with minimal PCB design effort.

Internally, the FB pin is connected to VOUT through metal routing on the silicon. As a result of the fixed metal

sizing of this connection, the FB pin has limited current carrying capability; therefore, adhere to Section 6.1 for

continuous operation.

R

F

1

2

3

4

8

7

6

5

+

PD

FB

IN

IN+

VS+

VS

NC

OUT

Figure 7-4. RF Connection Between FB and IN– Pins

7.3.3 Decompensated Architecture With Wide Gain-Bandwidth Product

Figure 7-5 shows the open-loop gain and phase response of the OPA818. The GBWP of an op amp is measured

in the 20-dB/decade constant slope region of the AOL magnitude plot. The open-loop gain of 60 dB for the

OPA818 is along this 20-dB/decade slope and the corresponding frequency intercept is at 2.7 MHz. Converting

60 dB to linear units (1000 V/V) and multiplying by the 2.7-MHz frequency intercept gives the GBWP of the

OPA818 as 2.7 GHz. The AOL Bode plot shows that the second pole in the AOL response occurs before the AOL

magnitude drops to less than 0 dB (1 V/V). This result shows a phase change of more than 180° at 0 dB AOL,

indicating that the amplifier can not be stable in a gain of 1 V/V. Amplifiers, such as the OPA818, that are not

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OPA818

SBOS940B – MAY 2019 – REVISED DECEMBER 2025

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Product Folder Links: OPA818