The following bullets list the considerations for this design example:

• Input Impedance: The JFET-input stage of the OPA814 offers gigaohms of input impedance, and therefore enables the front-end to be terminated with a 1-MΩ resistor while achieving excellent precision. A 50-Ω resistance can also be switched in, offering matched termination for high-frequency signals. Thus, the OPA814 enables the designer to use both 1-MΩ and 50-Ω termination in the same signal chain.
• Noise: The total noise of the front-end amplifier is a function of the voltage and current noise of the OPA814, input termination, and the resistors thermal noise. However, in 50-Ω mode, the dominant noise source is contributed by the voltage noise of the OPA814 due to the presence of voltage noise across the complete bandwidth. Therefore, the total RMS noise of the front-end amplifier is approximately equal to the voltage noise of the OPA814 over 180 MHz.

  The specified input-referred voltage noise of the OPA814 is 5.3 nV/√Hz; see also Section 6.5. The total integrated RMS noise at the input in a bandwidth of 180 MHz is given by the following equation:

  Equation 2. ${\mathrm{E}}_{\mathrm{N}\mathrm{R}\mathrm{M}\mathrm{S}}=5.3\mathrm{ }\mathrm{n}\mathrm{V}/\surd \mathrm{H}\mathrm{z}\mathrm{ }\times \surd \left(180\mathrm{ }\mathrm{M}\mathrm{H}\mathrm{z}\mathrm{ }\times 1.57\right)\mathrm{ }=90\mathrm{ }\mathrm{\mu }{\mathrm{V}}_{\mathrm{R}\mathrm{M}\mathrm{S}}$

  The brickwall correction factor of 1.57 is applied, assuming the bandwidth is limited to 180 MHz with a single-pole RC filter before digitizing the signal with the ADC. Detailed calculations are found at TI Precision Labs – Op Amp Noise: Spectral Density.

• Optimizing Overshoot: The OPA814 features an internal slew-boost circuit to deliver fast rise-time in applications that require high slew rates, such as when configured as a transimpedance amplifier. For applications where overshoot must be limited, limit the input slew rates by introducing a series resistance (R_S); see also Figure 8-4. Resistor R_S forms a low-pass filter with an input capacitance of approximately 2.5 pF at the noninverting pin of the OPA814, thus limiting the input slew rate to the amplifier. Figure 8-5 shows how limiting the input slew rate to the amplifier results in good overshoot performance. Figure 8-6 shows how this configuration achieves a small-signal and large-signal bandwidth of 180 MHz.