The OPA814 features a unique combination of high GBWP, low-input voltage noise, and the dc precision of a trimmed JFET-input stage to provide a high input impedance for a voltage-feedback amplifier. Figure 8-2 shows how the very high GBWP of 250 MHz and high large signal bandwidth of 200 MHz are used to either deliver wide signal bandwidths at high gains or to extend the achievable bandwidth or gain in typical high-speed, high-input-impedance data-acquisition front-end applications. To achieve the full performance of the OPA814, careful attention to the printed circuit board (PCB) layout and component selection is required, as discussed in the following sections of this data sheet. The OPA814 also features a wider supply range, thereby enabling a wider common-mode input range to support higher input-signal swings.

Figure 8-1 shows the noninverting gain of a +2‑V/V circuit used as the basis for most of the Typical Characteristics. Most of the curves are characterized using signal sources with 50-Ω driving impedance, and with measurement equipment presenting a 50-Ω load impedance. As Figure 8-1 shows, the 49.9-Ω shunt resistor at the V_IN terminal matches the source impedance of the test generator, while the 49.9-Ω series resistor at the V_O terminal provides a matching resistor for the measurement equipment load. Generally, data-sheet voltage-swing specifications are at the output pin (V_O in Figure 8-1); whereas, output power specifications are at the matched 50-Ω load. Figure 8-1 shows that the total 100-Ω load at the output combined with the 500-Ω total feedback network load presents the OPA814 with an effective output load of 83.3 Ω for the circuit.

Figure 8-1 Noninverting G = +2 V/V Configuration and Test Circuit

Figure 8-2 High Input Impedance DAQ Front-End

Voltage-feedback operational amplifiers, unlike current-feedback amplifiers, use a wide range of resistor values to set the gain. As Figure 8-1 shows, the parallel combination of R_F || R_G must always be kept to a lower value to retain a controlled frequency response for the noninverting voltage amplifier. In the noninverting configuration, the parallel combination of R_F || R_G form a pole with the parasitic input capacitance at the inverting node of the OPA814 (including layout parasitic capacitance). For best performance, this pole must be at a frequency greater than the closed-loop bandwidth for the OPA814.