Figure 7-5 shows the open-loop gain and phase response of the OPA817. The GBWP of an op amp is measured in the 20-dB/decade constant slope region of the A_OL magnitude plot. The open-loop gain of 60 dB for the OPA817 is along this 20-dB/decade slope, and the corresponding frequency intercept is at 400 kHz. Converting 60 dB to linear units (1000 V/V) and multiplying by the 400-kHz frequency intercept gives a GBWP of 400 MHz for the OPA817. The A_OL Bode plot shows that the second pole in the A_OL response occurs after A_OL magnitude drops to less than 0 dB (1 V/V). The location of second pole results in phase change of less than 180° at 0 dB A_OL, indicating that the amplifier is stable in a gain of 1 V/V. Amplifiers such as the OPA817 that are JFET-input, low noise and unity-gain stable can be used as high-input-impedance buffers and gain stages with minimal degradation in SNR. The device has 800 MHz of SSBW in a gain-of-1-V/V configuration with a phase margin of approximately 55°.

The low input offset voltage and offset voltage drift of OPA817 makes the device an excellent amplifier for high-precision, high-input-impedance, wideband, data-acquisition-system front-ends. Figure 8-2 shows that the system benefits from the low noise JFET input stage with pico-amperes of input bias current to achieve higher precision at 1-MΩ input impedance setting and higher SNR at 50-Ω input impedance setting simultaneously in a typical data-acquisition front-end circuit.

RL = 100 Ω

Figure 7-5 Open-Loop Gain Magnitude and Phase vs Frequency

RL = 100 Ω

Figure 7-6 Open-Loop Gain Magnitude vs Temperature