SBOSAA5B April 2022 – September 2022 OPA2675
The 51 Ω input matching resistor is optional in this case. The input signal is ac-coupled to the 5 V dc reference voltage developed through the resistor divider from the +10 V power supply. This first stage acts as a gain of +1 V/V voltage buffer for the signal where the 600-Ω feedback resistor is required for stability. This first stage easily drives the low input resistors required at the input of this high-frequency filter. The second stage is set for a dc gain of +1 V/V, carrying the 5-V operating point through to the output pin, and an ac gain of +4 V/V. The feedback resistor has been adjusted to optimize bandwidth for the amplifier itself. As Figure 9-1 and Figure 9-2 show, the OPA2675 in this configuration gives greater than 400 MHz small-signal bandwidth. The capacitor values were chosen as low as possible but adequate to override the parasitic input capacitance of the amplifier. The resistor values were slightly adjusted to give the desired filter frequency response while accounting for the approximate 1 ns propagation delay through each channel of the OPA2675.