The OPA3S2859 features integrated switches that can be used for implementing different gain configurations. The closed-loop bandwidth and noise of a TIA are affected by the transimpedance gain and photodiode capacitance. The OPA3S2859 has a higher bandwidth in its low-gain configuration for a given value of photodiode capacitance compared to the high-gain configuration. Increasing the gain of the TIA stage by a factor of X increases the output signal by a factor X, but the noise contribution from the resistor only increases by √X. The input-referred noise density of the low-gain configuration is therefore higher than the input-referred noise density of the high-gain configuration.

OPA3S2859 provides control for switching among three independently-configured external feedback networks using FB_x0, FB_x1, FB_x2 pins, and allows for up to four selectable gain configurations with an additional parallel non-switched feedback path. The internal switches minimize parasitic contributions to increase performance compared to external methods. Each switch is optimized for increasing feedback resistor values ranging from < 1 kΩ to > 100 kΩ for wide dynamic range applications. The selected switch path is controlled for both channels using a 2-wire parallel interface (SEL0 and SEL1).

In many systems it is typical that gain will switch sequentially (also known as adjacent gain switching). For example, the gain will switch low to medium to high or high to medium to low. When switching between adjacent gains, the switches feature make-before-break switching. When programmed to a different connection, the previous switch does not change to high impedance state until the new switch is closed (with a typical 80 ns to 230 ns delay when both switches are closed). This feature helps the amplifier from not operating in an open-loop state when the switches are used in a switched-gain transimpedance configuration.