ZHCSQB4A May 2022 – August 2022 OPA3S2859
PRODUCTION DATA
The OPA3S2859 meets the growing demand for wideband, low-noise photodiode amplifiers. The closed-loop bandwidth of a transimpedance amplifier is a function of the following:
Figure 9-1 shows the OPA3S2859 configured as programmable gain TIA using different feedback paths through the switch network. The feedback resistance (RF) and the input capacitance (CIN) form a zero in the noise gain that results in instability if left unchecked. To counteract the effect of the zero, a pole is inserted into the noise gain transfer function by adding the feedback capacitor (CF). The Transimpedance Considerations for High-Speed Amplifiers Application Report application report discusses theories and equations that show how to compensate a transimpedance amplifier for a particular transimpedance gain and input capacitance. The bandwidth and compensation equations from the application report are available in an Excel® calculator. What You Need To Know About Transimpedance Amplifiers – Part 1 provides a link to the calculator.
The equations and calculators in the referenced application report and blog posts are used to model the bandwidth (f–3dB) and noise performance of the OPA3S2859 configured as a TIA. For this setup, to emulate an ideal current source, choose an RIN value that is 1 to 10x greater than RF so that the resulting low frequency noise gain is closer to 1 V/V than to 2 V/V (RF = 1 kΩ, 10 kΩ, or 100 kΩ, RIN = 10 kΩ, 100 kΩ, or 100 kΩ; respectively). Figure 9-2 shows the resultant performance. To maximize bandwidth, make sure to reduce any stray parasitic capacitance from the PCB. Increasing RF results in lower bandwidth. To maximize the signal-to-noise ratio (SNR) in an optical front-end system, maximize the gain in the TIA stage.