OPA354_16 Datasheet, PDF (22/44 Pages) Texas Instruments – 250-MHz, Rail-to-Rail I/O, CMOS Operational Amplifiers

OPA354, OPA2354, OPA4354
SBOS233F – MARCH 2002 – REVISED JUNE 2016
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C_(F) is optional to prevent gain peaking. C_(F) includes the stray capacitance of R_(F).
9.2.2 Detailed Design Procedure
To achieve a maximally-flat, second-order Butterworth frequency response, set the feedback pole using
Equation 3.
(3)
Calculate the bandwidth using Equation 4.
(4)
9.2.2.1 Optimizing the Transimpedance Circuit
To achieve the best performance, components must be selected according to the following guidelines:
1. For lowest noise, select R_(F) to create the total required gain. Using a lower value for R_(F) and adding gain after
the transimpedance amplifier generally produces poorer noise performance. The noise produced by R_(F)
increases with the square-root of R_(F), whereas the signal increases linearly. Therefore, signal-to-noise ratio
improves when all the required gain is placed in the transimpedance stage.
2. Minimize photodiode capacitance and stray capacitance at the summing junction (inverting input). This
capacitance causes the voltage noise of the op amp to be amplified (increasing amplification at high frequency).
Using a low-noise voltage source to reverse-bias a photodiode can significantly reduce the capacitance. Smaller
photodiodes have lower capacitance. Use optics to concentrate light on a small photodiode.
3. Noise increases with increased bandwidth. Limit the circuit bandwidth to only that required. Use a capacitor
across the R_(F) to limit bandwidth, even if not required for stability.
4. Circuit board leakage can degrade the performance of an otherwise well-designed amplifier. Clean the circuit
board carefully. A circuit board guard trace that encircles the summing junction and is driven at the same voltage
can help control leakage.
9.2.3 Application Curve
Figure 38. AC Transfer Function
22
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