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THS4631DGNR Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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THS4631DGNR Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 34 page  _ + RF CF λ −V(Bias) RL Photodiode Circuit THS4631 www.ti.com SLOS451B – DECEMBER 2004 – REVISED AUGUST 2011 APPLICATION INFORMATION The large gain-bandwidth product of the THS4631 INTRODUCTION provides the capability for simultaneously achieving both high-transimpedance gain, wide bandwidth, high The THS4631 is a high-speed, FET-input operational slw rate, and low noise. In addition, the high-power amplifier. The combination of: high gain bandwidth supply rails provide the potential for a very wide product of 210 MHz, high slew rate of 1000 V/ µs, and dynamic range at the output, allowing for the use of trimmed dc precision makes the device an excellent input sources which possess wide dynamic range. design option for a wide variety of applications, The combination of these characteristics makes the including test and measurement, optical monitoring, THS4631 a design option for systems that require transimpedance gain circuits, and high-impedance transimpedance amplification of wideband, low-level buffers. The applications section of the data sheet input signals. A standard transimpedance circuit is discusses these particular applications in addition to shown in Figure 32. general information about the device and its features TRANSIMPEDANCE FUNDAMENTALS FET-input amplifiers are often used in transimpedance applications because of their extremely high input impedance. A transimpedance block accepts a current as an input and converts this current to a voltage at the output. The high-input impedance associated with FET-input amplifiers minimizes errors in this process caused by the input bias currents, IIB, of the amplifier. DESIGNING THE TRANSIMPEDANCE Figure 32. Wideband Photodiode CIRCUIT Transimpedance Amplifier Typically, design of a transimpedance circuit is driven by the characteristics of the current source that As indicated, the current source typically sets the provides the input to the gain block. A photodiode is requirements for gain, speed, and dynamic range of the most common example of a capacitive current the amplifier. For a given amplifier and source source that interfaces with a transimpedance gain combination, achievable performance is dictated by block. Continuing with the photodiode example, the the following parameters: the amplifier system designer traditionally chooses a photodiode gain-bandwidth product, the amplifier input based on two opposing criteria: speed and sensitivity. capacitance, the source capacitance, the Faster photodiodes cause a need for faster gain transimpedance gain, the amplifier slew rate, and the stages, and more sensitive photodiodes require amplifier output swing. From this information, the higher gains in order to develop appreciable signal optimal performance of a transimpedance circuit levels at the output of the gain stage. using a given amplifier is determined. Optimal is These parameters affect the design of the defined here as providing the required transimpedance circuit in a few ways. First, the speed transimpedance gain with a maximized flat frequency of the photodiode signal determines the required response. bandwidth of the gain circuit. Second, the required For the circuit shown in Figure 32, all but one of the gain, based on the sensitivity of the photodiode, limits design parameters is known; the feedback capacitor the bandwidth of the circuit. Third, the larger (CF) must be determined. Proper selection of the capacitance associated with a more sensitive signal feedback capacitor prevents an unstable design, source also detracts from the achievable speed of the controls pulse response characteristics, provides gain block. The dynamic range of the input signal maximized flat transimpedance bandwidth, and limits also places requirements on the amplifier dynamic broadband integrated noise. The maximized flat range. Knowledge of the source output current levels, frequency response results with CF calculated as coupled with a desired voltage swing on the output, shown in Equation 1, where CF is the feedback dictates the value of the feedback resistor, RF. The capacitor, RF is the feedback resistor, CS is the total transfer function from input to output is VOUT = IINRF. source capacitance (including amplifier input capacitance and parasitic capacitance at the inverting node), and GBP is the gain-bandwidth product of the amplifier in hertz. Copyright © 2004–2011, Texas Instruments Incorporated 9 |
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