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THS4631DGN Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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THS4631DGN Datasheet(HTML) 10 Page - National Semiconductor (TI) |
10 / 34 page C F + 1 p R F G B P ) 1 p R F G B P 2 ) 4C S p R F G B P 2 F *3dB + G B P 2 p R F CS ) CF Gain AOL 20 dB/Decade Rate-of-Closure GBP 20 dB/ Decade Noise Gain −20 dB/ Decade f Pole Zero 0 _ + CI(DIFF) RF CF CP CD I(DIODE) CI(CM) CS = CI(CM) + CI(DIFF) + CP + CD THS4631 SLOS451B – DECEMBER 2004 – REVISED AUGUST 2011 www.ti.com (1) Once the optimal feedback capacitor has been selected, the transimpedance bandwidth can be calculated with Equation 2. (2) Figure 34. Transimpedance Circuit Bode Plot The performance of the THS4631 has been measured for a variety of transimpedance gains with a variety of source capacitances. The achievable bandwidths of the various circuit configurations are summarized numerically in Table 1. The frequency responses are presented in Figure 35, Figure 36, and Figure 37. A. The total source capacitance is the sum of Note that the feedback capacitances do not several distinct capacitances. correspond exactly with the values predicted by the Figure 33. Transimpedance Analysis Circuit equation. They have been tuned to account for the parasitic capacitance of the feedback resistor (typically 0.2 pF for 0805 surface mount devices) as Where: well as the additional capacitance associated with the CI(CM) is the common-mode input capacitance. PC board. The equation should be used as a starting CI(DIFF) is the differential input capacitance. point for the design, with final values for CF optimized in the laboratory. CD is the diode capacitance. CP is the parasitic capacitance at the inverting node. Table 1. Transimpedance Performance Summary The feedback capacitor provides a pole in the noise for Various Configurations gain of the circuit, counteracting the zero in the noise gain caused by the source capacitance. The pole is SOURCE TRANS- FEEDBACK -3 dB CAPACITANCE IMPEDANCE CAPACITANCE FREQUENCY set such that the noise gain achieves a 20-dB per (PF) GAIN ( Ω) (PF) (MHZ) decade rate-of-closure with the open-loop gain 18 10 k 2 15.8 response of the amplifier, resulting in a stable circuit. 18 100 k 0.5 3 As indicated, the formula given provides the feedback 18 1 M 0 1.2 capacitance for maximized flat bandwidth. Reduction in the value of the feedback capacitor can increase 47 10 k 2.2 8.4 the signal bandwidth, but this occurs at the expense 47 100 k 0.7 2.1 of peaking in the ac response. 47 1 M 0.2 0.52 100 10 k 3 5.5 100 100 k 1 1.4 100 1 M 0.2 0.37 10 Copyright © 2004–2011, Texas Instruments Incorporated |
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