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THS4631 Datasheet(PDF) 10 Page - Texas Instruments |
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THS4631 Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 27 page  www.ti.com 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 (1) F *3dB + G B P 2 p R F CS ) CF (2) 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 SLOS451A – DECEMBER 2004 – REVISED MARCH 2005 Once the optimal feedback capacitor has been selec- ted, the transimpedance bandwidth can be calculated with Equation 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 corre- several distinct capacitances. spond 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 CD is the diode capacitance. in the laboratory. CP is the parasitic capacitance at the inverting node. Table 1. Transimpedance Performance Summary for Various Configurations The feedback capacitor provides a pole in the noise gain of the circuit, counteracting the zero in the noise SOURCE TRANS- FEEDBACK -3 dB CAPACITANCE IMPEDANCE CAPACITANCE FREQUENCY gain caused by the source capacitance. The pole is (PF) GAIN ( Ω) (PF) (MHZ) set such that the noise gain achieves a 20-dB per 18 10 k 2 15.8 decade rate-of-closure with the open-loop gain re- 18 100 k 0.5 3 sponse of the amplifier, resulting in a stable circuit. 18 1 M 0 1.2 As indicated, the formula given provides the feedback 47 10 k 2.2 8.4 capacitance for maximized flat bandwidth. Reduction in the value of the feedback capacitor can increase 47 100 k 0.7 2.1 the signal bandwidth, but this occurs at the expense 47 1 M 0.2 0.52 of peaking in the ac response. 100 10 k 3 5.5 100 100 k 1 1.4 100 1 M 0.2 0.37 10 |
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