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OPA657 Datasheet(PDF) 11 Page - Texas Instruments |
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OPA657 Datasheet(HTML) 11 Page - Texas Instruments |
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11 / 35 page  65 70 75 80 85 10 k 100 k 1 M 10 M 1 G f − Frequency − Hz VS = ±15 V RL = 1 k RF = 10 k CS = 100 PF CF = 3 PF CS = 18 PF CF = 2 PF CS = 47 PF CF = 2.2 PF 50 W 50 W RS VS C1 C2 IO Network Analizer I O V S (s) + 1 2 R S 1 ) C1 C2 (Above the Pole Frequency) I O V S (s) + s 2 R S 1) C1 C2 s ) 1 2 R S C1)C2 85 90 95 100 105 10 k 100 k 1 M 10 M 1 G f − Frequency − Hz VS = ±15 V RL = 1 k RF = 100 k CS = 47 PF CF = 0.7 PF CS = 100 PF CF = 1 PF CS = 18 PF CF = 0.5 PF 1 2 R S (C1 ) C2) 1 2 R S 1 ) C1 C2 95 100 110 120 10 k 100 k 1 M 10 M f − Frequency − Hz 125 VS = ±15 V RL = 1 k RF = 1 M 115 105 CS = 18 PF CF = 0 PF CS = 47 PF CF = 0.2 PF CS = 100 PF CF = 0.2 PF Z O(s) + C1 ) C2 C1 C2 s ) 1 2 R S C1)C2 s s ) 1 2 R S C1 Z O [ 1 s C2 THS4631 www.ti.com SLOS451B – DECEMBER 2004 – REVISED AUGUST 2011 Table 1. Transimpedance Performance Summary is difficult to measure the frequency response with for Various Configurations (continued) traditional laboratory equipment because the circuit requires a current as an input rather than a voltage. 10-k Ω TRANSIMPEDANCE RESPONSES Also, the capacitance of the current source has a direct effect on the frequency response. A simple interface circuit can be used to emulate a capacitive current source with a network analyzer. With this circuit, trans- impedance bandwidth measurements are simplified, making amplifier evaluation easier and faster. Figure 35. A. The interface network creates a capacitive, 100-k Ω TRANSIMPEDANCE RESPONSES constant current source from a network analyzer and properly terminates the network analyzer at high frequencies. Figure 38. Emulating a Capacitive Current Source With a Network Analyzer The transconductance transfer function of the interface circuit is: (3) The transfer function contains a zero at dc and a pole Figure 36. 1-M Ω TRANSIMPEDANCE RESPONSES at: . The transconductance is constant at: , above the pole frequency, providing a controllable ac-current source. This circuit also properly terminates the network analyzer with 50 Ω at high frequencies. The second requirement for this current source is to provide the desired output impedance, emulating the output impedance of a photodiode or other current source. The output impedance of this circuit is given by: Figure 37. (4) Assuming C1 >> C2, the equation reduces to: MEASURING TRANSIMPEDANCE BANDWIDTH , giving the appearance of a capacitive While there is no substitute for measuring the source at a higher frequency. performance of a particular circuit under the exact conditions that are used in the application, the Capacitor values should be chosen to satisfy two complete system environment often makes requirements. First, C2 represents the anticipated measurements harder. For transimpedance circuits, it capacitance of the true source. Second C1 is chosen Copyright © 2004–2011, Texas Instruments Incorporated 11 |
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