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LT6600-20 Datasheet(PDF) 8 Page - Linear Technology |
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LT6600-20 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LT6600-20 8 66002fb APPLICATIONS INFORMATION In Figure 3 the LT6600-20 is providing 12dB of gain. The gain resistor has an optional 62pF in parallel to improve the passband flatness near 20MHz. The common mode output voltage is set to 2V. Use Figure 4 to determine the interface between the LT6600-20 and a current output DAC. The gain, or “trans- impedance,” is defined as A = VOUT/IIN. To compute the transimpedance, use the following equation: A = 402 •R1 R1 +R2 () Ω () By setting R1 + R2 = 402Ω, the gain equation reduces to A = R1(Ω). The voltage at the pins of the DAC is determined by R1, R2, the voltage on Pin 7 and the DAC output current. Consider Figure 4 with R1 = 49.9Ω and R2 = 348Ω. The voltage at Pin 7 is 1.65V. The voltage at the DAC pins is given by: VDAC = VPIN7 • R1 R1 +R2+ 402 +IIN • R1•R2 R1 +R2 = 26mV +IIN • 48.3Ω IIN is IIN+ or IIN–. The transimpedance in this example is 50.4Ω. Evaluating the LT6600-20 The low impedance levels and high frequency operation ofthe LT6600-20 require some attention to the matching networks between the LT6600-20 and other devices. The previous examples assume an ideal (0Ω) source impedance and a large (1kΩ) load resistance. Among practical ex- amples where impedance must be considered is the evalu- ation of the LT6600-20 with a network analyzer. Figure 5 is a laboratory setup that can be used to characterize the LT6600-20 using single-ended instruments with 50 source impedance and 50Ω input impedance. For a unity gain configuration the LT6600-20 requires a 402Ω source resistance yet the network analyzer output is calibrated for a 50Ω load resistance. The 1:1 transformer, 53.6Ω and 388Ω resistors satisfy the two constraints above. The transformer converts the single-ended source into a differential stimulus. Similarly, the output of the LT6600-20 will have lower distortion with larger load resistance yet the analyzer input is typically 50Ω. The 4:1 turns (16:1 impedance) transformer and the two 402Ω resistors of Figure 5, present the output of the LT6600-20 with a 1600Ω differential load, or the equivalent of 800Ω to ground at each output. The impedance seen by the network analyzer input is still 50Ω, reducing reflections in the cabling be- tween the transformer and analyzer input. Differential and Common Mode Voltage Ranges The differential amplifiers inside the LT6600-20 contain circuitry to limit the maximum peak-to-peak differential voltage through the filter. This limiting function prevents excessive power dissipation in the internal circuitry and provides output short-circuit protection. The limiting function begins to take effect at output signal levels above 2VP-P and it becomes noticeable above 3.5VP-P. This is illustrated in Figure 6; the LT6600-20 was configured with unity passband gain and the input of the filter was driven with a 1MHz signal. Because this voltage limiting takes Figure 4 Figure 5 – + 0.1μF 3.3V – + LT6600-20 3 4 1 0.01μF CURRENT OUTPUT DAC 7 2 8 5 VOUT + VOUT – 66002 F04 6 R2 R1 IIN – IIN + R2 R1 – + 0.1μF 0.1μF 2.5V –2.5V – + LT6600-20 3 4 1 7 2 8 5 6 66002 F05 402Ω 402Ω NETWORK ANALYZER INPUT 50Ω COILCRAFT TTWB-16A 4:1 NETWORK ANALYZER SOURCE COILCRAFT TTWB-1010 1:1 50Ω 53.6Ω 388Ω 388Ω |
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