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LT6600-2.5 Datasheet(PDF) 7 Page - Linear Technology |
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LT6600-2.5 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LT6600-2.5 7 660025i APPLICATIO S I FOR ATIO In Figure 3 the LT6600-2.5 is providing 12dB of gain. The common mode output voltage is set to 2V. Use Figure 4 to determine the interface between the LT6600-2.5 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 R RR = + () Ω () 1580 1 12 • By setting R1 + R2 = 1580 Ω, 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 = 49.9 Ω and R2 = 1540Ω. The voltage at Pin 7 is 1.65V. The voltage at the DAC pins is given by: VV R RR I RR RR mV I DAC PIN IN IN = ++ + + =+ Ω 7 1 1 2 1580 12 12 26 48 3 •• • •. IIN is IIN+ or IIN–. The transimpedance in this example is 49.6 Ω. Evaluating the LT6600-2.5 The low impedance levels and high frequency operation of the LT6600-2.5 require some attention to the matching networks between the LT6600-2.5 and other devices. The previous examples assume an ideal (0 Ω) source imped- ance and a large (1k Ω) load resistance. Among practical examples where impedance must be considered is the evaluation of the LT6600-2.5 with a network analyzer. Figure 5 Figure 5 is a laboratory setup that can be used to charac- terize the LT6600-2.5 using single-ended instruments with 50 Ω source impedance and 50Ω input impedance. For a 12dB gain configuration the LT6600-2.5 requires a 402 Ω source resistance yet the network analyzer output is calibrated for a 50 Ω load resistance. The 1:1 transformer, 53.6 Ω and 388Ω resistors satisfy the two constraints above. The transformer converts the single-ended source into a differential stimulus. Similarly, the output of the LT6600-2.5 will have lower distortion with larger load resistance yet the analyzer input is typically 50 Ω. The 4:1 turns (16:1 impedance) transformer and the two 402 Ω resistors of Figure 5, present the output of the LT6600-2.5 with a 1600 Ω differential load, or the equivalent of 800Ω to ground at each output. The impedance seen by the network analyzer input is still 50 Ω, reducing reflections in the cabling between the transformer and analyzer input. Differential and Common Mode Voltage Ranges The rail-to-rail output stage of the LT6600-2.5 can process large differential signal levels. On a 3V supply, the output signal can be 5.1VP-P. Similarly, a 5V supply can support signals as large as 8.8VP-P. To prevent excessive power dissipation in the internal circuitry, the user must limit differential signal levels to 9VP-P. The two amplifiers inside the LT6600-2.5 have indepen- dent control of their output common mode voltage (see the “Block Diagram” section). The following guidelines will optimize the performance of the filter. Pin 7 can be allowed to float; Pin 7 must be bypassed to an AC ground with a 0.01 µF capacitor or some instability may be observed. Pin 7 can be driven from a low impedance Figure 4 – + 0.1 µF 3.3V – + LT6600-2.5 3 4 1 0.01 µF CURRENT OUTPUT DAC 7 2 8 5 VOUT + VOUT – 660025 F04 6 R2 R1 IIN – IIN + R2 R1 – + 0.1 µF 0.1 µF 2.5V – 2.5V – + LT6600-2.5 3 4 1 7 2 8 5 6 660025 F05 402 Ω 402 Ω NETWORK ANALYZER INPUT 50 Ω COILCRAFT TTWB-16A 4:1 NETWORK ANALYZER SOURCE COILCRAFT TTWB-1010 1:1 50 Ω 53.6 Ω 388 Ω 388 Ω |
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