Datasheet LT6600-20 (Analog Devices) - 8

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