Datasheet LT6600-15 (Analog Devices) - 8

LT6600-15
APPLICATIONS INFORMATION
In Figure 3 the LT6600-15 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 LT6600-15 with a network analyzer. Figure 5 the passband fl atness near 15MHz. The common mode is a laboratory setup that can be used to characterize the output voltage is set to 2V. LT6600-15 using single-ended instruments with 50Ω source impedance and 50Ω input impedance. For a unity Use Figure 4 to determine the interface between the gain confi guration the LT6600-15 requires a 536Ω source LT6600-15 and a current output DAC. The gain, or “tran- resistance yet the network analyzer output is calibrated simpedance,” is defi ned as A = VOUT/IIN. To compute the for a 50Ω load resistance. The 1:1 transformer, 52.3Ω transimpedance, use the following equation: and 523Ω resistors satisfy the two constraints above. A = 536 •R1 ( ) The transformer converts the single-ended source into a R1 ( +R2) Ω differential stimulus. Similarly, the output of the LT6600-15 will have lower distortion with larger load resistance yet By setting R1 + R2 = 536Ω, 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, Figure 5, present the output of the LT6600-15 with a 1600Ω R2, the voltage on Pin 7 and the DAC output current. differential load, or the equivalent of 800Ω to ground at Consider Figure 4 with R1 = 49.9Ω and R2 = 487Ω. The each output. The impedance seen by the network analyzer voltage at Pin 7 is 1.65V. The voltage at the DAC pins is input is still 50Ω, reducing refl ections in the cabling be- given by: tween the transformer and analyzer input. R1 R1• R2 V
Differential and Common Mode Voltage Ranges
DAC = VPIN7 • +I R1+ R2 + 536 IN • R1+ R2 The differential amplifi ers inside the LT6600-15 contain = 77mV +IIN • 45.3Ω circuitry to limit the maximum peak-to-peak differential I + – voltage through the fi lter. This limiting function prevents IN is IIN or IIN . The transimpedance in this example is 49.8Ω. excessive power dissipation in the internal circuitry and provides output short-circuit protection. The limiting
Evaluating the LT6600-15
function begins to take effect at output signal levels above 2V The low impedance levels and high frequency operation P-P and it becomes noticeable above 3.5VP-P. This is illustrated in Figure 6; the LT6600-15 was confi gured with of the LT6600-15 require some attention to the matching unity passband gain and the input of the fi lter was driven networks between the LT6600-15 and other devices. The with a 1MHz signal. Because this voltage limiting takes previous examples assume an ideal (0Ω) source impedance place well before the output stage of the fi lter reaches the and a large (1kΩ) load resistance. Among practical ex- 2.5V 0.1μF CURRENT 3.3V OUTPUT COILCRAFT COILCRAFT NETWORK NETWORK 0.1μF DAC TTWB-1010 TTWB-16A ANALYZER ANALYZER 3 SOURCE 1:1 523Ω 1 4:1 INPUT – 4 402Ω – 3 I R2 7 + IN 1 – 50Ω + LT6600-15 7 + 4 VOUT 52.3Ω 2 50Ω R1 0.011μF 8 – 2 LT6600-15 + 5 402Ω I + IN 8 – – 523Ω + V 6 5 OUT 0.1μF 660015 F05 R2 6 R1 660015 F04 –2.5V
Figure 4 Figure 5
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