Datasheet LT6604-5 (Analog Devices) - 10

LT6604-5
APPLICATIONS INFORMATION
2.5V CURRENT 3.3V 0.1μF OUTPUT 0.1μF DAC COILCRAFT COILCRAFT NETWORK NETWORK TTWB-1010 TTWB-16A ANALYZER ANALYZER 25 25 1:1 787Ω 4:1 SOURCE 4 INPUT I – R2 402Ω IN 4 – 27 – 34 1/2 + 34 1/2 + 27 V + OUT 50Ω LT6604-5 R1 LT6604-5 51.1Ω 6 50Ω 0.01μF 6 2 – I + IN 2 – – + 29 + VOUT 29 402Ω 787Ω 7 0.1μF 66045 F04 R2 R1 7 66045 F05 V + – OUT – VOUT 806 • R1 = I + – IN – IIN R1 + R2 –2.5V
Figure 4 Figure 5
Use Figure 4 to determine the interface between the gain confi guration the LT6604-5 requires an 806Ω source LT6604-5 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, 51.1Ω transimpedance, use the following equation: and 787Ω resistors satisfy the two constraints above. The transformer converts the single-ended source into a A = 806 •R1Ω differential stimulus. Similarly, the output of the LT6604-5 R1+R2 will have lower distortion with larger load resistance yet By setting R1 + R2 = 806Ω, the gain equation reduces to A the analyzer input is typically 50Ω. The 4:1 turns (16:1 = R1(Ω). The voltage at the pins of the DAC is determined impedance) transformer and the two 402Ω resistors of by R1, R2, the voltage on V Figure 5, present the output of the LT6604-5 with a 1600Ω MID and the DAC output current. Consider Figure 4 with R1 = 49.9Ω and R2 = 750Ω. The differential load, or the equivalent of 800Ω to ground at voltage at V each output. The impedance seen by the network analyzer MID, for VS = 3.3V, 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 = VMID • +I R1+R2 +806 IN R1+R2
Differential and Common Mode Voltage Ranges
= 51mV +I IN 46.8Ω The differential amplifi ers inside the LT6604-5 contain circuitry to limit the maximum peak-to-peak differential
Evaluating the LT6604-5
voltage through the fi lter. This limiting function prevents The low impedance levels and high frequency operation excessive power dissipation in the internal circuitry and of the LT6604-5 require some attention to the matching provides output short-circuit protection. The limiting networks between the LT6604-5 and other devices. The function begins to take effect at output signal levels previous examples assume an ideal (0Ω) source impedance above 2VP-P and it becomes noticeable above 3.5VP-P. and a large (1k) load resistance. Among practical examples This is illustrated in Figure 6; the LT6604-5 channel was where impedance must be considered is the evaluation of confi gured with unity passband gain and the input of the the LT6604-5 with a network analyzer. fi lter was driven with a 1MHz signal. Because this voltage limiting takes place well before the output stage of the Figure 5 is a laboratory setup that can be used to character- fi lter reaches the supply rails, the input/output behavior ize the LT6604-5 using single-ended instruments with 50Ω of the IC shown in Figure 6 is relatively independent of source impedance and 50Ω input impedance. For a unity the power supply voltage. 66045fa 10