Datasheet LT6600-5 (Analog Devices) - 8

LT6600-5
APPLICATIONS INFORMATION
the passband fl atness near 5MHz. The common mode Figure 5 is a laboratory setup that can be used to character- output voltage is set to 2V. ize the LT6600-5 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 LT6600-5 requires a 806Ω source LT6600-5 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 Ω. To compute for a 50Ω load resistance. The 1:1 transformer, 51.1Ω the 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 R1+ R2 differential stimulus. Similarly, the output the LT6600-5 will have lower distortion with larger load resistance yet By setting R1 + R2 = 806Ω, the gain equation reduces the analyzer input is typically 50Ω. The 4:1 turns (16:1 to A = R1Ω. 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 LT6600-5 with a 1600Ω R2, the voltage on Pin 7 and the DAC output current differential load, or the equivalent of 800Ω to ground at (I + – IN or IIN ). Consider Figure 4 with R1 = 49.9Ω and R2 each output. The impedance seen by the network analyzer = 750Ω. The voltage at Pin 7 is 1.65V. The voltage at the input is still 50Ω, reducing refl ections in the cabling be- DAC pins is given by: tween the transformer and analyzer input. R1 R1• R2 2.5V V = V • +I DAC PIN7 0.1μF R1+ R2 + 806 IN R1+ R2 COILCRAFT COILCRAFT = NETWORK NETWORK TTWB-1010 TTWB-16A 51mV +I 46.8Ω IN ANALYZER ANALYZER 3 SOURCE 1:1 787Ω 1 4:1 INPUT – 4 402Ω 7 + I – + IN is IIN or IIN . The transimpedance in this example is 50Ω LT6600-5 51.1Ω 2 50Ω 50.3Ω. 402Ω 8 – + 5 787Ω 6 0.1μF 66005 F05 CURRENT 3.3V OUTPUT 0.1μF DAC –2.5V – 3 IIN R2 1 – 4
Figure 5
7 + V + OUT R1 0.01μF 2 LT6600-5
Differential and Common Mode Voltage Ranges
I + R2 – IN 8 – + VOUT 5 The differential amplifi ers inside the LT6600-5 contain 6 R1 circuitry to limit the maximum peak-to-peak differential 66005 F04 voltage through the fi lter. This limiting function prevents
Figure 4
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
Evaluating the LT6600-5
2VP-P and it becomes noticeable above 3.5VP-P. This is The low impedance levels and high frequency operation illustrated in Figure 6; the LTC6600-5 was confi gured with of the LT6600-5 require some attention to the matching unity passband gain and the input of the fi lter was driven networks between the LT6600-5 and other devices. The with a 1MHz signal. Because this voltage limiting takes previous examples assume an ideal (0Ω) source imped- place well before the output stage of the fi lter reaches the ance and a large (1kΩ) load resistance. Among practi- supply rails, the input/output behavior of the IC shown cal examples where impedance must be considered is in Figure 6 is relatively independent of the power supply the evaluation of the LT6600-5 with a network analyzer. voltage. 66005fb 8