Datasheet LT6600-2.5 (Analog Devices) - 9

LT6600-2.5
APPLICATIONS INFORMATION
Use Figure 4 to determine the interface between the Figure 5 is a laboratory setup that can be used to charac- LT6600-2.5 and a current output DAC. The gain, or “trans- terize the LT6600-2.5 using single-ended instruments impedance,” is defi ned as A = VOUT/IIN. To compute the with 50Ω source impedance and 50Ω input impedance. transimpedance, use the following equation: For a 12dB gain confi guration the LT6600-2.5 requires a 402Ω source resistance yet the network analyzer output is 1580 • R1 A = (Ω) calibrated for a 50Ω load resistance. The 1:1 transformer, R1 + R2 ( ) 53.6Ω and 388Ω resistors satisfy the two constraints By setting R1 + R2 = 1580Ω, the gain equation reduces above. The transformer converts the single-ended source to A = R1(Ω). into a differential stimulus. Similarly, the output of the LT6600-2.5 will have lower distortion with larger load The voltage at the pins of the DAC is determined by R1, resistance yet the analyzer input is typically 50Ω. The 4:1 R2, the voltage on VMID and the DAC output current. turns (16:1 impedance) transformer and the two 402Ω Consider Figure 4 with R1 = 49.9Ω and R2 = 1540Ω. The resistors of Figure 5, present the output of the LT6600-2.5 voltage at VMID, for VS = 3.3V, is 1.65V. The voltage at the with a 1600Ω differential load, or the equivalent of 800Ω DAC pins is given by: to ground at each output. The impedance seen by the R1 R1• R2 network analyzer input is still 50Ω, reducing refl ections in V • • DAC = VPIN7 +I the cabling between the transformer and analyzer input. R1+ R2 + 1580 IN R1+ R2 • 48.3 = 26mV +IIN Ω
Differential and Common Mode Voltage Ranges
I + – The rail-to-rail output stage of the LT6600-2.5 can process IN is IIN or IIN . The transimpedance in this example is 49.6Ω. large differential signal levels. On a 3V supply, the output signal can be 5.1VP-P. Similarly, a 5V supply can support
Evaluating the LT6600-2.5
signals as large as 8.8VP-P. To prevent excessive power dissipation in the internal circuitry, the user must limit The low impedance levels and high frequency operation differential signal levels to 9V of the LT6600-2.5 require some attention to the matching P-P. networks between the LT6600-2.5 and other devices. The The two amplifi ers inside the LT6600-2.5 have indepen- previous examples assume an ideal (0Ω) source imped- dent control of their output common mode voltage (see ance and a large (1kΩ) load resistance. Among practical the Block Diagram section). The following guidelines will examples where impedance must be considered is the optimize the performance of the fi lter. evaluation of the LT6600-2.5 with a network analyzer. 2.5V CURRENT 3.3V 0.1μF OUTPUT 0.1μF DAC COILCRAFT COILCRAFT NETWORK NETWORK TTWB-1010 TTWB-16A ANALYZER ANALYZER 3 SOURCE 1:1 388Ω 1 4:1 INPUT – 3 402Ω I R2 – IN 1 4 – 7 + + 7 + 4 VOUT 50Ω LT6600-2.5 R1 53.6Ω 2 50Ω 0.01μF 402Ω 2 LT6600-2.5 8 – I + + 5 IN 8 – – + V 5 OUT 388Ω 6 0.1μF 660025 F04 R2 6 R1 660025 F05 V + – OUT – VOUT 1580 • R1 = I + – IN – IIN R1 + R2 –2.5V
Figure 4. (S8 Pin Numbers) Figure 5. (S8 Pin Numbers)
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