DEMO CIRCUIT DC1299 QUICK START GUIDE DEMO CIRCUIT 1299 LOW NOISE/DISTORTION DUAL DIFFERENTIAL ADC DRIVER QUICK START GUIDE LTC6420 and LTC6421 Low Noise/Distortion Dual Differential ADC Drivers DESCRIPTION Demonstration circuit 1299 features the LTC6420 and with almost no difference in distortion performance. Two LTC6421 Dual ADC Drivers. It incorporates a variety of pas- standout features of the LTC6420/1 are the ability to inde- sive components to support configurations for varied appli- pendently adjust the output common-mode voltage of both cations. Single-ended or differential input and output confi- amplifiers independently without any additional components gurations are possible, and there is a calibration circuit to and the ability to shut each amplifier down into a low-power null out the effects of other circuit components. state independently of the other. The LTC6420/1 is a high-speed dual differential amplifier Design files for this circuit board are available. Call with superior distortion and noise performance, perfect for the LTC factory. demanding communications transceivers, cellular base- , LTC and LT are registered trademarks of Linear Technology Corporation. stations, and other high-speed signal chain applications. The LTC6420/1 accepts single-ended or differential inputs QUICK START PROCEDURE Table 1: DC1299 Board Connector Descriptions Table 1 shows the function of each SMA connector on the CONNECTOR FUNCTION board. Refer to Figure 1 for the connection diagram and J1,J7 Board Inputs. Use this connector to supply an input to follow the procedure below: (+INA,+INB) the DC1299. Drive from a 50 signal source, no exter- 1. Connect the power supply as shown in Figure 1. The nal termination necessary. power labels of V+ and GND directly correspond to the J2 (-INA) Differential Input. Not connected by default. Capacitor V+, and V- pins of the LTC6420/1. Jumpers JP1 and JP2 C25 can be installed and C26 removed to drive this channel differentially. shut down the amplifiers within the LTC6420/1. J3,J8 Board Outputs. Impedance-matched to 50, can be 2. Apply input signals to J1 and J7. A network ana- (+OUTA,+OUTB) used to drive a network/spectrum analyzer input. lyzer output or RF signal source with an external bandpass filter will yield the best performance. J4 (-OUTA) Differential Output. Not connected by default. Capaci- tor C24 can be installed and C28 removed to receive DC1299s input is impedance-matched to 50 no this output differentially. external termination is necessary. J5 (-INB) Differential Input. Not connected by default. Capacitor C31 can be installed and C30 removed to drive this 3. Observe the outputs via J3 and J8. The outputs are im- channel differentially. pedance-matched to 50, suitable for the input of a net- J6 (-OUTB) Differential Output. Not connected by default. Capaci- work or spectrum analyzer. Note that the schematic cir- tor C35 can be installed and C33 removed to receive cuit includes impedance transformers and series resis- this output differentially. tance that lowers the gain of the overall circuit. VOCMA, Turrets can be driven from a DC voltage source to set VOCMB the input and output common-mode levels of the 4. The VOCMA and VOCMB turrets may be used to adjust LTC6420/1. Input impedance is approximately 600 the common-mode output voltages of the LTC6420/1. See the LTC6420/1 datasheet for more information. 1 DEMO CIRCUIT DC1299 QUICK START GUIDE LOW NOISE/DISTORTION DUAL DIFFERENTIAL ADC DRIVER Figure 1: Proper Measurement Equipment Setup ADDITIONAL INFORMATION Although the DC1299 demo board is ready to use out of the box, it has features that you can access by adding, re- DRIVING THE INPUTS WITH DC COUPLING moving or changing components on the board. These are It is possible to drive the DC1299 inputs differentially with described below. DC coupling. The input capacitors (C22/C25 and C31/C36) should be shorted or replaced with 0 resistors. As a DEMO BOARD VERSIONS warning, the low input impedance of the LTC6420/1 can DC1299 has versions to support the different options of the cause large input bias currents if DC1299 is driven DC LTC6420/1 family. DC1299A-A contains the LTC6420-20, coupled. If transformers T1/T3 are transmission-line trans- and DC1299A-B contains the LTC6421-20. See the sche- formers (the transformers included on-board are), DC vol- matic for component differences between the versions. tages and currents will propagate through. See the LTC6420/1 datasheet for additional information about DC coupling and input bias currents. DIFFERENTIAL INPUTS To drive the inputs differentially, remove the capacitors DIFFERENTIAL OUTPUTS C26/C30 and install them in locations C25/C31. J1/J2 and To use the outputs of DC1299 differentially, move capaci- J7/J5 can now be driven differentially from 50 signal tors C28/C33 to positions C24/C35. The outputs now ap- sources. The inputs to the DC1299 will still be matched to pear differentially across J3/J4 and J8/J6. The outputs of 50 (differentially) when driven in this manner. 2