DEMO CIRCUIT DC987 QUICK START GUIDE DEMO CIRCUIT 987 LOW NOISE/DISTORTION DIFFERENTIAL ADC DRIVER QUICK START GUIDE LTC6400 and LTC6401 Low Noise/Distortion Differential ADC Drivers DESCRIPTION Demonstration circuit 987 features the LTC6400 and The LTC6400/1 accepts single-ended or differential inputs LTC6401 ADC Drivers. It incorporates a variety of passive with almost no difference in distortion performance. One components to support configurations for varied applica- standout feature of the LTC6400/1 is the ability to indepen- tions. Single-ended or differential input and output configu- dently adjust the output common-mode voltage without any rations are possible, and there is a calibration circuit to null additional components. out the effects of other circuit components. Design files for this circuit board are available. Call The LTC6400/1 is a high-speed differential amplifier with the LTC factory. superior distortion and noise performance, perfect for de- , LTC and LT are registered trademarks of Linear Technology Corporation. manding communications transceivers, cellular base- stations, and other high-speed signal chain applications. QUICK START PROCEDURE Table 1 shows the function of each SMA connector on the ance-matched to 50, suitable for the input of a network board. Refer to Figure 1 and Table 1, and follow the proce- or spectrum analyzer. dure below: Table 1: DC987 SMA Connector Descriptions 1. Connect the power supply VCC. The power labels of VCC, CONNECTOR FUNCTION EN, and GND on the board directly correspond to the J1 (+IN) Differential Input. Use this connector to supply an input to the DC987. Drive from a 50ohm signal source, no VCC, EN, and VEE pins of the LTC6400/1, respectively. external termination necessary. Jumper JP1 enables or shuts down the LTC6400/1. J2 (-IN) Differential Input. Not connected by default. Resistor 2. For network analyzer use, calibrate out board and trans- R6 can installed and R2 removed to drive the DC987 differentially. former effects using J6 and J7. There is a 1:1 impedance TP53 (VOCM) Output Common-Mode Adjust. By default, 1.25V is transfer from J6 to J7, so external termination may be supplied to this pin by two resistors (VCC=3V). Over- necessary for a 50 match. ride this voltage with any DC voltage source. 3. Apply an input signal to J1. A low-distortion, low J4 (+OUT) Output. Impedance-matched to 50 ohms, can be used to drive a 50ohm network/spectrum analyzer input. noise signal source with an external high-order filter J5 (-OUT) Differential Output. Not connected by default. Resistor will yield the best performance. DC987s input is R12 can be installed and R14 removed for differential impedance-matched to 50 no external termination output drive. is necessary. J6 (TEST IN) Calibration network input. Use with network analyzer to calibrate out effects of board trace parasitics, transfor- 4. Observe the output via J4. By default, the unfiltered out- mers, and SMA connectors. puts of the LTC6400/1 are used. The output is imped- J7 (TEST OUT) Calibration network output. 1 DEMO CIRCUIT DC987 QUICK START GUIDE LOW NOISE/DISTORTION DIFFERENTIAL ADC DRIVER Picture of DC987 Top Silkscreen ADDITIONAL INFORMATION Although the DC987 demo board is ready to use out of the 50 signal source. The input to the DC987 will still be box, it has features that you can access by adding, remov- matched to 50 (differential) when driven in this manner. ing or changing components on the board. These are de- scribed below. DRIVING THE INPUTS WITH DC COUPLING It is possible to drive the DC987 inputs differentially with DC DEMO BOARD VERSIONS coupling. Capacitors C1 and C2 should be shorted or re- DC987 has eight versions to support all of the different gain placed with 0 resistors. As a warning, the low input im- options of the LTC6400/1. DC987B-A through DC987B-D pedance of the LTC6400/1 can cause large input bias cur- contain the LTC6400-8, LTC6400-14, LTC6400-20, and rents if DC987 is driven DC coupled. If transformer T1 is a LTC6400-26, respectively. DC987B-E through DC987B-H transmission-line transformer (the transformer included on- contain the LTC6401-8, LTC6401-14, LTC6401-20, and board is a TLT), DC voltages and currents will propagate LTC6401-26. See the schematic (last page of this guide) for through. See the LTC6400/1 datasheet for additional infor- component differences between the versions. mation about DC coupling and input bias currents. DIFFERENTIAL OUTPUTS DIFFERENTIAL INPUTS To use the both outputs of the DC987, remove resistor R13 To drive the inputs differentially, remove resistor R1 and and install R11. The output now appears differentially install R5. J1 and J2 can now be driven differentially from a 2