QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC954 LOW DISTORTION, LOW NOISE DIFFERENTIAL AMPLIFIER / ADC DRIVER LT6402-6/-12/-20 DESCRIPTION Demonstration circuit 954 features the LT6402 Differential other high-speed mixed signal applications. The LT6402 Amplifier and ADC Driver. It incorporates a variety of pas- accepts single-ended or differential inputs with almost no sive components to support configurations for varied ap- difference in distortion performance. Another standout plications. Single-ended or differential input and output feature of the LT6402 is the ability to independently adjust configurations are possible, and there is a calibration cir- the output common-mode voltage, which allows output cuit to null out the effects of other circuit components. level shifting without any additional components. See the DC954A-A contains the LT6402-6, DC954A-B contains LT6402 datasheet for more information. the LT6402-12, and DC954A-C contains the LT6402-20. Design files for this circuit board are available. Call The LT6402 is a high-speed differential amplifier with su- the LTC factory. perior distortion and noise performance, perfect for de- , LTC and LT are registered trademarks of Linear Technology Corporation. manding communications transceiver applications and QUICK START PROCEDURE Table 1 shows the function of each SMA connector on the Table 1: DC954 SMA Connector Descriptions board. Refer to Figure 1 for the connection diagram and CONNECTOR FUNCTION follow the procedure below: J1 (-IN) Differential Input. Connected to GND through resistor R2 by default. Resistor can be removed to drive the 1. Connect the power supply as shown. The power labels DC954 differentially. of VCC, EN, and GND directly correspond to the VCC, J2 (+IN) Differential Input. Use this connector to supply an in- EN, and VEE pins of the LT6402, respectively. Voltages put to the DC954. Drive from a 50ohm signal source, other than +5V may be used as long as the voltage be- no external termination necessary. tween VCC and GND is between 4.5V and 5.5V. Switch J3 (VOCM) Output Common-Mode Pin. By default, 0.44VCC is SW1 enables or shuts down the LT6402 to enable the supplied to this pin by a 14k resistor and an 11k re- part externally, see the Enable (Shutdown) section of sistor. Override this voltage with any low-impedance this Quick Start Guide. DC voltage source. J4 (-OUT) Differential Output. Connected to GND through 0WW 2. For network analyzer use, calibrate out board and trans- WW resistor R14 by default. Resistor can be removed for former effects using J6 and J7. There is a 1:1 imped- differential output drive. ance transfer from J6 to J7, so external termination J5 (+OUT) Differential Output. Resistively impedance-matched to may be necessary for a 50W match. 50 ohms, can be used to drive a 50ohm network ana- lyzer input. 3. Apply an input signal to J2. A low-distortion, low noise signal source with an external high-order filter will yield J6 (TEST IN) Calibration network input. Use with network analyzer to calibrate out effects of board trace parasitics, trans- the best performance. formers, and SMA connectors. NOTE: DC954s input is impedance-matched to 50W. No J7 (TEST OUT) Calibration network output. external termination is necessary. 4. Observe the output via J5. By default, the unfiltered outputs of the LT6402 are used. The output is imped- ance-matched to 50W, suitable for the input of a net- work or spectrum analyzer. 1 QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC954 LOW DISTORTION, LOW NOISE DIFFERENTIAL AMPLIFIER / ADC DRIVER Figure 1. Proper Measurement Equipment Setup ADDITIONAL INFORMATION Although the DC954 demo board is ready to use out of the DRIVING THE INPUTS WITH DC COUPLING box (See Quick Start Procedure above), it has features that you can access by adding, removing or changing It is possible to drive the DC954 inputs differentially with components on the board. Here are some ways that you DC coupling. Capacitors C1 and C2 should be shorted or can change the configuration to take advantage of these replaced with 0W resistors. As a warning, the low input features. impedance of the LT6402 can cause large input bias cur- rents if DC954 is driven DC coupled. Since transformer T1 DIFFERENTIAL INPUTS is a transmission-line transformer, DC voltages (and cur- rents) will propagate through. Removing T1 would delete To drive the inputs differentially, remove resistor R2. J1 the simple impedance transformation, but free up space to and J2 can now be driven differentially from a 50W signal place extra components on T1s pads. See the LT6402 source. The input to the DC954 will still be matched to datasheet for additional information about DC coupling 50W (differential) when driven in this manner. and input bias currents. 2