QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC800 LOW DISTORTION, LOW NOISE DIFFERENTIAL AMPLIFIER / ADC DRIVER LT1993-2/-4/-10 DESCRIPTION Demonstration circuit 800 features the LT1993-X Differen- X accepts single-ended or differential inputs with almost tial Amplifier and ADC Driver. It incorporates a variety of no difference in distortion performance. One standout fea- passive components to support configurations for varied ture of the LTC1993-X is the ability to independently adjust applications. Single-ended or differential input and output the output common-mode voltage, which allows output configurations are possible, and there is a calibration cir- level shifting without any additional components. See the cuit to null out the effects of other circuit components. LT1993-X datasheet for information on typical perform- DC800A-A contains the LT1993-2, DC800A-B contains ance specifications to expect when using the DC800. the LT1993-4, and DC800A-C contains the LT1993-10. Design files for this circuit board are available. Call The LTC1993-X is a high-speed differential amplifier with the LTC factory. superior distortion and noise performance, perfect for , LTC and LT are registered trademarks of Linear Technology Corporation. demanding communications transceiver applications and other high-speed mixed signal applications. The LTC1993- QUICK START PROCEDURE Table 1 shows the function of each SMA connector on the Table 1: DC800 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 DC800 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 LT1993-X, respectively. Volt- put to the DC800. Drive from a 50ohm signal source, ages other than +5V may be used as long as the voltage no external termination necessary. between VCC and GND is between 4V and 5.5V. Switch J3 (VOCM) Output Common-Mode Pin. By default, 0.44VCC is SW1 enables or shuts down the LT1993-X to enable supplied to this pin by a 14k resistor and an 11k re- the part externally, see the Enable (Shutdown) section sistor. Override this voltage with any low-impedance of this Quick Start Guide. DC voltage source. 2. For network analyzer use, calibrate out board and trans- J4 (-OUT) Differential Output. Connected to GND through 0WWWW 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- 3. Apply an input signal to J2. A low-distortion, low noise lyzer input. signal source with an external high-order filter will yield J6 (TEST IN) Calibration network input. Use with network analyzer the best performance. to calibrate out effects of board trace parasitics, trans- formers, and SMA connectors. NOTE: DC800s 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 LT1993-X 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 DC800 LOW DISTORTION, LOW NOISE DIFFERENTIAL AMPLIFIER / ADC DRIVER Oscilloscope, Spectrum Analyzer, Net- work Analyzer, etc. Output Impedance = 50W Use a SHORT Cable or Barrel Figure 1. Proper Measurement Equipment Setup ADDITIONAL INFORMATION Although the DC800 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 DC800 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 LT1993-X can cause large input bias currents if DC800 is driven DC coupled. Since transformer DIFFERENTIAL INPUTS T1 is a transmission-line transformer, DC voltages (and currents) will propagate through. Removing T1 would de- To drive the inputs differentially, remove resistor R2. J1 lete the simple impedance transformation, but free up and J2 can now be driven differentially from a 50W signal space to place extra components on T1s pads. See the source. The input to the DC800 will still be matched to LT1993-X datasheet for additional information about DC 50W (differential) when driven in this manner. coupling and input bias currents. 2